Generic constraint on boxing

valuetype.cs

using System;

interface IAdd
{
    void Add(int val);
}

struct Foo : IAdd
{
    public int value;

    void IAdd.Add(int val)
    {
        value += val;
    }

    public void AddValue(int val)
    {
        value += val;
    }

    public void Print(string msg)
    {
        Console.WriteLine(msg + ":" + value);
    }
}

class Program
{
    static void Add_WithoutConstraints<T>(ref T foo, int val)
    {
        ((IAdd)foo).Add(val);
    }

    static void Add_WithConstraints<T>(ref T foo, int val) where T : IAdd
    {
        foo.Add(val);
    }
    
    static void Main(string[] args)
    {
        Foo f = new Foo();
        f.value = 10;
        
        f.Print("Initial Value");

        f.AddValue(10);
        f.Print("After calling AddValue");

        ((IAdd) f).Add(10);
        f.Print("After calling IAdd.Add");

        Add_WithoutConstraints<Foo>(ref f, 10);
        f.Print("After Add_WithoutConstrats");

        Add_WithConstraints<Foo>(ref f, 10);
        f.Print("After Add_WithConstraints");
     }
}