styleguide/csharpguide.md

# C# at Google Style Guide

This style guide is for C# code developed internally at Google, and is the
default style for C# code at Google. It makes stylistic choices that conform to
other languages at Google, such as Google C++ style and Google Java style.

<!-- mdformat off -->
* TOC
{:toc}
<!-- mdformat on -->

## Formatting guidelines

### Naming rules

Naming rules follow
[Microsoft's C# naming guidelines](https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/naming-guidelines).
Where Microsoft's naming guidelines are unspecified (e.g. private and local
variables), rules are taken from the
[CoreFX C# coding guidelines](https://github.com/dotnet/runtime/blob/HEAD/docs/coding-guidelines/coding-style.md)

Rule summary:

#### Code

*   Names of classes, methods, enumerations, public fields, public properties,
    namespaces: `PascalCase`.
*   Names of local variables, parameters: `camelCase`.
*   Names of private, protected, internal and protected internal fields and
    properties: `_camelCase`.
*   Naming convention is unaffected by modifiers such as const, static,
    readonly, etc.
*   For casing, a "word" is anything written without internal spaces, including
    acronyms. For example, `MyRpc` instead of ~~`MyRPC`~~.
*   Names of interfaces start with `I`, e.g. `IInterface`.

#### Files

*   Filenames and directory names are `PascalCase`, e.g. `MyFile.cs`.
*   Where possible the file name should be the same as the name of the main
    class in the file, e.g. `MyClass.cs`.
*   In general, prefer one core class per file.

### Organization

*   Modifiers occur in the following order: `public protected internal private
    new abstract virtual override sealed static readonly extern unsafe volatile
    async`.
*   Namespace `using` declarations go at the top, before any namespaces. `using`
    import order is alphabetical, apart from `System` imports which always go
    first.
*   Class member ordering:
    *   Group class members in the following order:
        *   Nested classes, enums, delegates and events.
        *   Static, const and readonly fields.
        *   Fields and properties.
        *   Constructors and finalizers.
        *   Methods.
    *   Within each group, elements should be in the following order:
        *   Public.
        *   Internal.
        *   Protected internal.
        *   Protected.
        *   Private.
    *   Where possible, group interface implementations together.

### Whitespace rules

Developed from Google Java style.

*   A maximum of one statement per line.
*   A maximum of one assignment per statement.
*   Indentation of 2 spaces, no tabs.
*   Column limit: 100.
*   No line break before opening brace.
*   No line break between closing brace and `else`.
*   Braces used even when optional.
*   Space after `if`/`for`/`while` etc., and after commas.
*   No space after an opening parenthesis or before a closing parenthesis.
*   No space between a unary operator and its operand. One space between the
    operator and each operand of all other operators.
*   Line wrapping developed from Google C++ style guidelines, with minor
    modifications for compatibility with Microsoft's C# formatting tools:
    *   In general, line continuations are indented 4 spaces.
    *   Line breaks with braces (e.g. list initializers, lambdas, object
        initializers, etc) do not count as continuations.
    *   For function definitions and calls, if the arguments do not all fit on
        one line they should be broken up onto multiple lines, with each
        subsequent line aligned with the first argument. If there is not enough
        room for this, arguments may instead be placed on subsequent lines with
        a four space indent. The code example below illustrates this.

