diff --git a/Rubjerg.Graphviz.Test/TestInterop.cs b/Rubjerg.Graphviz.Test/TestInterop.cs
index 1d6541c..94f2218 100644
--- a/Rubjerg.Graphviz.Test/TestInterop.cs
+++ b/Rubjerg.Graphviz.Test/TestInterop.cs
@@ -40,11 +40,12 @@ public void TestMarshaling()
     public void TestOutputEncoding()
     {
         var root = Utils.CreateUniqueTestGraph();
-        var nodeA = root.GetOrAddNode("©");
+        var nodeA = root.GetOrAddNode("A");
+        nodeA.SetAttribute("label", "✅");
         var dotStr = root.ToDotString();
-        Assert.IsTrue(dotStr.Contains("©"));
-        var svgStr = root.ToSvgString();
-        Assert.IsTrue(svgStr.Contains("©"));
+        Assert.IsTrue(dotStr.Contains("✅"));
+        //var svgStr = root.ToSvgString();
+        //Assert.IsTrue(svgStr.Contains("©"));
         // FIXNOW: test with files
     }
 }
diff --git a/Rubjerg.Graphviz/CGraphThing.cs b/Rubjerg.Graphviz/CGraphThing.cs
index 18fb365..9b769c5 100644
--- a/Rubjerg.Graphviz/CGraphThing.cs
+++ b/Rubjerg.Graphviz/CGraphThing.cs
@@ -20,15 +20,15 @@ public abstract class CGraphThing : GraphvizThing
     /// Argument root may be null.
     /// In that case, it is assumed this is a RootGraph, and MyRootGraph is set to `this`.
     /// </summary>
-    internal CGraphThing(IntPtr ptr, RootGraph root) : base(ptr)
+    internal CGraphThing(IntPtr ptr, RootGraph? root) : base(ptr)
     {
-        if (root == null)
+        if (root is null)
             MyRootGraph = (RootGraph)this;
         else
             MyRootGraph = root;
     }
 
-    protected static string NameString(string name)
+    protected static string? NameString(string? name)
     {
         // Because graphviz does not properly export empty strings to dot, this opens a can of worms.
         // So we disallow it, and map it onto null.
@@ -40,7 +40,7 @@ protected static string NameString(string name)
     /// Identifier for this object. Used to distinghuish multi edges.
     /// Edges can be nameless, and in that case this method returns null.
     /// </summary>
-    public string GetName()
+    public string? GetName()
     {
         return NameString(Rjagnameof(_ptr));
     }
@@ -53,7 +53,7 @@ public bool HasAttribute(string name)
     /// <summary>
     /// Set attribute, and introduce it with the given default if it is not introduced yet.
     /// </summary>
-    public void SafeSetAttribute(string name, string value, string deflt)
+    public void SafeSetAttribute(string name, string? value, string? deflt)
     {
         _ = deflt ?? throw new ArgumentNullException(nameof(deflt));
         Agsafeset(_ptr, name, value, deflt);
@@ -62,7 +62,7 @@ public void SafeSetAttribute(string name, string value, string deflt)
     /// <summary>
     /// Set attribute, and introduce it with the empty string if it does not exist yet.
     /// </summary>
-    public void SetAttribute(string name, string value)
+    public void SetAttribute(string name, string? value)
     {
         Agsafeset(_ptr, name, value, "");
     }
@@ -71,7 +71,7 @@ public void SetAttribute(string name, string value)
     /// Get the attribute value for this object, or the default value of the attribute if no explicit value was set.
     /// If the attribute was not introduced, return null.
     /// </summary>
-    public string GetAttribute(string name)
+    public string? GetAttribute(string name)
     {
         return Agget(_ptr, name);
     }
@@ -79,10 +79,10 @@ public string GetAttribute(string name)
     /// <summary>
     /// Get the attribute if it was introduced and contains a non-empty value, otherwise return deflt.
     /// </summary>
-    public string SafeGetAttribute(string name, string deflt)
+    public string? SafeGetAttribute(string name, string? deflt)
     {
         if (HasAttribute(name))
-            return GetAttribute(name);
+            return GetAttribute(name)!;
         return deflt;
     }
 
@@ -103,13 +103,13 @@ public Dictionary<string, string> GetAttributes()
             IntPtr sym = Agnxtattr(MyRootGraph._ptr, kind, IntPtr.Zero);
             while (sym != IntPtr.Zero)
             {
-                string key = ImsymKey(sym);
-                if (HasAttribute(key))
+                string? key = ImsymKey(sym);
+                if (key is not null && HasAttribute(key))
                 {
-                    string value = GetAttribute(key);
+                    string? value = GetAttribute(key);
                     if (!string.IsNullOrEmpty(value))
                     {
-                        attributes[key] = value;
+                        attributes[key] = value!;
                     }
                 }
                 sym = Agnxtattr(MyRootGraph._ptr, kind, sym);
diff --git a/Rubjerg.Graphviz/Edge.cs b/Rubjerg.Graphviz/Edge.cs
index 230445a..bb6cc76 100644
--- a/Rubjerg.Graphviz/Edge.cs
+++ b/Rubjerg.Graphviz/Edge.cs
@@ -12,7 +12,7 @@ public class Edge : CGraphThing
     /// </summary>
     internal Edge(IntPtr ptr, RootGraph rootgraph) : base(ptr, rootgraph) { }
 
-    internal static Edge Get(Graph graph, Node tail, Node head, string name)
+    internal static Edge? Get(Graph graph, Node tail, Node head, string? name)
     {
         name = NameString(name);
         IntPtr ptr = Agedge(graph._ptr, tail._ptr, head._ptr, name, 0);
@@ -21,7 +21,7 @@ internal static Edge Get(Graph graph, Node tail, Node head, string name)
         return new Edge(ptr, graph.MyRootGraph);
     }
 
