Error with in this and ref this as Parameters in Static Method Calls

[aa89227]

Hello everyone,

I'm working on a C# 12 project and encountering errors when using in this and ref this as parameters in static method calls. Below is a snippet of the code that's causing the issues:

public class ClassA
{
    public void MethodA()
    {
        ClassWithStaticMethod.StaticMethodWithIn(this);
        
        // Error occurs here
        ClassWithStaticMethod.StaticMethodWithIn(in this);
        
        // Error occurs here
        ClassWithStaticMethod.StaticMethodWithRefReadonly(ref this);
        
        ClassWithStaticMethod.StaticMethodWithRefReadonly(this);
    }
}

public class ClassWithStaticMethod
{
    public static void StaticMethodWithIn(in ClassA classA)
    {
        Console.WriteLine("Static Method with in");
    }

    public static void StaticMethodWithRefReadonly(ref readonly ClassA classA)
    {
        Console.WriteLine("Static Method with ref readonly");
    }
}

When attempting to pass this with in and ref readonly modifiers in StaticMethodWithIn and StaticMethodWithRefReadonly respectively, the compiler throws errors. I would appreciate any insights on the following:

1. Why does the compiler not accept in this and ref this in these contexts?
2. Why does the compiler not throw any errors when just this is passed without any modifiers in these method calls?

Thank you for any explanations or workarounds you might suggest!

[iam3yal]

@aa89227 The question shouldn't be why the language not doing what you might expect or why the compiler enforces it but why do you think it should work? not to mention that both of the examples that do work implicitly pass reference by ref.

[aa89227]

I'm curious why passing this and in this results in different outcomes, given that both methods essentially pass a reference, yet the explicit keyword is not permitted.

[huoyaoyuan]

This is intentional and the whole purpose of ref readonly feature. in x must be used with a location (field or local variable). When passing value x, the compiler will implicitly assign a location y = x and pass in y. ref readonly disallows such implicit variable.

this is a value, not stored a particular location. The same applies to string literals etc. You can't say in "abc" either.

[aa89227]

Thank you for your explanation. I believe I now understand why it's not possible to use those features in this context. I appreciate your help in clarifying this matter!