For example, let's say that you have an integer for tracking some value and it can be positive and negative.
It is also possible for the value to be not set yet.
How do you detect that the value hasn't been set yet?
You typically come up with some arbitrary number like 99999999 and hope that the real value is never actually 999999999. A better way since .NET 2.0, Nullable Value Types Class [MyClass]
Private someValue As System.Nullable(Of Integer)
Public Sub New()someValue = Nothing
End Sub
Public Function GetSomeValue() As Integer
If someValue IsNot Nothing Then
Return someValue
Else
Throw New Exception("Some Value has not been set") End If
End Function
End Class
This is a much cleaner and safer way than checking for some arbitrary number like 999999999.
Following snippet from Microsoft explains :
http://msdn.microsoft.com/en-us/library/ms235245(v=VS.100).aspx
So what are Nullable Value Types ?
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(Of 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)
The variable 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 Function procedure.
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
Data Type Implementation (Visual Basic).
How to Use a Nullable Type Variable?
The most important members of a nullable type are its
HasValue and
Value properties.
For a variable of a nullable type,
HasValue tells you whether the variable contains a defined value.
If
HasValue is True, you can read the value from
Value.
Note that both
HasValue and
Value are ReadOnly properties.
Default Values
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 numberOfChildreninitially 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.
Storing ValuesYou 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.
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
Comparison that uses the equal sign, someVar = Nothing, always evaluates to Nothing.
You can test the variable's HasValue property for False, or test by using the Is or IsNot operator. Retrieving Values
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
HasValue is 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
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.
See Also
TasksTroubleshooting Data Types (Visual Basic)Reference
InvalidOperationExceptionIf Operator (Visual Basic)Is Operator (Visual Basic)IsNot Operator (Visual Basic)HasValueOther ResourcesData Type Implementation (Visual Basic)
Watch: 
![clip_image001[4]](http://lh3.ggpht.com/_7b3sXkSrn-0/TKxDFgFYv2I/AAAAAAAAAZI/CfQxmg6v_VM/s1600-h/clip_image001%5B4%5D%5B7%5D.gif "clip_image001[4]")