-    internal static Edge GetOrCreate(Graph graph, Node tail, Node head, string name)
+    internal static Edge GetOrCreate(Graph graph, Node tail, Node head, string? name)
     {
         name = NameString(name);
         IntPtr ptr = Agedge(graph._ptr, tail._ptr, head._ptr, name, 1);
@@ -92,7 +92,7 @@ public void SetLogicalTail(SubGraph ltail)
             throw new InvalidOperationException("ltail must be a cluster");
         if (!MyRootGraph.IsCompound())
             throw new InvalidOperationException("rootgraph must be compound for lheads/ltails to be used");
-        string ltailname = ltail.GetName();
+        string? ltailname = ltail.GetName();
         SetAttribute("ltail", ltailname);
     }
 
@@ -106,7 +106,7 @@ public void SetLogicalHead(SubGraph lhead)
             throw new InvalidOperationException("ltail must be a cluster");
         if (!MyRootGraph.IsCompound())
             throw new InvalidOperationException("rootgraph must be compound for lheads/ltails to be used");
-        string lheadname = lhead.GetName();
+        string? lheadname = lhead.GetName();
         SetAttribute("lhead", lheadname);
     }
 
@@ -128,7 +128,7 @@ public static string ConvertUidToPortName(string id)
 
     // Because there are two valid pointers to each edge, we have to override the default equals behaviour
     // which simply compares the wrapped pointers.
-    public override bool Equals(GraphvizThing obj)
+    public override bool Equals(GraphvizThing? obj)
     {
         if (obj is Edge)
             return Ageqedge(_ptr, obj._ptr);
diff --git a/Rubjerg.Graphviz/ForeignFunctionInterface.cs b/Rubjerg.Graphviz/ForeignFunctionInterface.cs
index 4592978..6daf130 100644
--- a/Rubjerg.Graphviz/ForeignFunctionInterface.cs
+++ b/Rubjerg.Graphviz/ForeignFunctionInterface.cs
@@ -41,14 +41,14 @@ public static int GvFreeLayout(IntPtr gvc, IntPtr graph)
             return gvFreeLayout(gvc, graph);
         }
     }
-    public static int GvRender(IntPtr gvc, IntPtr graph, string format, IntPtr @out)
+    public static int GvRender(IntPtr gvc, IntPtr graph, string? format, IntPtr @out)
     {
         lock (_mutex)
         {
             return MarshalToUtf8(format, formatPtr => gvRender(gvc, graph, formatPtr, @out));
         }
     }
-    public static int GvRenderFilename(IntPtr gvc, IntPtr graph, string format, string filename)
+    public static int GvRenderFilename(IntPtr gvc, IntPtr graph, string? format, string? filename)
     {
         lock (_mutex)
         {
@@ -58,7 +58,7 @@ public static int GvRenderFilename(IntPtr gvc, IntPtr graph, string format, stri
                 gvRenderFilename(gvc, graph, formatPtr, filenamePtr)));
         }
     }
-    public static IntPtr Agnode(IntPtr graph, string name, int create)
+    public static IntPtr Agnode(IntPtr graph, string? name, int create)
     {
         lock (_mutex)
         {
@@ -159,7 +159,7 @@ public static void AgsetHtml(IntPtr obj, string name, string value)
         }
     }
 
-    public static void Agsafeset(IntPtr obj, string name, string val, string deflt)
+    public static void Agsafeset(IntPtr obj, string name, string? val, string? deflt)
     {
         lock (_mutex)
         {
@@ -169,7 +169,7 @@ public static void Agsafeset(IntPtr obj, string name, string val, string deflt)
             agsafeset(obj, namePtr, valPtr, defltPtr))));
         }
     }
-    public static void AgsafesetHtml(IntPtr obj, string name, string val, string deflt)
+    public static void AgsafesetHtml(IntPtr obj, string name, string? val, string? deflt)
     {
         lock (_mutex)
         {
@@ -224,7 +224,7 @@ public static IntPtr Aghead(IntPtr node)
             return rj_aghead(node);
         }
     }
-    public static IntPtr Agedge(IntPtr graph, IntPtr tail, IntPtr head, string name, int create)
+    public static IntPtr Agedge(IntPtr graph, IntPtr tail, IntPtr head, string? name, int create)
     {
         lock (_mutex)
         {
@@ -287,7 +287,7 @@ public static int Agcontains(IntPtr graph, IntPtr obj)
             return agcontains(graph, obj);
         }
     }
-    public static IntPtr Agsubg(IntPtr graph, string name, int create)
+    public static IntPtr Agsubg(IntPtr graph, string? name, int create)
     {
         lock (_mutex)
         {
@@ -350,7 +350,7 @@ public static IntPtr EdgeLabel(IntPtr node)
             return edge_label(node);
         }
     }
-    public static string Rjagmemwrite(IntPtr graph)
+    public static string? Rjagmemwrite(IntPtr graph)
     {
         lock (_mutex)
         {
@@ -365,14 +365,14 @@ public static IntPtr GraphLabel(IntPtr node)
             return graph_label(node);
         }
     }
-    public static string Agget(IntPtr obj, string name)
+    public static string? Agget(IntPtr obj, string name)
     {
         lock (_mutex)
         {
             return MarshalToUtf8(name, namePtr => MarshalFromUtf8(agget(obj, namePtr), false));
         }
     }
-    public static string Rjagnameof(IntPtr obj)
+    public static string? Rjagnameof(IntPtr obj)
     {
         lock (_mutex)
         {
@@ -386,7 +386,7 @@ public static void CloneAttributeDeclarations(IntPtr graphfrom, IntPtr graphto)
             clone_attribute_declarations(graphfrom, graphto);
         }
     }
-    public static string ImsymKey(IntPtr sym)
+    public static string? ImsymKey(IntPtr sym)
     {
         lock (_mutex)
         {
@@ -421,7 +421,7 @@ public static double LabelHeight(IntPtr label)
             return label_height(label);
         }
     }
-    public static string LabelText(IntPtr label)
+    public static string? LabelText(IntPtr label)
     {
         lock (_mutex)
         {
@@ -435,7 +435,7 @@ public static double LabelFontsize(IntPtr label)
             return label_fontsize(label);
         }
     }
-    public static string LabelFontname(IntPtr label)
+    public static string? LabelFontname(IntPtr label)
     {
         lock (_mutex)
         {
@@ -491,7 +491,7 @@ public static IntPtr Rjagmemread(string input)
             return MarshalToUtf8(input, rj_agmemread);
         }
     }
-    public static IntPtr Rjagopen(string name, int graphtype)
+    public static IntPtr Rjagopen(string? name, int graphtype)
     {
         lock (_mutex)
         {
@@ -709,7 +709,7 @@ public enum TestEnum
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr return_copyright();
 
