Why are const parameters not allowed in C#?

user_303756 0
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It looks strange especially for C++ developers. In C++ we used to mark a parameter as const in order to be sure that its state will not be changed in the method. There are also other C++ specific reasons, like passing const ref in order to pass by ref and be sure that state will not be changed. But why can't we mark as method parameters const in C#?

Why can't I declare my method like the following?

    ....
    static void TestMethod1(const MyClass val)
    {}
    ....
    static void TestMethod2(const int val)
    {}
    ....
language-design language-agnostic c#
5 Answers

This is possible since C# 7.2 (December 2017 with Visual Studio 2017 15.5).

For Structs and basic types only!, not for members of classes.

You must use in to send the argument as an input by reference. See:

See: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/in-parameter-modifier

For your example:

....
static void TestMethod1(in MyStruct val)
{
    val = new MyStruct;  // Error CS8331 Cannot assign to variable 'in MyStruct' because it is a readonly variable
    val.member = 0;  // Error CS8332 Cannot assign to a member of variable 'MyStruct' because it is a readonly variable
}
....
static void TestMethod2(in int val)
{
    val = 0;  // Error CS8331 Cannot assign to variable 'in int' because it is a readonly variable
}
....
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