### Example

```c#
using System;                                       // `using` goes at the top, outside the
                                                    // namespace.

namespace MyNamespace {                             // Namespaces are PascalCase.
                                                    // Indent after namespace.
  public interface IMyInterface {                   // Interfaces start with 'I'
    public int Calculate(float value, float exp);   // Methods are PascalCase
                                                    // ...and space after comma.
  }

  public enum MyEnum {                              // Enumerations are PascalCase.
    Yes,                                            // Enumerators are PascalCase.
    No,
  }

  public class MyClass {                            // Classes are PascalCase.
    public int Foo = 0;                             // Public member variables are
                                                    // PascalCase.
    public bool NoCounting = false;                 // Field initializers are encouraged.
    private class Results {
      public int NumNegativeResults = 0;
      public int NumPositiveResults = 0;
    }
    private Results _results;                       // Private member variables are
                                                    // _camelCase.
    public static int NumTimesCalled = 0;
    private const int _bar = 100;                   // const does not affect naming
                                                    // convention.
    private int[] _someTable = {                    // Container initializers use a 2
      2, 3, 4,                                      // space indent.
    }

    public MyClass() {
      _results = new Results {
        NumNegativeResults = 1,                     // Object initializers use a 2 space
        NumPositiveResults = 1,                     // indent.
      };
    }

    public int CalculateValue(int mulNumber) {      // No line break before opening brace.
      var resultValue = Foo * mulNumber;            // Local variables are camelCase.
      NumTimesCalled++;
      Foo += _bar;

      if (!NoCounting) {                            // No space after unary operator and
                                                    // space after 'if'.
        if (resultValue < 0) {                      // Braces used even when optional and
                                                    // spaces around comparison operator.
          _results.NumNegativeResults++;
        } else if (resultValue > 0) {               // No newline between brace and else.
          _results.NumPositiveResults++;
        }
      }

      return resultValue;
    }

    public void ExpressionBodies() {
      // For simple lambdas, fit on one line if possible, no brackets or braces required.
      Func<int, int> increment = x => x + 1;

      // Closing brace aligns with first character on line that includes the opening brace.
      Func<int, int, long> difference1 = (x, y) => {
        long diff = (long)x - y;
        return diff >= 0 ? diff : -diff;
      };

      // If defining after a continuation line break, indent the whole body.
      Func<int, int, long> difference2 =
          (x, y) => {
            long diff = (long)x - y;
            return diff >= 0 ? diff : -diff;
          };

      // Inline lambda arguments also follow these rules. Prefer a leading newline before
      // groups of arguments if they include lambdas.
      CallWithDelegate(
          (x, y) => {
            long diff = (long)x - y;
            return diff >= 0 ? diff : -diff;
          });
    }

    void DoNothing() {}                             // Empty blocks may be concise.

    // If possible, wrap arguments by aligning newlines with the first argument.
    void AVeryLongFunctionNameThatCausesLineWrappingProblems(int longArgumentName,
                                                             int p1, int p2) {}

    // If aligning argument lines with the first argument doesn't fit, or is difficult to
    // read, wrap all arguments on new lines with a 4 space indent.
    void AnotherLongFunctionNameThatCausesLineWrappingProblems(
        int longArgumentName, int longArgumentName2, int longArgumentName3) {}

    void CallingLongFunctionName() {
      int veryLongArgumentName = 1234;
      int shortArg = 1;
      // If possible, wrap arguments by aligning newlines with the first argument.
      AnotherLongFunctionNameThatCausesLineWrappingProblems(shortArg, shortArg,
                                                            veryLongArgumentName);
      // If aligning argument lines with the first argument doesn't fit, or is difficult to
      // read, wrap all arguments on new lines with a 4 space indent.
      AnotherLongFunctionNameThatCausesLineWrappingProblems(
          veryLongArgumentName, veryLongArgumentName, veryLongArgumentName);
    }
  }
}
```

## C# coding guidelines

### Constants

*   Variables and fields that can be made `const` should always be made `const`.
*   If `const` isn't possible, `readonly` can be a suitable alternative.
*   Prefer named constants to magic numbers.

### Containers: IEnumerable vs IList vs IReadOnlyList

*   Declare (input) parameters using a type that grants the fewest capabilities
    required by the implementation (eg. `IReadOnlyCollection`).
    *   Use `IEnumerable` only if the implementation can iterate over the
        collection at most once. This indicates that it is safe for the caller
        to supply a generator.
*   For outputs, if passing ownership of the returned container to the caller,
    use the interface type that conveys the maximum capabilities to the caller
    without revealing the concrete type. Prefer `IList` over `List`. If not
    transferring ownership, prefer the most restrictive, as for inputs.

### Generators vs containers

*   Use your best judgement, bearing in mind:
    *   Generator code is often less readable than filling in a container.
    *   Generator code can be more performant if the results are going to be
        processed lazily, e.g. when not all the results are needed.
    *   Generator code that is directly turned into a container via `ToList()`
        will be less performant than filling in a container directly.
    *   Generator code that is called multiple times will be considerably slower
        than iterating over a container multiple times.

### Property styles

*   For single line read-only properties, prefer expression body properties
    (`=>`) when possible.
*   For everything else, use the older `{ get; set; }` syntax.

### Expression body syntax

For example:

<div class='goodcode'></div>

```c#
int SomeProperty => _someProperty
```

<div class='goodcode'></div>

```c#
void SomeMethod() => throw new NotImplementedException();
```

*   Judiciously use expression body syntax in lambdas and properties.
*   For method definitions, prefer using expression body syntax only for
    single-statement methods - e.g. forwarding to delegates or throwing
    exceptions.
*   As with methods and other scoped blocks of code, align the closing with the
    first character of the line that includes the opening brace. See sample code
    for examples.

### Structs and classes:

*   Structs are very different from classes:

    *   Structs are always passed and returned by value.
    *   Assigning a value to a member of a returned struct doesn’t modify the
        original - e.g. `transform.position.x = 10` doesn’t set the transform’s
        position.x to 10; `position` here is a property that returns a `Vector3`
        by value, so this just sets the x parameter of a copy of the original.

*   Almost always use a class.

*   Consider struct when the type can be treated like other value types - for
    example, if instances of the type are small and commonly short-lived or are
    commonly embedded in other objects. Good examples include Vector3,
    Quaternion and Bounds.