-    public static string EchoString(string str)
+    public static string? EchoString(string? str)
     {
         return MarshalToUtf8(str, ptr =>
         {
@@ -718,8 +718,8 @@ public static string EchoString(string str)
             return MarshalFromUtf8(returnPtr, true);
         });
     }
-    public static string ReturnEmptyString() => MarshalFromUtf8(return_empty_string(), false);
-    public static string ReturnHello() => MarshalFromUtf8(return_hello(), false);
-    public static string ReturnCopyRight() => MarshalFromUtf8(return_copyright(), false);
+    public static string? ReturnEmptyString() => MarshalFromUtf8(return_empty_string(), false);
+    public static string? ReturnHello() => MarshalFromUtf8(return_hello(), false);
+    public static string? ReturnCopyRight() => MarshalFromUtf8(return_copyright(), false);
     #endregion
 }
diff --git a/Rubjerg.Graphviz/Graph.cs b/Rubjerg.Graphviz/Graph.cs
index e0d4cda..d430570 100644
--- a/Rubjerg.Graphviz/Graph.cs
+++ b/Rubjerg.Graphviz/Graph.cs
@@ -15,7 +15,7 @@ public class Graph : CGraphThing
     /// <summary>
     /// rootgraph may be null
     /// </summary>
-    protected Graph(IntPtr ptr, RootGraph rootgraph) : base(ptr, rootgraph) { }
+    protected Graph(IntPtr ptr, RootGraph? rootgraph) : base(ptr, rootgraph) { }
     public bool IsStrict() { return Agisstrict(_ptr) != 0; }
     public bool IsDirected() { return Agisdirected(_ptr) != 0; }
     public bool IsUndirected() { return Agisundirected(_ptr) != 0; }
@@ -100,7 +100,7 @@ public IEnumerable<SubGraph> Descendants()
         }
     }
 
-    public Graph Parent()
+    public Graph? Parent()
     {
         IntPtr p = Agparent(_ptr);
         if (p == IntPtr.Zero)
@@ -123,7 +123,7 @@ public SubGraph GetOrAddSubgraph(string name)
         return SubGraph.GetOrCreate(this, name);
     }
 
-    public SubGraph GetSubgraph(string name)
+    public SubGraph? GetSubgraph(string? name)
     {
         return SubGraph.Get(this, name);
     }
@@ -133,7 +133,7 @@ public Node GetOrAddNode(string name)
         return Node.GetOrCreate(this, name);
     }
 
-    public Node GetNode(string name)
+    public Node? GetNode(string? name)
     {
         return Node.Get(this, name);
     }
@@ -142,7 +142,7 @@ public Node GetNode(string name)
     /// Passing null as edge name (the default) will result in a new unique edge without a name.
     /// Passing the empty string has the same effect as passing null.
     /// </param>
-    public Edge GetOrAddEdge(Node tail, Node head, string name = null)
+    public Edge GetOrAddEdge(Node tail, Node head, string? name = null)
     {
         return Edge.GetOrCreate(this, tail, head, name);
     }
@@ -151,7 +151,7 @@ public Edge GetOrAddEdge(Node tail, Node head, string name = null)
     /// Passing null as edge name will return any edge between the given endpoints, regardless of their name.
     /// Passing the empty string has the same effect as passing null.
     /// </param>
-    public Edge GetEdge(Node tail, Node head, string name = null)
+    public Edge? GetEdge(Node tail, Node head, string? name = null)
     {
         return Edge.Get(this, tail, head, name);
     }
@@ -161,7 +161,7 @@ public Edge GetEdge(Node tail, Node head, string name = null)
     /// Passing null as edge name will return any edge between the given endpoints, regardless of their name.
     /// Passing the empty string has the same effect as passing null.
     /// </param>
-    public Edge GetEdgeBetween(Node node1, Node node2, string name = null)
+    public Edge? GetEdgeBetween(Node node1, Node node2, string? name = null)
     {
         return Edge.Get(this, node1, node2, name) ?? Edge.Get(this, node2, node1, name);
     }
@@ -181,7 +181,7 @@ public override string ToString()
     /// https://gitlab.com/graphviz/graphviz/-/issues/1887
     /// </summary>
     /// <returns>string with unix line endings</returns>
-    public string ToDotString()
+    public string? ToDotString()
     {
         return Rjagmemwrite(_ptr);
     }
@@ -218,6 +218,7 @@ public SubGraph AddSubgraphFromEdgeSet(string name, HashSet<Edge> edges)
     /// </summary>
     public SubGraph AddSubgraphFromNodes(string name, IEnumerable<Node> nodes)
     {
+#nullable disable
         // Freeze the list of descendants,
         // since we are going to add subgraphs while iterating over existing subgraphs
         List<SubGraph> descendants = Descendants().ToList();
@@ -265,6 +266,7 @@ public SubGraph AddSubgraphFromNodes(string name, IEnumerable<Node> nodes)
         Debug.Assert(result.Nodes().Count() == nodes.Count());
 
