ByRefExtension.cs

using System;

static class Program
{
    private static void Main()
    {
        Intermediary<C>();
        Console.WriteLine();
        Intermediary<S>();
    }

    private static void Intermediary<T>() where T : I<T>, new()
    {
        T x = new T();
        ByValueExtension(x);
        x.Dump();
        ByReferenceExtension1(ref x);
        x.Dump();
        ByReferenceExtension2(ref x);
        x.Dump();
    }

    private static void ByValueExtension<T>(this T x) where T : I<T>
    {
        x.M1();
    }

    // you want to allow this for value types
    private static void ByReferenceExtension1<T>(ref T x) where T : I<T>
    {
        x.M1();
    }

    // but forbid this for reference types
    private static void ByReferenceExtension2<T>(ref T x) where T : I<T>
    {
        x = x.M2();
    }

    private static void Dump(this object o)
    {
        Console.WriteLine(o);
    }
}

interface I<T> where T : I<T>
{
    void M1();
    T M2();
}

class C : I<C>
{
    public int Field;

    public void M1()
    {
        Field++;
    }

    public C M2()
    {
        return new C { Field = 10 * Field };
    }

    public override string ToString()
    {
        return Field.ToString();
    }
}

struct S : I<S>
{
    public int Field;

    public void M1()
    {
        Field++;
    }

    public S M2()
    {
        return new S { Field = 10 * Field };
    }

    public override string ToString()
    {
        return Field.ToString();
    }
}