Updated: July 20, 2015
For the latest documentation on Visual Studio 2017, see Visual Studio 2017 Documentation.
Sometimes you work with a value type that does not have a defined value in certain circumstances. For example, a field in a database might have to distinguish between having an assigned value that is meaningful and not having an assigned value. Value types can be extended to take either their normal values or a null value. Such an extension is called a nullable type.
Each nullable type is constructed from the generic
Nullable<T> structure. Consider a database that tracks work-related activities. The following example constructs a nullable
Boolean type and declares a variable of that type. You can write the declaration in three ways:
Dim ridesBusToWork1? As Boolean Dim ridesBusToWork2 As Boolean? Dim ridesBusToWork3 As Nullable(Of Boolean)
ridesBusToWork can hold a value of
True, a value of
False, or no value at all. Its initial default value is no value at all, which in this case could mean that the information has not yet been obtained for this person. In contrast,
False could mean that the information has been obtained and the person does not ride the bus to work.
You can declare variables and properties with nullable types, and you can declare an array with elements of a nullable type. You can declare procedures with nullable types as parameters, and you can return a nullable type from a
You cannot construct a nullable type on a reference type such as an array, a
String, or a class. The underlying type must be a value type. For more information, see
Value Types and Reference Types.
The most important members of a nullable type are its
Value properties. For a variable of a nullable type,
HasValue tells you whether the variable contains a defined value. If
True, you can read the value from
Value. Note that both
When you declare a variable with a nullable type, its
HasValue property has a default value of
False. This means that by default the variable has no defined value, instead of the default value of its underlying value type. In the following example, the variable
numberOfChildren initially has no defined value, even though the default value of the
Integer type is 0.
Dim numberOfChildren? As Integer
A null value is useful to indicate an undefined or unknown value. If
numberOfChildren had been declared as
Integer, there would be no value that could indicate that the information is not currently available.
You store a value in a variable or property of a nullable type in the typical way. The following example assigns a value to the variable
numberOfChildren declared in the previous example.
numberOfChildren = 2
If a variable or property of a nullable type contains a defined value, you can cause it to revert to its initial state of not having a value assigned. You do this by setting the variable or property to
Nothing, as the following example shows.
numberOfChildren = Nothing
Although you can assign
To retrieve the value of a variable of a nullable type, you should first test its
HasValue property to confirm that it has a value. If you try to read the value when
False, Visual Basic throws an
InvalidOperationException exception. The following example shows the recommended way to read the variable
numberOfChildren of the previous examples.
If numberOfChildren.HasValue Then MsgBox("There are " & CStr(numberOfChildren) & " children.") Else MsgBox("It is not known how many children there are.") End If
Boolean variables are used in Boolean expressions, the result can be
Nothing. The following is the truth table for
b2 now have three possible values, there are nine combinations to evaluate.
|b1||b2||b1 And b2||b1 Or b2|
When the value of a Boolean variable or expression is
Nothing, it is neither
false. Consider the following example.
Dim b1? As Boolean Dim b2? As Boolean b1 = True b2 = Nothing ' The following If statement displays "Expression is not true". If (b1 And b2) Then Console.WriteLine("Expression is true") Else Console.WriteLine("Expression is not true") End If ' The following If statement displays "Expression is not false". If Not (b1 And b2) Then Console.WriteLine("Expression is false") Else Console.WriteLine("Expression is not false") End If
In this example,
b1 And b2 evaluates to
Nothing. As a result, the
Else clause is executed in each
If statement, and the output is as follows:
Expression is not true
Expression is not false
If one or both of the operands of an arithmetic, comparison, shift, or type operation is nullable, the result of the operation is also nullable. If both operands have values that are not
Nothing, the operation is performed on the underlying values of the operands, as if neither were a nullable type. In the following example, variables
sum1 are implicitly typed. If you rest the mouse pointer over them, you will see that the compiler infers nullable types for both of them.
' Variable n is a nullable type, but both m and n have proper values. Dim m As Integer = 3 Dim n? As Integer = 2 ' The comparison evaluated is 3>2, but compare1 is inferred to be of ' type Boolean?. Dim compare1 = m > n ' The values summed are 3 and 2, but sum1 is inferred to be of type Integer?. Dim sum1 = m + n ' The following line displays: 3 * 2 * 5 * True Console.WriteLine(m & " * " & n & " * " & sum1 & " * " & compare1)
If one or both operands have a value of
Nothing, the result will be
' Change the value of n to Nothing. n = Nothing Dim compare2 = m > n Dim sum2 = m + n ' Because the values of n, compare2, and sum2 are all Nothing, the ' following line displays 3 * * * Console.WriteLine(m & " * " & n & " * " & sum2 & " * " & compare2)
A database is one of the most important places to use nullable types. Not all database objects currently support nullable types, but the designer-generated table adapters do. See "TableAdapter Support for Nullable Types" in TableAdapter Overview.
Using Nullable Types
Value Types and Reference Types
Troubleshooting Data Types
Local Type Inference