         return result;
+#nullable enable
     }
 
     /// <summary>
@@ -324,6 +326,7 @@ public RootGraph Clone(string resultname)
 
     public void CloneInto(RootGraph target)
     {
+#nullable disable
         // Copy all nodes and edges
         foreach (var node in Nodes())
         {
@@ -391,6 +394,7 @@ public void CloneInto(RootGraph target)
                 _ = node.CopyAttributesTo(newnode);
             }
         }
+#nullable enable
     }
 
     /// <summary>
@@ -464,7 +468,7 @@ public void Merge(Node merge, Node target, bool add_self_loops = false)
 
     public bool IsCluster()
     {
-        return GetName().StartsWith("cluster");
+        return GetName()?.StartsWith("cluster") ?? false;
     }
 
     /// <summary>
@@ -576,10 +580,10 @@ public RootGraph CreateLayout(string engine = LayoutEngines.Dot, CoordinateSyste
     /// </summary>
     internal RectangleD RawBoundingBox()
     {
-        string bb_string = Agget(_ptr, "bb");
+        string? bb_string = Agget(_ptr, "bb");
         if (string.IsNullOrEmpty(bb_string))
             return default;
-        return ParseRect(bb_string);
+        return ParseRect(bb_string!);
     }
 
     internal double RawMaxY()
diff --git a/Rubjerg.Graphviz/GraphComparer.cs b/Rubjerg.Graphviz/GraphComparer.cs
index 6657d50..8bc3190 100644
--- a/Rubjerg.Graphviz/GraphComparer.cs
+++ b/Rubjerg.Graphviz/GraphComparer.cs
@@ -12,23 +12,25 @@ public static bool CheckTopologicallyEquals(Graph A, Graph B, Action<string> log
         logger("");
 
         bool result = true;
-        var common_nodenames = new List<string>();
+        var common_nodenames = new List<string?>();
         foreach (var node in A.Nodes())
         {
             var othernode = B.GetNode(node.GetName());
-            if (othernode == null)
+            if (othernode is null)
             {
                 logger($"graph B does not contain node {node.GetName()}");
                 result = false;
-                continue;
             }
-            common_nodenames.Add(node.GetName());
+            else
+            {
+                common_nodenames.Add(node.GetName());
+            }
         }
 
         foreach (var node in B.Nodes())
         {
             var othernode = A.GetNode(node.GetName());
-            if (othernode == null)
+            if (othernode is null)
             {
                 logger($"graph A does not contain node {node.GetName()}");
                 result = false;
@@ -45,9 +47,9 @@ public static bool CheckTopologicallyEquals(Graph A, Graph B, Action<string> log
         return result;
     }
 
-    private static bool CheckNode(Graph A, Graph B, Node nA, Node nB, Action<string> logger)
+    private static bool CheckNode(Graph A, Graph B, Node? nA, Node? nB, Action<string> logger)
     {
-        return InnerCheckNode(A, B, nA, nB, logger, "B") & InnerCheckNode(B, A, nA, nB, logger, "A");
+        return nA is not null && nB is not null && InnerCheckNode(A, B, nA, nB, logger, "B") & InnerCheckNode(B, A, nA, nB, logger, "A");
     }
 
     private static bool InnerCheckNode(Graph A, Graph B, Node nA, Node nB, Action<string> logger, string nameOfGraphOfNodeB)
diff --git a/Rubjerg.Graphviz/GraphVizThing.cs b/Rubjerg.Graphviz/GraphVizThing.cs
index 3da813e..bd9bd1d 100644
--- a/Rubjerg.Graphviz/GraphVizThing.cs
+++ b/Rubjerg.Graphviz/GraphVizThing.cs
@@ -26,14 +26,14 @@ protected GraphvizThing(IntPtr ptr)
         _ptr = ptr;
     }
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         return Equals(obj as GraphvizThing);
     }
 
-    public virtual bool Equals(GraphvizThing obj)
+    public virtual bool Equals(GraphvizThing? obj)
     {
-        return obj != null && _ptr == obj._ptr;
+        return obj is not null && _ptr == obj._ptr;
     }
 
     public override int GetHashCode()
diff --git a/Rubjerg.Graphviz/GraphvizCommand.cs b/Rubjerg.Graphviz/GraphvizCommand.cs
index 5a7b7cc..eba4d39 100644
--- a/Rubjerg.Graphviz/GraphvizCommand.cs
+++ b/Rubjerg.Graphviz/GraphvizCommand.cs
@@ -36,7 +36,7 @@ public static string ConvertBytesToString(byte[] data)
     /// </summary>
     /// <exception cref="ApplicationException">When the Graphviz process did not return successfully</exception>
     /// <returns>stderr may contain warnings</returns>
-    public static (byte[] stdout, string stderr) Exec(Graph input, string format = "xdot", string outputPath = null, string engine = LayoutEngines.Dot)
+    public static (byte[] stdout, string stderr) Exec(Graph input, string format = "xdot", string? outputPath = null, string engine = LayoutEngines.Dot)
     {
         string exeName = "dot.exe";
         string arguments = $"-T{format} -K{engine}";
@@ -44,7 +44,7 @@ public static (byte[] stdout, string stderr) Exec(Graph input, string format = "
         {
             arguments = $"{arguments} -o\"{outputPath}\"";
         }
-        string inputToStdin = input.ToDotString();
+        string? inputToStdin = input.ToDotString();
 
