in-parameter-modifier.md

title	in parameter modifier (C# Reference)
ms.date	03/06/2018
ms.prod	.net
ms.technology	devlang-csharp
ms.topic	article
helpviewer_keywords	parameters [C#], in in parameters [C#]
author	BillWagner
ms.author	wiwagn

in parameter modifier (C# Reference)

The in keyword causes arguments to be passed by reference. It is like the ref or out keywords, except that in arguments cannot be modified by the called method, whereas ref arguments may be modified, out arguments must be modified by the caller, and those modifications are observable in the calling context.

[!code-csharp-interactivecs-in-keyword]

The preceding example demonstrates that the in modifier is unnecessary at the call site. It is only required in the method declaration.

Note

The in keyword can also be used with a generic type parameter to specify that the type parameter is contravariant, as part of a foreach statement, or as part of a join clause in a LINQ query. For more information on the use of the in keyword in these contexts, see in which provides links to all those uses.

Variables passed as in arguments must be initialized before being passed in a method call. However, the called method may not assign a value or modify the argument.

Although the in, ref, and out keywords cause different run-time behavior, they are not considered part of the method signature at compile time. Therefore, methods cannot be overloaded if the only difference is that one method takes a ref or in argument and the other takes an out argument. The following code, for example, will not compile:

class CS0663_Example
{
    // Compiler error CS0663: "Cannot define overloaded 
    // methods that differ only on in, ref and out".
    public void SampleMethod(in int i) { }
    public void SampleMethod(ref int i) { }
}

Overloading based on the presence of in is allowed, but generates a compiler warning:

class InOverloads
{
    // Discouraged. Calling SampleMethod(value) is ambiguous.
    public void SampleMethod(in int i) { }
    public void SampleMethod(int i) { }
}

Properties or constants may be passed as in parameters, because the calling method may not modify their values.

You can't use the in, ref, and out keywords for the following kinds of methods:

• Async methods, which you define by using the async modifier.

• Iterator methods, which include a yield return or yield break statement.

You typically declare in arguments to avoid the copy operations necessary for passing arguments by value. This is most useful when arguments are structures or arrays of structures.

C# Language Specification

[!INCLUDECSharplangspec]

See Also

C# Reference
C# Programming Guide
C# Keywords
Method Parameters