TryCast
Introduces a type conversion operation that does not throw an exception.
Remarks
If an attempted conversion fails, CType and DirectCast both throw an InvalidCastException error. This can adversely affect the performance of your application. TryCast returns Nothing (Visual Basic), so that instead of having to handle a possible exception, you need only test the returned result against Nothing.
You use the TryCast keyword the same way you use the CType Function and the DirectCast keyword. You supply an expression as the first argument and a type to convert it to as the second argument. TryCast operates only on reference types, such as classes and interfaces. It requires an inheritance or implementation relationship between the two types. This means that one type must inherit from or implement the other.
Errors and Failures
TryCast 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, TryCast returns Nothing (Visual Basic).
Conversion Keywords
A comparison of the type conversion keywords is as follows.
Keyword |
Data types |
Argument relationship |
Run-time failure |
|---|---|---|---|
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 |
Reference types only |
One type must inherit from or implement the other type |
Returns Nothing (Visual Basic) |
Example
The following example shows how to use TryCast.
Function PrintTypeCode(ByVal obj As Object) As String
Dim objAsConvertible As IConvertible = TryCast(obj, IConvertible)
If objAsConvertible Is Nothing Then
Return obj.ToString() & " does not implement IConvertible"
Else
Return "Type code is " & objAsConvertible.GetTypeCode()
End If
End Function