Syntax

• Nullable<int> i = 10;
• int? j = 11;
• int? k = null;
• DateTime? DateOfBirth = DateTime.Now;
• decimal? Amount = 1.0m;
• bool? IsAvailable = true;
• char? Letter = ‘a’;
• (type)? variableName

Remarks

Nullable types can represent all the values of an underlying type as well as null.

The syntax T? is shorthand for Nullable<T>

Nullable values are System.ValueType objects actually, so they can be boxed and unboxed. Also, null value of a nullable object is not the same as null value of a reference object, it’s just a flag.

When a nullable object boxing, the null value is converted to null reference, and non-null value is converted to non-nullable underlying type.

DateTime? dt = null;
var o = (object)dt;
var result = (o == null); // is true

DateTime? dt = new DateTime(2015, 12, 11);
var o = (object)dt;
var dt2 = (DateTime)dt; // correct cause o contains DateTime value

The second rule leads to correct, but paradoxical code:

DateTime? dt = new DateTime(2015, 12, 11);
var o = (object)dt;
var type = o.GetType(); // is DateTime, not Nullable<DateTime>

In short form:

DateTime? dt = new DateTime(2015, 12, 11);
var type = dt.GetType(); // is DateTime, not Nullable<DateTime>