C# Coding Style
===============

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

1. We use [Allman style](http://en.wikipedia.org/wiki/Indent_style#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](https://github.com/dotnet/runtime/issues/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](https://editorconfig.org "EditorConfig homepage") 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 [.NET Codeformatter Tool](https://github.com/dotnet/codeformatter) 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:``

```C#
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:``

```C#
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](https://devblogs.microsoft.com/scripting/tag/powertip) and [best practices](https://devblogs.microsoft.com/scripting/tag/best-practices).
