using System;

using System.Linq.Expressions;

public class Program

{

    public static void Main()

    {

        var paramX = Expression.Parameter(typeof(decimal?), "x");

        var paramY = Expression.Parameter(typeof(decimal), "y");

        var lambda = Expression.Lambda<Action<decimal?, decimal>>(

            Expression.Block(

                Expression.Assign(paramX, Expression.Convert(paramY, typeof(decimal?)))

                , Expression.Call(typeof(Console), "WriteLine", new Type[0], Expression.Convert(paramX, typeof(decimal)))

            ),

            paramX, paramY);

        var action = lambda.Compile();

        action(null, 10); // with null

        action(5, 15); // with not null

    }

}