using System;

using System.Linq;

using System.Linq.Expressions;

​

​

public class Program

{

    public delegate void Add<T>(ref T a, T b) where T : class;

    public static Expression<Add<T>> BuildExpression<T>() where T : class

    {

        var t = typeof(T);

        var a = Expression.Parameter(t.MakeByRefType(), "a");

        var b = Expression.Parameter(t, "b");

        return Expression.Lambda<Add<T>>(

            Expression.Block(

                t.GetFields().Select(f => 

                    Expression.AddAssign(Expression.Field(a, f), Expression.Field(b, f)))),

            a, b

        );

    }

​

​

    public class MyClass

    {

        public int Field1;

    }

​

    public static void Main()

    {

        var lambda = BuildExpression<MyClass>();

        var add = lambda.Compile();

        var a = new MyClass { Field1 = 10 };

        var b = new MyClass { Field1 = 15 };

        add(ref a, b);      

        Console.WriteLine(a.Field1); // expect 25 but was 10

    }

}