Can Extension Methods Use the `ref` Modifier for Their First Parameter?

Extension method with ref modifier

In extension methods, it is forbidden to use the this modifier on the first parameter (usually called the ref parameter). The main reasons are as follows:

Incompatible with C# language design principles:

Extension methods are designed to provide a way to inherit without modifying the original type. Allowing ref for the first argument would violate this principle, as it would allow direct modification of the original instance.

Potential type safety issues:

Passing a value type (struct) by reference through the first parameter may allow an extension method to modify the original instance, even if the method is declared in a static class. This can lead to inconsistencies and unexpected behavior.

However, in C# 7.2 and above:

The C# language specification has been updated to allow the use of ref for the first argument of extension methods. This functionality is limited to value types (structs) and allows modification of the original instance. It is worth noting that this feature does not work with reference types (classes, interfaces, records).

Example:

The following example illustrates the use of ref within an extension method:

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

public static class MyExtensions
{
    public static void ChangeMyValue(this ref MyProperties myProperties)
    {
        myProperties.MyValue = "来自MyExtensions的问候";
    }
}

public class MyClass
{
    public MyClass()
    {
        MyProperties myProperties = new MyProperties();
        myProperties.MyValue = "你好，世界";
        myProperties.ChangeMyValue(); //调用扩展方法修改值
    }
}

In this example, the ChangeMyValue extension method modifies the MyProperties property of the original MyValue instance, which is a value type. By using ref, the method can directly access the instance and thus change its state.

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