coding-style.md

C# Coding Style

The general rule we follow is "use Visual Studio defaults".

1. We use Allman style braces, where each brace begins on a new line. A single line statement block can go without braces but the block must be properly indented on its own line and must not be nested in other statement blocks that use braces (See rule 18 for more details). One exception is that a using statement is permitted to be nested within another using statement by starting on the following line at the same indentation level, even if the nested using contains a controlled block.
2. We use four spaces of indentation (no tabs).
3. We use _camelCase for internal and private fields and use readonly where possible. Prefix internal and private instance fields with _, static fields with s_ and thread static fields with t_. When used on static fields, readonly should come after static (e.g. static readonly not readonly static). Public fields should be used sparingly and should use PascalCasing with no prefix when used.
4. We avoid this. unless absolutely necessary.
5. We always specify the visibility, even if it's the default (e.g. private string _foo not string _foo). Visibility should be the first modifier (e.g. public abstract not abstract public).
6. Namespace imports should be specified at the top of the file, outside of namespace declarations, and should be sorted alphabetically, with the exception of System.* namespaces, which are to be placed on top of all others.
7. Avoid more than one empty line at any time. For example, do not have two blank lines between members of a type.
8. Avoid spurious free spaces. For example avoid if (someVar == 0)..., where the dots mark the spurious free spaces. Consider enabling "View White Space (Ctrl+R, Ctrl+W)" or "Edit -> Advanced -> View White Space" if using Visual Studio to aid detection.
9. If a file happens to differ in style from these guidelines (e.g. private members are named m_member rather than _member), the existing style in that file takes precedence.
10. We only use var when the type is explicitly named on the right-hand side, typically due to either new or an explicit cast, e.g. var stream = new FileStream(...) not var stream = OpenStandardInput().
    • Similarly, target-typed new() can only be used when the type is explicitly named on the left-hand side, in a variable definition statement or a field definition statement. e.g. FileStream stream = new(...);, but not stream = new(...); (where the type was specified on a previous line).
11. We use language keywords instead of BCL types (e.g. int, string, float instead of Int32, String, Single, etc) for both type references as well as method calls (e.g. int.Parse instead of Int32.Parse). See issue #13976 for examples.
12. We use PascalCasing to name all our constant local variables and fields. The only exception is for interop code where the constant value should exactly match the name and value of the code you are calling via interop.
13. We use PascalCasing for all method names, including local functions.
14. We use nameof(...) instead of "..." whenever possible and relevant.
15. Fields should be specified at the top within type declarations.
16. When including non-ASCII characters in the source code use Unicode escape sequences (\uXXXX) instead of literal characters. Literal non-ASCII characters occasionally get garbled by a tool or editor.
17. When using labels (for goto), indent the label one less than the current indentation.
18. When using a single-statement if, we follow these conventions:
    • Never use single-line form (for example: if (source == null) throw new ArgumentNullException("source");)
    • Using braces is always accepted, and required if any block of an if/else if/.../else compound statement uses braces or if a single statement body spans multiple lines.
    • Braces may be omitted only if the body of every block associated with an if/else if/.../else compound statement is placed on a single line.
19. Make all internal and private types static or sealed unless derivation from them is required. As with any implementation detail, they can be changed if/when derivation is required in the future.

An EditorConfig file (.editorconfig) has been provided at the root of the runtime repository, enabling C# auto-formatting conforming to the above guidelines.

We also use the dotnet-format tool to ensure the code base maintains a consistent style over time, the tool automatically fixes the code base to conform to the guidelines outlined above.

Example File:

ObservableLinkedList`1.cs:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Diagnostics;
using Microsoft.Win32;

namespace System.Collections.Generic
{
    public partial class ObservableLinkedList<T> : INotifyCollectionChanged, INotifyPropertyChanged
    {
        private ObservableLinkedListNode<T> _head;
        private int _count;

        public ObservableLinkedList(IEnumerable<T> items)
        {
            if (items == null)
                throw new ArgumentNullException(nameof(items));

            foreach (T item in items)
            {
                AddLast(item);
            }
        }

        public event NotifyCollectionChangedEventHandler CollectionChanged;

        public int Count
        {
            get { return _count; }
        }

        public ObservableLinkedListNode AddLast(T value)
        {
            var newNode = new LinkedListNode<T>(this, value);

            InsertNodeBefore(_head, node);
        }

        protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
        {
            NotifyCollectionChangedEventHandler handler = CollectionChanged;
            if (handler != null)
            {
                handler(this, e);
            }
        }

        private void InsertNodeBefore(LinkedListNode<T> node, LinkedListNode<T> newNode)
        {
            ...
        }

        ...
    }
}

ObservableLinkedList`1.ObservableLinkedListNode.cs:

using System;

namespace System.Collections.Generics
{
    partial class ObservableLinkedList<T>
    {
        public class ObservableLinkedListNode
        {
            private readonly ObservableLinkedList<T> _parent;
            private readonly T _value;

            internal ObservableLinkedListNode(ObservableLinkedList<T> parent, T value)
            {
                Debug.Assert(parent != null);

                _parent = parent;
                _value = value;
            }

            public T Value
            {
                get { return _value; }
            }
        }

        ...
    }
}

For other languages, our current best guidance is consistency. When editing files, keep new code and changes consistent with the style in the files. For new files, it should conform to the style for that component. If there is a completely new component, anything that is reasonably broadly accepted is fine. For script files, please refer to the scripting blog for tips and best practices.