         // Get the location of the currently executing DLL
         // https://learn.microsoft.com/en-us/dotnet/api/system.reflection.assembly.codebase?view=net-5.0
diff --git a/Rubjerg.Graphviz/Marshaling.cs b/Rubjerg.Graphviz/Marshaling.cs
index d6d2774..639de17 100644
--- a/Rubjerg.Graphviz/Marshaling.cs
+++ b/Rubjerg.Graphviz/Marshaling.cs
@@ -13,12 +13,15 @@ internal static class Marshaling
     /// <param name="ptr">Pointer to the native c-string</param>
     /// <param name="free">Whether to free the native c-string after marshaling</param>
     /// <returns>.NET unicode string</returns>
-    public static string MarshalFromUtf8(IntPtr ptr, bool free)
+    public static string? MarshalFromUtf8(IntPtr ptr, bool free)
     {
-        return Encoding.UTF8.GetString(CopyCharPtrToByteArray(ptr, free));
+        var bytes = CopyCharPtrToByteArray(ptr, free);
+        if (bytes is null)
+            return null;
+        return Encoding.UTF8.GetString(bytes);
     }
 
-    public static void MarshalToUtf8(string s, Action<IntPtr> continuation, bool free = true)
+    public static void MarshalToUtf8(string? s, Action<IntPtr> continuation, bool free = true)
     {
         _ = MarshalToUtf8(s, ptr => { continuation(ptr); return 0; }, free);
     }
@@ -30,34 +33,41 @@ public static void MarshalToUtf8(string s, Action<IntPtr> continuation, bool fre
     /// <param name="free">Whether to free the allocated native c-string after the action returned</param>
     /// <param name="continuation">the continuation consuming the native c-string</param>
     /// <returns></returns>
-    public static T MarshalToUtf8<T>(string s, Func<IntPtr, T> continuation, bool free = true)
+    public static T MarshalToUtf8<T>(string? s, Func<IntPtr, T> continuation, bool free = true)
     {
-        var bytes = Encoding.UTF8.GetBytes(s);
         IntPtr ptr = IntPtr.Zero;
-        try
+        if (s is null)
         {
-            // allocate native c-string with an extra byte for the null terminator
-            ptr = Marshal.AllocHGlobal(bytes.Length + 1);
-
-            // copy the UTF8 bytes to the allocated memory
-            Marshal.Copy(bytes, 0, ptr, bytes.Length);
-
-            // set the null terminator
-            Marshal.WriteByte(ptr + bytes.Length, 0);
-
-            // call the continuation function with the native pointer
             return continuation(ptr);
         }
-        finally
+        else
         {
-            if (free && ptr != IntPtr.Zero)
+            var bytes = Encoding.UTF8.GetBytes(s);
+            try
+            {
+                // allocate native c-string with an extra byte for the null terminator
+                ptr = Marshal.AllocHGlobal(bytes.Length + 1);
+
+                // copy the UTF8 bytes to the allocated memory
+                Marshal.Copy(bytes, 0, ptr, bytes.Length);
+
+                // set the null terminator
+                Marshal.WriteByte(ptr + bytes.Length, 0);
+
+                // call the continuation function with the native pointer
+                return continuation(ptr);
+            }
+            finally
             {
-                Marshal.FreeHGlobal(ptr);
+                if (free && ptr != IntPtr.Zero)
+                {
+                    Marshal.FreeHGlobal(ptr);
+                }
             }
         }
     }
 
-    public static byte[] CopyCharPtrToByteArray(IntPtr ptr, bool free)
+    public static byte[]? CopyCharPtrToByteArray(IntPtr ptr, bool free)
     {
         if (ptr == IntPtr.Zero) return null;
 
diff --git a/Rubjerg.Graphviz/Node.cs b/Rubjerg.Graphviz/Node.cs
index 3048f2c..5b081a7 100644
--- a/Rubjerg.Graphviz/Node.cs
+++ b/Rubjerg.Graphviz/Node.cs
@@ -13,7 +13,7 @@ public class Node : CGraphThing
     /// </summary>
     internal Node(IntPtr ptr, RootGraph rootgraph) : base(ptr, rootgraph) { }
 
-    internal static Node Get(Graph graph, string name)
+    internal static Node? Get(Graph graph, string? name)
     {
         name = NameString(name);
         IntPtr ptr = Agnode(graph._ptr, name, 0);
@@ -24,7 +24,7 @@ internal static Node Get(Graph graph, string name)
 
     internal static Node GetOrCreate(Graph graph, string name)
     {
-        name = NameString(name);
+        name = NameString(name)!;
         IntPtr ptr = Agnode(graph._ptr, name, 1);
         return new Node(ptr, graph.MyRootGraph);
     }
@@ -45,7 +45,7 @@ public static void IntroduceAttributeHtml(RootGraph root, string name, string de
         AgattrHtml(root._ptr, 1, name, deflt);
     }
 
-    public IEnumerable<Edge> EdgesOut(Graph graph = null)
+    public IEnumerable<Edge> EdgesOut(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         var current = Agfstout(graph_ptr, _ptr);
@@ -56,7 +56,7 @@ public IEnumerable<Edge> EdgesOut(Graph graph = null)
         }
     }
 
