Can Ref and Out Modifiers Be Used in C# Extension Methods?

ref and out modifiers in C# extension methods

In an extension method that extends a reference type, the first parameter cannot use the ref or out modifier. However, starting with C# 7.2, it is possible to add the ref modifier to the first parameter of an extension method of an extended value type (struct).

The reason for this limitation is that the first parameter of an extension method is considered a receiver parameter, which represents the instance being extended. For reference types, the receiver parameter is always passed by reference, so using the ref or out modifier is redundant.

However, for value types, the receiver parameter is passed by value, which means that any changes made to it in the extension method will not be reflected in the original value type instance. By using the ref modifier, you can pass a value type instance by reference, allowing you to modify its state in an extension method.

Example:

public struct MyProperties
{
    public string MyValue { get; set; }
}

public static class MyExtensions
{
    public static void ChangeMyValue(this ref MyProperties myProperties)
    {
        myProperties.MyValue = "hello from MyExtensions";
    }
}

public class MyClass
{
    public MyClass()
    {
        MyProperties myProperties = new MyProperties();
        myProperties.MyValue = "hello world";
        myProperties.ChangeMyValue(); // Now modifies the original myProperties instance
    }
}

In this example, the ChangeMyValue extension method can modify the state of the ref value type instance using the MyProperties modifier. Note that the out modifier still cannot be used on the first parameter of an extension method.

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