Adding single statement if formatting rule to coding-style (dotnet/corefx#41906)

* Adding single statement if formatting rule to coding-style

cc: @stephentoub @carlossanlop 

This came up in a recent PR and I just wanted to make it more clear in our Coding guidelines.

* Apply suggestions from code review

Co-Authored-By: Sam Harwell <sam.harwell@microsoft.com>


Commit migrated from dab6a40e99

docs/libraries/coding-guidelines/coding-style.md

@@ -7,7 +7,7 @@ For non code files (xml, etc), our current best guidance is consistency. When ed
 
 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 issue [381](https://github.com/dotnet/corefx/issues/381) for examples). 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.
+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 17 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. 
@@ -30,6 +30,10 @@ The general rule we follow is "use Visual Studio defaults".
 14. Fields should be specified at the top within type declarations.
 15. 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.
 16. When using labels (for goto), indent the label one less than the current indentation.
+17. 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.
 
 An [EditorConfig](https://editorconfig.org "EditorConfig homepage") file (`.editorconfig`) has been provided at the root of the corefx repository, enabling C# auto-formatting conforming to the above guidelines.