JetBrains Rider 2019.3 Help

Code Syntax Style: Implicit/Explicit Typing ('var' Keyword)

Using implicitly typed local variables (also known as var keyword) introduced in C# 3.0 has become quite popular as it improves readability in many scenarios. By default, ReSharper also encourages using of var keyword, but preferences of its usage are flexibly configurable — for example, you can opt for using explicit types in specific cases or everywhere and JetBrains Rider will help you enforce your preferences.

Starting from C# 7.0, you can declare local variables when deconstructing tuples. If you prefer var in such declarations, you can additionally configure a style to use joined or separated notation, that is, for example: var (x, y) = GetTuple(); or (var x, var y) = GetTuple();.

Another C# 7.0 feature is discards, which also allows using var to make sure that there are no conflicts with variables in the scope that may be named _. JetBrains Rider allows you to configure a style to always use var with discards where appropriate.

Enforce preferences of using 'var' keyword

JetBrains Rider checks all local variables for compliance with your preferences and if they do not comply, JetBrains Rider highlights such declarations and suggests the corresponding quick-fix or fix in scope.

By default, JetBrains Rider's preferences say that 'var' keyword is preferred:

'Var' usage quick-fix
If you change your preference, JetBrains Rider will help you to use explicit types:
'Var' usage quick-fix

Another option to enforce preferences of 'var' keyword usage in a bulk mode is code cleanup. You can either run code cleanup with one of the default profiles Full Cleanup or Reformat & Apply Code Style, or create and run a custom profile solely targeted at your specific task as described below.

Apply preferences of using 'var' keyword with custom Code Cleanup profile

  1. Press Ctrl+Alt+S, or alternatively from the main menu, choose File | Settings on Windows and Linux or JetBrains Rider | Preferences on macOS.

  2. Go to the Code Cleanup settings page: Editor | Code Cleanup.

  3. Create a new profile as described in the To create a new custom cleanup profile section. In the Selected profile settings section for the new profile, tick the Enforce 'var' keyword usage settings checkbox. Optionally, you can enable other code cleanup tasks in this profile.

  4. Click Save to apply the modifications and let JetBrains Rider choose where to save them, or save the modifications to a specific settings layer using the Save To list. For more information, see manage and share jetbrains rider settings.

  5. Select the scope where you want to enforce your preferences:

    • Set the caret anywhere in the file to enforce your preferences to the file.

    • Select one or more items in the Solution Explorer to enforce your preferences in the files under these nodes and their child items.

  6. Choose Code | Code Cleanup in the main menu.

  7. In the Code Cleanup dialog that opens, select the newly created profile.

  8. Click OK. JetBrains Rider will enforce your preferences in the selected scope.

Configure preferences of using 'var' keyword

Your 'var' keyword usage preferences are saved using the mechanism of layer-based settings. Among other things, this mechanism allows you to maintain different preferences for different solutions as well as to keep these preferences under a VCS and automatically share them with your team members.

Configure preferences of using 'var' keyword in the Settings/Preferences dialog

  1. Go to the Editor | Code Style | C# page of JetBrains Rider settings Ctrl+Alt+S, and then select the Code Style tab.

  2. Modify settings in the 'var' usage in declarations category according to your coding practices/standards.

    You can set different preferences of using 'var' or explicit type for different types:

    • For built-in types — applies to C# built-in types.

    • For simple types — applies to types without generic parameters.

    • Elsewhere — applies to generic types and deconstruction declarations.

    For each of these preferences you can opt for using 'var', explicit type, or 'var' when evident.

  3. If you have chosen Use 'var' when evident in the previous step, you can opt to apply Visual Studio logic to decide which cases should be considered evident.

    To learn the differences between JetBrains Rider and Visual Studio logic, see Use 'var' when evident: what is considered evident?.

  4. By default, JetBrains Rider will suggest joined notation for multiple var's in deconstruction declarations, for example var (x, y) = GetTuple();. You can select Prefer separate declarations for deconstructed variables to opt for separate notation, for example (var x, var y) = GetTuple();.

  5. By default, JetBrains Rider suggests using standalone _ for discards, for example (_, _) = (0, 1);. You can select Use 'var' keyword for discards to always use var with discards where appropriate thus making sure that there are no conflicts with variables in the scope that may be named _.

  6. The selectors in the right column allow you to set severity levels of code inspections detecting code that differs from your preferences.

  7. Click Save to apply the modifications and let JetBrains Rider choose where to save them, or save the modifications to a specific settings layer using the Save To list. For more information, see manage and share jetbrains rider settings.

Use 'var' when evident: what is considered evident?

When configuring preferences of using 'var' keyword vs. explicit type, you can opt for Use 'var' when evident. This option seems self-explanatory but in some cases it might be unclear what is considered 'evident' and what is not.

Apart from that, there are some differences between what is considered evident (apparent) by JetBrains Rider and by Visual Studio when both products suggest using 'var' keyword or explicit type.

The table below shows in which cases initializers that could be declared 'var' are considered evident depending on the state of the Prefer Roslyn (Visual Studio) logic for type evidence checkbox on the Editor | Code Style | C# | Code Style page of JetBrains Rider settings Ctrl+Alt+S.

Initializer expression

Examples

Off

On

Object creation expressions

new MyClass()

Evident

Evident

Cast expressions

(MyClass) obj

Evident

Evident

as expressions

obj as MyClass

Evident

Evident

Literal expressions

"some string literal"
123
true

Evident

Evident

Default expressions

default(MyClass)

Evident

Evident

Tuple expressions

(1, 2, 3)

Evident when all component expressions are evident

Explicit array creation expressions

new int[] { 1, 2, 3 }
new MyClass[5]

Evident

Evident

Implicit array creation expressions

new [] { 1, 2, 3}
new [,] { {1, 2, 3}, {1, 2, 3}, {1, 2, 3} }

Evident when all element initializers (up to 42) are evident

Not evident

Non-generic factory (creation) methods (static methods declared in some type and returning a value of the same type)

MyClass.Build(someArgs) // returns MyClass

Evident when the method name contains a parent type name or one of the following substrings: "Create", "Build", "Construct", "Make", "Generate", "Produce", "New", "Instance"

Evident

Generic factory (creation) methods (static methods returning a generic type and declared in a class with the same name as the return type)

Tuple.Create(1, 2, 3)

Evident when all method call arguments are evident and the method name contains one of the following substrings or a class name: "Create", "Build", "Construct", "Make", "Generate", "Produce", "New", "Instance"

Evident

Conversion methods (methods with a name consisting of "To" plus the name of the return type)

obj.ToString() enumeration.ToList()

Evident when the return type is not generic

Evident

Generic methods with an explicit type argument returning the value of that type argument

obj.GetValue<MyClass>() // returns MyClass

Evident

Not evident

Enum members

KnownColor.Azure

Evident

Not evident

Singleton fields (static/constant fields returning the value of the type where they are declared)

int.MaxValue string.Empty

Evident when the field name contains the type name or one of the following substrings: "Empty", "Instance", "Default", "Value"

Not evident

Last modified: 21 February 2020