DirectCast Operator (Visual Basic)
Updated: July 20, 2015
For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.
Introduces a type conversion operation based on inheritance or implementation.
DirectCast does not use the Visual Basic run-time helper routines for conversion, so it can provide somewhat better performance than
CType when converting to and from data type
You use the
DirectCast keyword similar to the way you use the CType Function and the TryCast Operator keyword. You supply an expression as the first argument and a type to convert it to as the second argument.
DirectCast requires an inheritance or implementation relationship between the data types of the two arguments. This means that one type must inherit from or implement the other.
DirectCast generates a compiler error if it detects that no inheritance or implementation relationship exists. But the lack of a compiler error does not guarantee a successful conversion. If the desired conversion is narrowing, it could fail at run time. If this happens, the runtime throws an InvalidCastException error.
A comparison of the type conversion keywords is as follows.
|Keyword||Data types||Argument relationship||Run-time failure|
|CType Function||Any data types||Widening or narrowing conversion must be defined between the two data types||Throws InvalidCastException|
|Any data types||One type must inherit from or implement the other type||Throws InvalidCastException|
|TryCast Operator||Reference types only||One type must inherit from or implement the other type||Returns Nothing|
The following example demonstrates two uses of
DirectCast, one that fails at run time and one that succeeds.
Dim q As Object = 2.37 Dim i As Integer = CType(q, Integer) ' The following conversion fails at run time Dim j As Integer = DirectCast(q, Integer) Dim f As New System.Windows.Forms.Form Dim c As System.Windows.Forms.Control ' The following conversion succeeds. c = DirectCast(f, System.Windows.Forms.Control)
In the preceding example, the run-time type of
CType succeeds because
Double can be converted to
Integer. However, the first
DirectCast fails at run time because the run-time type of
Double has no inheritance relationship with
Integer, even though a conversion exists. The second
DirectCast succeeds because it converts from type Form to type Control, from which Form inherits.