C# 4.0 Pocket Reference, 3rd Edition

Nullable Types

Reference types can represent a nonexistent value with a null reference. Value types, however, cannot ordinarily represent null values. For example:

string s = null;   // OK - reference type.
int i = null;      // Compile error - int cannot be null.

To represent null in a value type, you must use a special construct called a nullable type. A nullable type is denoted with a value type followed by the ? symbol:

int? i = null;                     // OK - Nullable Type
Console.WriteLine (i == null);     // True

Nullable Basics

Nullable<T> struct

T? translates into System.Nullable<T>. Nullable<T> is a lightweight immutable structure, having only two fields, to represent Value and HasValue. The essence of System.Nullable<T> is very simple:

public struct Nullable<T> where T : struct
{
  public T Value {get;}
  public bool HasValue {get;}
  public T GetValueOrDefault();
  public T GetValueOrDefault (T defaultValue);
  ...
}

This code:

int? i = null;
Console.WriteLine (i == null);              // True

translates to:

Nullable<int> i = new Nullable<int>();
Console.WriteLine (! i.HasValue);           // True

Attempting to retrieve Value when HasValue is false throws an InvalidOperationException. GetValueOrDefault() returns Value if HasValue is true; otherwise, it returns new T() or a specified custom default value.

The default value of T? is null.

Implicit and explicit nullable conversions

The conversion from T to T? is implicit, and from T? to T is explicit. For example:

int? x = 5;        // implicit
int y = (int)x;    // explicit

The explicit cast is directly equivalent ...