Extension methods on a struct

Question

Can you add extension methods to a struct?

Answer 1

Yes, you can define an extension method on a struct/value type. However, they do not have the same behavior as extension methods on reference types.

For example, the GetA() extension method in the following C# code receives a copy of the struct, not a reference to the struct. This means that a C# extension method on a struct can't modify the original struct contents.

public static class TestStructExtensionMethods {
    public struct FooStruct {
        public int a;
    }
    public static int GetA(this FooStruct st) {
        return st.a;
    }
}

In order to modify the struct contents, the struct paramater needs to be declared as "ref". However, "this ref" is not allowed in C#. The best we can do is a static non-extension method like:

// this works, but is inefficient, because it copies the whole FooStruct
// just to return a
public static int GetA(ref FooStruct st) {
    return st.a;
}

In VB.NET, you can create this as a ByRef struct extension method, so it could modify the original struct:

' This is efficient, because it is handed a reference to the struct
 _ 
Public Sub GetA(ByRef [me] As FooStruct) As Integer
    Return [me].a
End Sub

' It is possible to change the struct fields, because we have a ref
 _ 
Public Sub SetA(ByRef [me] As FooStruct, newval As Integer) 
    [me].a = newval
End Sub