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.
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.
HasValue
property returns true or false. It indicates whether an underlying value is contained in the nullable.GetValueOrDefault
function safely returns the underlying value. If the nullable has no value, it returns the value type's default.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
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
.