What is Nullable types in C# ?

Use a nullable type when you need to represent the undefined value of an underlying type. A Boolean variable can have only two values: true and false. There is no "undefined" value. In many programming applications, most notably database interactions, a variable value can be undefined or missing. For example, a field in a database may contain the values true or false, or it may contain no value at all. You use a Nullable<bool> type in that case.

Nullable types represent value-type variables that can be assigned the null value. You cannot create a nullable type based on a reference type. (Reference types already support the null value.)

The syntax T? is shorthand for Nullable<T>. The two forms are interchangeable.

Assign a value to a nullable type just as you would for an underlying value type: int? x = 10; or double? d = 4.108;. You also can assign the null value: int? x = null;.

Use the Nullable<T>.HasValue and Nullable<T>.Value readonly properties to test for null and retrieve the value, as shown in the following example: if (x.HasValue) y = x.Value;

The HasValue property returns true if the variable contains a value, or false if it's null.

The Value property returns a value if HasValue returns true. Otherwise, an InvalidOperationException is thrown.

You can also use the == and != operators with a nullable type, as shown in the following example: if (x != null) y = x.Value;. If a and b are both null, a == b evaluates to true.

Beginning with C# 7.0, you can use pattern matching to both examine and get a value of a nullable type: if (x is int valueOfX) y = valueOfX;.

The default value of T? is an instance whose HasValue property returns false.

Use the GetValueOrDefault() method to return either the assigned value, or the default value of the underlying value type if the value of the nullable type is null.

Use the GetValueOrDefault(T) method to return either the assigned value, or the provided default value if the value of the nullable type is null.

Use the null-coalescing operator, ??, to assign a value to an underlying type based on a value of the nullable type: int? x = null; int y = x ?? -1;. In the example, since x is null, the result value of y is -1.

If a user-defined conversion is defined between two data types, the same conversion can also be used with the nullable versions of these data types.

Nested nullable types are not allowed. The following line doesn't compile: Nullable<Nullable<int>> n;