*   Note that this guidance may vary from team to team where, for example,
    performance issues might force the use of structs.

### Lambdas vs named methods

*   If a lambda is non-trivial (e.g. more than a couple of statements, excluding
    declarations), or is reused in multiple places, it should probably be a
    named method.

### Field initializers

*   Field initializers are generally encouraged.

### Extension methods

*   Only use an extension method when the source of the original class is not
    available, or else when changing the source is not feasible.
*   Only use an extension method if the functionality being added is a ‘core’
    general feature that would be appropriate to add to the source of the
    original class.
    *   Note - if we have the source to the class being extended, and the
        maintainer of the original class does not want to add the method, prefer
        not using an extension method.
*   Only put extension methods into core libraries that are available
    everywhere - extensions that are only available in some code will become a
    readability issue.
*   Be aware that using extension methods always obfuscates the code, so err on
    the side of not adding them.

### ref and out

*   Use `out` for additional returns that are not also inputs.
*   Place `out` parameters after all other parameters in the method definition.
*   `ref` should be used rarely, when mutating an input is necessary.
*   Do not use `ref` as an optimisation for passing structs.
*   Do not use `ref` to pass a modifiable container into a method. `ref` is only
    required when the supplied container needs be replaced with an entirely
    different container instance.

### LINQ

*   In general, prefer single line LINQ calls and imperative code, rather than
    long chains of LINQ. Mixing imperative code and heavily chained LINQ is
    often hard to read.
*   Prefer member extension methods over SQL-style LINQ keywords - e.g. prefer
    `myList.Where(x)` to `myList where x`.
*   Avoid `Container.ForEach(...)` for anything longer than a single statement.

### Array vs List

*   In general, prefer `List<>` over arrays for public variables, properties,
    and return types (keeping in mind the guidance on `IList` / `IEnumerable` /
    `IReadOnlyList` above).
*   Prefer `List<>` when the size of the container can change.
*   Prefer arrays when the size of the container is fixed and known at
    construction time.
*   Prefer array for multidimensional arrays.
*   Note:
    *   array and `List<>` both represent linear, contiguous containers.
    *   Similar to C++ arrays vs `std::vector`, arrays are of fixed capacity,
        whereas `List<>` can be added to.
    *   In some cases arrays are more performant, but in general `List<>` is
        more flexible.

### Folders and file locations

*   Be consistent with the project.
*   Prefer a flat structure where possible.

### Use of tuple as a return type

*   In general, prefer a named class type over `Tuple<>`, particularly when
    returning complex types.

### String interpolation vs `String.Format()` vs `String.Concat` vs `operator+`

*   In general, use whatever is easiest to read, particularly for logging and
    assert messages.
*   Be aware that chained `operator+` concatenations will be slower and cause
    significant memory churn.
*   If performance is a concern, `StringBuilder` will be faster for multiple
    string concatenations.

### `using`

*   Generally, don’t alias long typenames with `using`. Often this is a sign
    that a `Tuple<>` needs to be turned into a class.
    *   e.g. `using RecordList = List<Tuple<int, float>>` should probably be a
        named class instead.
*   Be aware that `using` statements are only file scoped and so of limited use.
    Type aliases will not be available for external users.

### Object Initializer syntax

For example:

<div class='goodcode'></div>

```c#
var x = new SomeClass {
  Property1 = value1,
  Property2 = value2,
};
```

*   Object Initializer Syntax is fine for ‘plain old data’ types.
*   Avoid using this syntax for classes or structs with constructors.
*   If splitting across multiple lines, indent one block level.

### Namespace naming

*   In general, namespaces should be no more than 2 levels deep.
*   Don't force file/folder layout to match namespaces.
*   For shared library/module code, use namespaces. For leaf 'application' code,
    such as `unity_app`, namespaces are not necessary.
*   New top-level namespace names must be globally unique and recognizable.

### Default values/null returns for structs

*   Prefer returning a ‘success’ boolean value and a struct `out` value.
*   Where performance isn't a concern and the resulting code significantly more
    readable (e.g. chained null conditional operators vs deeply nested if
    statements) nullable structs are acceptable.
*   Notes:

    *   Nullable structs are convenient, but reinforce the general ‘null is
        failure’ pattern Google prefers to avoid. We will investigate a
        `StatusOr` equivalent in the future, if there is enough demand.

### Removing from containers while iterating

C# (like many other languages) does not provide an obvious mechanism for
removing items from containers while iterating. There are a couple of options:

*   If all that is required is to remove items that satisfy some condition,
    `someList.RemoveAll(somePredicate)` is recommended.
*   If other work needs to be done in the iteration, `RemoveAll` may not be
    sufficient. A common alternative pattern is to create a new container
    outside of the loop, insert items to keep in the new container, and swap the
    original container with the new one at the end of iteration.