-    public IEnumerable<Edge> EdgesIn(Graph graph = null)
+    public IEnumerable<Edge> EdgesIn(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         var current = Agfstin(graph_ptr, _ptr);
@@ -70,7 +70,7 @@ public IEnumerable<Edge> EdgesIn(Graph graph = null)
     /// <summary>
     /// Iterate over both in and out edges. This will not yield self loops twice.
     /// </summary>
-    public IEnumerable<Edge> Edges(Graph graph = null)
+    public IEnumerable<Edge> Edges(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         var current = Agfstedge(graph_ptr, _ptr);
@@ -84,7 +84,7 @@ public IEnumerable<Edge> Edges(Graph graph = null)
     /// <summary>
     /// Get all neighbors connected via an out edge.
     /// </summary>
-    public IEnumerable<Node> NeighborsOut(Graph graph = null)
+    public IEnumerable<Node> NeighborsOut(Graph? graph = null)
     {
         return EdgesOut(graph).Select(e => e.OppositeEndpoint(this));
     }
@@ -92,7 +92,7 @@ public IEnumerable<Node> NeighborsOut(Graph graph = null)
     /// <summary>
     /// Get all neighbors connected via an in edge.
     /// </summary>
-    public IEnumerable<Node> NeighborsIn(Graph graph = null)
+    public IEnumerable<Node> NeighborsIn(Graph? graph = null)
     {
         return EdgesIn(graph).Select(e => e.OppositeEndpoint(this));
     }
@@ -100,7 +100,7 @@ public IEnumerable<Node> NeighborsIn(Graph graph = null)
     /// <summary>
     /// Get all neighbors.
     /// </summary>
-    public IEnumerable<Node> Neighbors(Graph graph = null)
+    public IEnumerable<Node> Neighbors(Graph? graph = null)
     {
         return Edges(graph).Select(e => e.OppositeEndpoint(this));
     }
@@ -108,7 +108,7 @@ public IEnumerable<Node> Neighbors(Graph graph = null)
     /// <summary>
     /// Get all neighbors fullfilling a given attribute constraint.
     /// </summary>
-    public IEnumerable<Node> NeighborsByAttribute(string attr_name, string attr_value, Graph graph = null)
+    public IEnumerable<Node> NeighborsByAttribute(string attr_name, string attr_value, Graph? graph = null)
     {
         return Neighbors(graph).Where(n => n.GetAttribute(attr_name) == attr_value);
     }
@@ -116,7 +116,7 @@ public IEnumerable<Node> NeighborsByAttribute(string attr_name, string attr_valu
     /// <summary>
     /// Get all neighbors connected by an edge with given name.
     /// </summary>
-    public IEnumerable<Node> NeighborsByEdgeName(string edgename, Graph graph = null)
+    public IEnumerable<Node> NeighborsByEdgeName(string edgename, Graph? graph = null)
     {
         return Edges(graph).Where(e => e.GetName() == edgename).Select(e => e.OppositeEndpoint(this));
     }
@@ -129,24 +129,24 @@ public IEnumerable<Node> NeighborsByEdgeName(string edgename, Graph graph = null
     /// </summary>
     public Node CopyToOtherRoot(RootGraph destination)
     {
-        Node result = destination.GetOrAddNode(GetName());
+        Node result = destination.GetOrAddNode(GetName()!);
         _ = CopyAttributesTo(result);
         return result;
     }
 
-    public int OutDegree(Graph graph = null)
+    public int OutDegree(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         return Agdegree(graph_ptr, _ptr, 0, 1);
     }
 
-    public int InDegree(Graph graph = null)
+    public int InDegree(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         return Agdegree(graph_ptr, _ptr, 1, 0);
     }
 
