Nullable

Consider an Integer in VB.NET. It cannot be null—it can only have a numeric value. But we can wrap the Integer inside a nullable Structure, which can logically be Nothing.

With properties and functions, we can test a Nullable. We specify a nullable type by adding a question mark after the type name. For the value, we can access Value.

Example

Here we see a nullable Integer named "element." We use the Integer in various ways. We use the Nothing value to indicate "null" in the VB.NET language.

Note The HasValue property returns true or false. It indicates whether an underlying value is contained in the nullable.

Note 2 The GetValueOrDefault function safely returns the underlying value. If the nullable has no value, it returns the value type's default.

Finally The Value property will throw an exception if the nullable has no value. Prefer GetValueOrDefault to avoid exceptions.

Module Module1

    Sub Main()
        ' A nullable can be Nothing.
        Dim element As Integer? = Nothing

        Console.WriteLine("HASVALUE: {0}", element.HasValue)
        Console.WriteLine("GETVALUEORDEFAULT: {0}", element.GetValueOrDefault())

        ' Use an Integer value for the nullable Integer.
        element = 100

        Console.WriteLine("VALUE: {0}", element.Value)
        Console.WriteLine("GETVALUEORDEFAULT: {0}", element.GetValueOrDefault())
    End Sub

End ModuleHASVALUE: False
GETVALUEORDEFAULT: 0
VALUE: 100
GETVALUEORDEFAULT: 100

Notes, `null`

In VB.NET the keyword "Nothing" is used to indicate null. This makes types like nullable somewhat confusing, because they are based on C# terms.

What is a null Integer or other null value type? A null thing can be used to indicate an uninitialized value—we can have uninitialized, or an actual number set.

In this way, nullable types can help eliminate ambiguous parts of programs. We do not need to use -1 as a special value meaning "not set." We can use null.

Nullable

Example

Notes, null

Notes, `null`