Nothing (Visual Basic)
Updated: July 2011
Represents the default value of any data type. For reference types, the default value is the null reference. For value types, the default value depends on whether the value type is nullable.
For non-nullable value types, Nothing in Visual Basic differs from null in C#. In Visual Basic, if you set a variable of a non-nullable value type to Nothing, the variable is set to the default value for its declared type. In C#, if you assign a variable of a non-nullable value type to null, a compile-time error occurs.
Nothing represents the default value of a data type. The default value depends on whether the variable is of a value type or of a reference type.
A variable of a value type directly contains its value. Value types include all numeric data types, Boolean, Char, Date, all structures, and all enumerations. A variable of a reference type stores a reference to an instance of the object in memory. Reference types include classes, arrays, delegates, and strings. For more information, see Value Types and Reference Types.
If a variable is of a value type, the behavior of Nothing depends on whether the variable is of a nullable data type. To represent a nullable value type, add a ? modifier to the type name. Assigning Nothing to a nullable variable sets the value to null. For more information and examples, see Nullable Value Types (Visual Basic).
If a variable is of a value type that is not nullable, assigning Nothing to it sets it to the default value for its declared type. If that type contains variable members, they are all set to their default values. The following example illustrates this for scalar types.
Module Module1 Sub Main() Dim ts As TestStruct Dim i As Integer Dim b As Boolean ' The following statement sets ts.Name to Nothing and ts.Number to 0. ts = Nothing ' The following statements set i to 0 and b to False. i = Nothing b = Nothing Console.WriteLine("ts.Name: " & ts.Name) Console.WriteLine("ts.Number: " & ts.Number) Console.WriteLine("i: " & i) Console.WriteLine("b: " & b) Console.ReadKey() End Sub Public Structure TestStruct Public Name As String Public Number As Integer End Structure End Module
If a variable is of a reference type, assigning Nothing to the variable sets it to a null reference of the variable's type. A variable that is set to a null reference is not associated with any object. The following example demonstrates this.
Module Module1 Sub Main() Dim testObject As Object ' The following statement sets testObject so that it does not refer to ' any instance. testObject = Nothing Dim tc As New TestClass tc = Nothing ' The fields of tc cannot be accessed. The following statement causes ' a NullReferenceException at run time. (Compare to the assignment of ' Nothing to structure ts in the previous example.) 'Console.WriteLine(tc.Field1) End Sub Class TestClass Public Field1 As Integer ' . . . End Class End Module
When checking whether a reference (or nullable value type) variable is null, do not use = Nothing or <> Nothing. Always use Is Nothing or IsNot Nothing.
For strings in Visual Basic, the empty string equals Nothing. Therefore, "" = Nothing is true.
The following example shows comparisons that use the Is and IsNot operators.
Module Module1 Sub Main() Dim testObject As Object testObject = Nothing Console.WriteLine(testObject Is Nothing) ' Output: True Dim tc As New TestClass tc = Nothing Console.WriteLine(tc IsNot Nothing) ' Output: False ' Declare a nullable value type. Dim n? As Integer Console.WriteLine(n Is Nothing) ' Output: True n = 4 Console.WriteLine(n Is Nothing) ' Output: False n = Nothing Console.WriteLine(n IsNot Nothing) ' Output: False Console.ReadKey() End Sub Class TestClass Public Field1 As Integer Private field2 As Boolean End Class End Module
If you declare a variable without using an As clause and set it to Nothing, the variable has a type of Object. An example of this is Dim something = Nothing. A compile-time error occurs in this case when Option Strict is on and Option Infer is off.
When you assign Nothing to an object variable, it no longer refers to any object instance. If the variable had previously referred to an instance, setting it to Nothing does not terminate the instance itself. The instance is terminated, and the memory and system resources associated with it are released, only after the garbage collector (GC) detects that there are no active references remaining.
Nothing differs from the DBNull object, which represents an uninitialized variant or a nonexistent database column.