C# Nullable DateTime

A nullable DateTime can be null. The DateTime struct itself does not provide a null option. But the DateTime? nullable type allows you to assign the null literal to the DateTime type. It provides another level of indirection in the object model.

Example

First, this program illustrates how you can use a nullable DateTime instance in various ways. A nullable DateTime is most easily specified using the question mark syntax, which results in the type "DateTime?". A DateTime? is actually a struct that wraps a DateTime struct, providing another level of indirection that can simplify some null checks and control flow layouts.

This C# example program uses a nullable DateTime? instance. Nullable DateTimes can be null.

Program that uses null DateTime? struct [C#]

using System;

class Program
{
    static void Main()
    {
	//
	// Declare a nullable DateTime instance and assign to null.
	// ... Change the DateTime and use the Test method (below).
	//
	DateTime? value = null;
	Test(value);
	value = DateTime.Now;
	Test(value);
	value = DateTime.Now.AddDays(1);
	Test(value);
	//
	// You can use the GetValueOrDefault method on nulls.
	//
	value = null;
	Console.WriteLine(value.GetValueOrDefault());
    }

    static void Test(DateTime? value)
    {
	//
	// This method uses the HasValue property.
	// ... If there is no value, the number zero is written.
	//
	if (value.HasValue)
	{
	    Console.WriteLine(value.Value);
	}
	else
	{
	    Console.WriteLine(0);
	}
    }
}

Output

0
9/29/2009 9:56:21 AM
9/30/2009 9:56:21 AM
1/1/0001 12:00:00 AM

Question mark syntax. This program introduces two methods: the Main entry point where control flow begins, and the Test method that uses the nullable DateTime? as a parameter. The DateTime? variable is first declared in the Main method body. It is passed as a parameter to the Test method. The DateTime? variable is reassigned to different values, including DateTime.Now, and tomorrow's date is computed.

Using HasValue and GetValueOrDefault on DateTime? type. The HasValue instance method is available on any nullable type in the C# language. It returns true if the nullable type is logically non-null, and false if the null field on the nullable type is activated. The GetValueOrDefault will return DateTime.MinValue for a null DateTime.

Null DateTime

Let's resolve the differences between the DateTime? type, followed by a question mark, and the DateTime struct type, which can be wrapped by the nullable type. The DateTime? type has special semantics in the language that allows you to use it as null, but it is actually a value type struct. The DateTime type has no way to be null because it is a value type and stored in the memory location of the variable itself. You can research this topic more on this site.

Summary

We looked at the nullable DateTime? type in the C# language, seeing how you can specify a trailing question mark to transform the DateTime struct to a nullable DateTime struct wrapper, which adds utility for handling null and invalid data more easily. Finally, we explored null DateTime instances in general.