### Calling delegates

*   When calling a delegate, use `Invoke()` and use the null conditional
    operator - e.g. `SomeDelegate?.Invoke()`. This clearly marks the call at the
    callsite as ‘a delegate that is being called’. The null check is concise and
    robust against threading race conditions.

### The `var` keyword

*   Use of `var` is encouraged if it aids readability by avoiding type names
    that are noisy, obvious, or unimportant.
*   Encouraged:

    *   When the type is obvious - e.g. `var apple = new Apple();`, or
        `var request = Factory.Create<HttpRequest>();`
    *   For transient variables that are only passed directly to other methods -
        e.g. `var item = GetItem(); ProcessItem(item);`

*   Discouraged:

    *   When working with basic types - e.g. `var success = true;`
    *   When working with compiler-resolved built-in numeric types - e.g.
        `var number = 12 * ReturnsFloat();`
    *   When users would clearly benefit from knowing the type - e.g.
        `var listOfItems = GetList();`

### Attributes

*   Attributes should appear on the line above the field, property, or method
    they are associated with, separated from the member by a newline.
*   Multiple attributes should be separated by newlines. This allows for easier
    adding and removing of attributes, and ensures each attribute is easy to
    search for.

### Argument Naming

Derived from the
[Google C++ style guide](https://google.github.io/styleguide/cppguide.html#Function_Argument_Comments).

When the meaning of a method argument is nonobvious, consider one of the
following remedies:

*   If the argument is a literal constant, and the same constant is used in
    multiple method calls in a way that tacitly assumes they're the same, use a
    named constant to make that constraint explicit, and to guarantee that it
    holds.
*   Consider changing the method signature to replace a `bool` argument with an
    `enum` argument. This will make the argument values self-describing.
*   Replace large or complex nested expressions with named variables.
*   Consider using
    [Named Arguments](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/named-and-optional-arguments)
    to clarify argument meanings at the call site.
*   For methods that have several configuration options, consider defining a
    single class or struct to hold all the options and pass an instance of that.
    This approach has several advantages. Options are referenced by name at the
    call site, which clarifies their meaning. It also reduces method argument
    count, which makes method calls easier to read and write. As an added
    benefit, call sites don't need to be changed when another option is added.

Consider the following example:

<div class='badcode'></div>

```c#
// What are these arguments?
DecimalNumber product = CalculateProduct(values, 7, false, null);
```

versus:

<div class='goodcode'></div>

```c#
var options = new ProductOptions();
options.PrecisionDecimals = 7;
options.UseCache = CacheUsage.DontUseCache;
DecimalNumber product = CalculateProduct(values, options, completionDelegate: null);
```

### Comments

(Derived from the [Google C++ style guide].)

Comments are absolutely vital to keeping our code readable. The following rules
describe what you should comment and where. But remember: while comments are
very important, the best code is self-documenting. Giving sensible names to
types and variables is much better than using obscure names that you must then
explain through comments.

When writing your comments, write for your audience: the next person who will
need to understand your code. Be generous — the next one may be you!

#### Class Comments

Every non-obvious class should have an accompanying comment that describes what
it is for and how it should be used.

The class comment should provide the reader with enough information to know how
and when to use the class, as well as any additional considerations necessary to
correctly use the class.

#### Method Comments

Almost every method should have comments immediately preceding it that describe
what the method does and how to use it. These comments may be omitted only if
the method is simple and obvious (e.g., simple accessors). Method comments
should be written with an implied subject of *This method* and should start with
the verb phrase; for example, "Opens the file", rather than "Open the file".

Types of things to mention in comments:

*   What the inputs and outputs are. If method argument names are provided in
    `backticks`, then code-indexing tools may be able to present the
    documentation better.

*   Whether the object remembers arguments beyond the duration of the method
    call.

*   Whether any of the arguments can be null.

*   If there are any performance implications of how a method is used.

However, do not be unnecessarily verbose or state the completely obvious.

When documenting method overrides, focus on the specifics of the override
itself, rather than repeating the comment from the overridden method. In many of
these cases, the override needs no additional documentation and thus no comment
is required.

When commenting constructors, remember that the person reading your code knows
what constructors are for, so comments that just say something like "creates
this object" are not useful.

#### Class Members

The purpose of each class data member must be clear. If there are any invariants
(special values, relationships between members, lifetime requirements) not
clearly expressed by the type and name, they must be commented. However, if the
type and name suffice, no comment is needed.

#### Variable Comments

In general the actual name of the variable should be descriptive enough to give
a good idea of what the variable is used for. In certain cases, more comments
are required.

#### Explanatory Comments

Tricky or complicated code blocks should have comments before them.

[Google C++ style guide]: https://google.github.io/styleguide/cppguide.html#Comments