Effect of null conditional operator in comparison with default

Question

I am wondering about the behavioral difference that can be seen in output for Test 2 on example below.

I'm on .NET Core 3.1 for this scenario.

using System;
                    
public class Program
{
    public static void Main()
    {
        var a = new Customer();
        
        if (a.BirthDate == default)
            Console.WriteLine("Test 1 Result: default");
        else
            Console.WriteLine("Test 1 Result: NOT default");
            
        
        if (a?.BirthDate == default)
            Console.WriteLine("Test 2 Result: default");
        else
            Console.WriteLine("Test 2 Result: NOT default");
        
        
        if (a.BirthDate == default(DateTime))
            Console.WriteLine("Test 3 Result: default");
        else
            Console.WriteLine("Test 3 Result: NOT default");
        
        
        if (a?.BirthDate == default(DateTime))
            Console.WriteLine("Test 4 Result: default");
        else
            Console.WriteLine("Test 4 Result: NOT default");
    }
}

public class Customer
{
    public DateTime BirthDate {get;set;}
}

Output:

Test 1 Result: default
Test 2 Result: NOT default
Test 3 Result: default
Test 4 Result: default

I was expecting all the outputs would be "default".

Answer 1

The type of default literal is inferred by the compiler. In the second test, the inferred type is Nullable<DateTime>, so it's equivalent to

a?.BirthDate == default(Nullable<DateTime>)

The default value of a Nullable<T> type is null, you can check with the following code

Console.WriteLine(default(Nullable<DateTime>) == null); // true

Because the left operand is a real DateTime object, the comparison result is false.