Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class User
    {
        public int? Age { get; set; }
        public int? ID { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            User user = new User();
            user.Age = null;        // no warning or error
            user.ID  = (int?)null;  // no warning or error

            string result = string.Empty;
            User user2 = new User
                             {
                Age = string.IsNullOrEmpty(result) ? null : Int32.Parse(result),
                // Error    1   Type of conditional expression cannot be determined 
                // because there is no implicit conversion between '<null>' and 'int'   
                // ConsoleApplication1\ConsoleApplication1\Program.cs   23  71  ConsoleApplication1

                ID = string.IsNullOrEmpty(result) ? (int?)null : Int32.Parse(result) // // no warning or error
                             };
        }
    }
}

Question:

Why the following line doesn’t work?

Age = string.IsNullOrEmpty(result) ? null : Int32.Parse(result)

// Correction one is

Age = string.IsNullOrEmpty(result) ? (int?) null : Int32.Parse(result)

Why the following line work?

user.Age = null;        // no warning or error

Answer 1

Age = string.IsNullOrEmpty(result) ? null : Int32.Parse(result)

Doesn’t work because the string.IsNullOrEmpty(result) ? null : Int32.Parse(result) is evaluated separately from the Age = part.

The compiler can’t figure out what type string.IsNullOrEmpty(result) ? null : Int32.Parse(result) is supposed to be.

It first sees null which indicates that it’s a reference type, and it sees an int which is a value type which seems incompatible. The fact that there exists a type with an implicit cast operator from int to int? isn’t inferred by the compiler.

It could in theory have enough information to figure it out but the compiler would need to be a lot more sophisticated.