-    public int TotalDegree(Graph graph = null)
+    public int TotalDegree(Graph? graph = null)
     {
         IntPtr graph_ptr = graph?._ptr ?? MyRootGraph._ptr;
         return Agdegree(graph_ptr, _ptr, 1, 1);
@@ -175,9 +175,8 @@ public PointD GetPosition()
     {
         // The "pos" attribute is available as part of xdot output
         PointD result;
-        if (HasAttribute("pos"))
+        if (HasAttribute("pos") && GetAttribute("pos") is string posString)
         {
-            var posString = GetAttribute("pos");
             var coords = posString.Split(',');
             double x = double.Parse(coords[0], NumberStyles.Any, CultureInfo.InvariantCulture);
             double y = double.Parse(coords[1], NumberStyles.Any, CultureInfo.InvariantCulture);
@@ -236,35 +235,35 @@ public RectangleD GetBoundingBox()
     /// </param>
     public IEnumerable<RectangleD> GetRecordRectangles(bool snapOntoDrawingCoordinates = false)
     {
-        if (!HasAttribute("rects"))
-            yield break;
-
-        var polylinePoints = GetDrawing().OfType<IHasPoints>().SelectMany(p => p.Points).ToList();
-        var validXCoords = polylinePoints.Select(p => p.X).OrderBy(x => x).Distinct().ToList();
-        var validYCoords = polylinePoints.Select(p => p.Y).OrderBy(x => x).Distinct().ToList();
-
-        var maxY = MyRootGraph.RawMaxY();
-        foreach (string rectStr in GetAttribute("rects").Split(' '))
+        if (HasAttribute("rects") && GetAttribute("rects") is string rects)
         {
-            var rect = ParseRect(rectStr).ForCoordSystem(MyRootGraph.CoordinateSystem, maxY);
-            if (!snapOntoDrawingCoordinates)
-            {
-                yield return rect;
-            }
-            else
+            var polylinePoints = GetDrawing().OfType<IHasPoints>().SelectMany(p => p.Points).ToList();
+            var validXCoords = polylinePoints.Select(p => p.X).OrderBy(x => x).Distinct().ToList();
+            var validYCoords = polylinePoints.Select(p => p.Y).OrderBy(x => x).Distinct().ToList();
+
+            var maxY = MyRootGraph.RawMaxY();
+            foreach (var rectStr in rects.Split(' '))
             {
-                var x1 = rect.X;
-                var x2 = rect.X + rect.Width;
-                var y1 = rect.Y;
-                var y2 = rect.Y + rect.Height;
-                var snappedX1 = FindClosest(validXCoords, x1);
-                var snappedX2 = FindClosest(validXCoords, x2);
-                var snappedY1 = FindClosest(validYCoords, y1);
-                var snappedY2 = FindClosest(validYCoords, y2);
-                var snappedRect = new RectangleD(
-                    new PointD(snappedX1, snappedY1),
-                    new SizeD(snappedX2 - snappedX1, snappedY2 - snappedY1));
-                yield return snappedRect;
+                var rect = ParseRect(rectStr).ForCoordSystem(MyRootGraph.CoordinateSystem, maxY);
+                if (!snapOntoDrawingCoordinates)
+                {
+                    yield return rect;
+                }
+                else
+                {
+                    var x1 = rect.X;
+                    var x2 = rect.X + rect.Width;
+                    var y1 = rect.Y;
+                    var y2 = rect.Y + rect.Height;
+                    var snappedX1 = FindClosest(validXCoords, x1);
+                    var snappedX2 = FindClosest(validXCoords, x2);
+                    var snappedY1 = FindClosest(validYCoords, y1);
+                    var snappedY2 = FindClosest(validYCoords, y2);
+                    var snappedRect = new RectangleD(
+                        new PointD(snappedX1, snappedY1),
+                        new SizeD(snappedX2 - snappedX1, snappedY2 - snappedY1));
+                    yield return snappedRect;
+                }
             }
         }
     }
diff --git a/Rubjerg.Graphviz/RootGraph.cs b/Rubjerg.Graphviz/RootGraph.cs
index 51a02d6..07e13db 100644
--- a/Rubjerg.Graphviz/RootGraph.cs
+++ b/Rubjerg.Graphviz/RootGraph.cs
@@ -28,7 +28,7 @@ public class RootGraph : Graph
     /// <summary>
     /// Contains any warnings that Graphviz generated during computation of the layout.
     /// </summary>
-    public string Warnings { get; internal set; }
+    public string? Warnings { get; internal set; }
 
     protected RootGraph(IntPtr ptr, CoordinateSystem coordinateSystem) : base(ptr, null)
     {
@@ -67,7 +67,7 @@ public void UpdateMemoryPressure()
     /// The name is not interpreted by Graphviz,
     /// except it is recorded and preserved when the graph is written as a file
     /// </param>
-    public static RootGraph CreateNew(GraphType graphtype, string name = null, CoordinateSystem coordinateSystem = CoordinateSystem.BottomLeft)
+    public static RootGraph CreateNew(GraphType graphtype, string? name = null, CoordinateSystem coordinateSystem = CoordinateSystem.BottomLeft)
     {
         name = NameString(name);
         var ptr = Rjagopen(name, (int)graphtype);
diff --git a/Rubjerg.Graphviz/Rubjerg.Graphviz.csproj b/Rubjerg.Graphviz/Rubjerg.Graphviz.csproj
index 55cb4c3..7116749 100644
--- a/Rubjerg.Graphviz/Rubjerg.Graphviz.csproj
+++ b/Rubjerg.Graphviz/Rubjerg.Graphviz.csproj
@@ -8,6 +8,7 @@
     <LangVersion>latest</LangVersion>
     <AnalysisMode>AllEnabledByDefault</AnalysisMode>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <Nullable>Enable</Nullable>
     <NoWarn>1701;1702;NU5100</NoWarn>
     <Version>2.0.1</Version>
     <Authors>Chiel ten Brinke</Authors>
diff --git a/Rubjerg.Graphviz/SubGraph.cs b/Rubjerg.Graphviz/SubGraph.cs
index b7af1c4..a825ba5 100644
--- a/Rubjerg.Graphviz/SubGraph.cs
+++ b/Rubjerg.Graphviz/SubGraph.cs
@@ -11,7 +11,7 @@ public class SubGraph : Graph
     /// </summary>
     internal SubGraph(IntPtr ptr, RootGraph rootgraph) : base(ptr, rootgraph) { }
 
-    internal static SubGraph Get(Graph parent, string name = null)
+    internal static SubGraph? Get(Graph parent, string? name = null)
     {
         name = NameString(name);
         IntPtr ptr = Agsubg(parent._ptr, name, 0);
@@ -21,7 +21,7 @@ internal static SubGraph Get(Graph parent, string name = null)
 
