using System;

using System.Linq.Expressions;

​

public class Program

{

    public class A

    {

        public B B{get;set;}

    }

    public class B

    {

        public C C{get;set;}

    }

    public class C

    {

        public int N=>1;

    }

    public static void Main()

    {

        Console.WriteLine("EO!!!");

        var message = new A{B=new B{C=new C()}};

        Expression<Func<A,int>> exp = a => a.B.C.N;

        var x = exp.CheckForNull("KK D L VK");

        var func = x.Compile();

        func(message);

        var message2 = new A{B=new B()};

        func(message2);

​

        Console.WriteLine("OE!!!");

    }

}

    public static class ExpressionExt

    {

        public static TExp? CheckForNull<TExp>(this TExp orig, string? message) where TExp : Expression => (TExp?)(new NullTestVisitor(message).Visit(orig));

    }

​

    /// <summary>

    /// ExpressionVisitor to replace a obj.member Expression with a null test ((obj == null) ? null : obj.member)

    /// </summary>

    public class NullTestVisitor : ExpressionVisitor

    {

        private readonly string? _message;

​

        public NullTestVisitor(string? message)

        {

            _message = message;

        }

​

        public override Expression? Visit(Expression? node)

        {

            if (node is MemberExpression nme)

                //return Expression.Condition(

                //    Expression.MakeBinary(ExpressionType.Equal, nme.Expression.CheckForNull(_message) ?? throw new ArgumentNullException(_message ?? $"{nme.Member.Name} is null."), Expression.Constant(null, nme.Expression.Type)),

                //    Expression.Constant(null, nme.Type),

                //    nme);

                //return Expression.Condition(

                //    Expression.MakeBinary(ExpressionType.Equal, nme.Expression.CheckForNull(_message) ?? throw new ArgumentNullException(_message ?? $"{nme.Member.Name} is null."), Expression.Constant(null, nme.Expression.Type)),

                //    Expression.Constant(null, GetNullableType(nme.Type)),

                //    nme);

                return Expression.Condition(

                    Expression.MakeBinary(ExpressionType.Equal, nme.Expression.CheckForNull(_message), Expression.Constant(null, nme.Expression.Type)),

                    //Expression.Throw(Expression.New(typeof(ArgumentNullException)), nme.Type),

                    Expression.Throw(Expression.Constant(new ArgumentNullException(_message ?? $"{nme.Member.Name} is null.")), nme.Type),

                    nme);

            else

                return base.Visit(node);

        }

    }

​