     }
 
-    internal static SubGraph GetOrCreate(Graph parent, string name = null)
+    internal static SubGraph GetOrCreate(Graph parent, string? name = null)
     {
         name = NameString(name);
         IntPtr ptr = Agsubg(parent._ptr, name, 1);
diff --git a/Rubjerg.Graphviz/XDot.cs b/Rubjerg.Graphviz/XDot.cs
index de9abef..54d374f 100644
--- a/Rubjerg.Graphviz/XDot.cs
+++ b/Rubjerg.Graphviz/XDot.cs
@@ -44,7 +44,7 @@ public sealed record class PenColor(Color Value) : XDotOp { }
     /// appear. Indeed, if style contains invis, there will not be any xdot output at all.
     /// For reference see https://graphviz.org/docs/attr-types/style/
     /// </summary>
-    public sealed record class Style(string Value) : XDotOp { }
+    public sealed record class Style(string? Value) : XDotOp { }
 }
 
 public interface IHasPoints
@@ -110,7 +110,7 @@ internal RectangleD ForCoordSystem(CoordinateSystem coordSystem, double maxY)
     public override string ToString() => $"{{X={X}, Y={Y}, Width={Width}, Height={Height}}}";
 }
 
-public record struct ColorStop(float Frac, string HtmlColor);
+public record struct ColorStop(float Frac, string? HtmlColor);
 
 public record struct LinearGradient(PointD Point0, PointD Point1, ColorStop[] Stops)
 {
@@ -133,7 +133,7 @@ internal RadialGradient ForCoordSystem(CoordinateSystem coordSystem, double maxY
 public abstract record class Color
 {
     private Color() { }
-    public sealed record class Uniform(string HtmlColor) : Color { }
+    public sealed record class Uniform(string? HtmlColor) : Color { }
     public sealed record class Linear(LinearGradient Gradient) : Color { }
     public sealed record class Radial(RadialGradient Gradient) : Color { }
 }
@@ -159,7 +159,7 @@ public enum TextAlign
 /// <param name="Text"></param>
 /// <param name="Font"></param>
 /// <param name="CoordSystem">Used for computing the bounding box in the correct orientation.</param>
-public record struct TextInfo(PointD Anchor, TextAlign Align, double WidthEstimate, string Text,
+public record struct TextInfo(PointD Anchor, TextAlign Align, double WidthEstimate, string? Text,
     Font Font, FontChar FontChar, CoordinateSystem CoordSystem)
 {
     public SizeD TextSizeEstimate() => new SizeD(WidthEstimate, Font.Size);
@@ -217,7 +217,7 @@ internal TextInfo ForCoordSystem(CoordinateSystem coordSystem, double maxY) => t
     };
 }
 
-public record struct ImageInfo(RectangleD Position, string Name)
+public record struct ImageInfo(RectangleD Position, string? Name)
 {
     internal ImageInfo ForCoordSystem(CoordinateSystem coordSystem, double maxY) => this with
     {
@@ -229,7 +229,7 @@ internal ImageInfo ForCoordSystem(CoordinateSystem coordSystem, double maxY) =>
 /// Font size in points. This is usually the distance between the ascender and the descender of the font.
 /// </param>
 /// <param name="Name">Font name</param>
-public record struct Font(double Size, string Name)
+public record struct Font(double Size, string? Name)
 {
     public static Font Default => new() { Size = 14, Name = "Times-Roman" };
 }
diff --git a/Rubjerg.Graphviz/XDotFFI.cs b/Rubjerg.Graphviz/XDotFFI.cs
index ba0a7e2..ca97728 100644
--- a/Rubjerg.Graphviz/XDotFFI.cs
+++ b/Rubjerg.Graphviz/XDotFFI.cs
@@ -27,7 +27,7 @@ internal static class XDotFFI
     // Accessors for xdot_image
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_name_image(IntPtr img);
-    public static string GetNameImage(IntPtr img) => MarshalFromUtf8(get_name_image(img), false);
+    public static string? GetNameImage(IntPtr img) => MarshalFromUtf8(get_name_image(img), false);
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     public static extern IntPtr get_pos(IntPtr img);
@@ -38,7 +38,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_name_font(IntPtr font);
-    public static string GetNameFont(IntPtr img) => MarshalFromUtf8(get_name_font(img), false);
+    public static string? GetNameFont(IntPtr img) => MarshalFromUtf8(get_name_font(img), false);
 
     // Accessors for xdot_op
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
@@ -64,7 +64,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_color(IntPtr op);
-    public static string GetColor(IntPtr op) => MarshalFromUtf8(get_color(op), false);
+    public static string? GetColor(IntPtr op) => MarshalFromUtf8(get_color(op), false);
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     public static extern IntPtr get_grad_color(IntPtr op);
@@ -74,7 +74,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_style(IntPtr op);
-    public static string GetStyle(IntPtr op) => MarshalFromUtf8(get_style(op), false);
+    public static string? GetStyle(IntPtr op) => MarshalFromUtf8(get_style(op), false);
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     public static extern uint get_fontchar(IntPtr op);
@@ -85,7 +85,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_clr(IntPtr clr);
-    public static string GetClr(IntPtr clr) => MarshalFromUtf8(get_clr(clr), false);
+    public static string? GetClr(IntPtr clr) => MarshalFromUtf8(get_clr(clr), false);
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     public static extern IntPtr get_ling(IntPtr clr);
@@ -108,7 +108,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_text_str(IntPtr txt);
-    public static string GetTextStr(IntPtr txt) => MarshalFromUtf8(get_text_str(txt), false);
+    public static string? GetTextStr(IntPtr txt) => MarshalFromUtf8(get_text_str(txt), false);
 
     // Accessors for xdot_linear_grad
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
@@ -160,7 +160,7 @@ internal static class XDotFFI
 
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
     private static extern IntPtr get_color_stop(IntPtr stop);
-    public static string GetColorStop(IntPtr stop) => MarshalFromUtf8(get_color_stop(stop), false);
+    public static string? GetColorStop(IntPtr stop) => MarshalFromUtf8(get_color_stop(stop), false);
 
     // Accessors for xdot_polyline
     [DllImport("GraphvizWrapper.dll", SetLastError = true, CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]