Try...Catch...Finally Statement (Visual Basic)
Provides a way to handle some or all possible errors that may occur in a given block of code, while still running code.
Try [ tryStatements ] [ Exit Try ] [ Catch [ exception [ As type ] ] [ When expression ] [ catchStatements ] [ Exit Try ] ] [ Catch ... ] [ Finally [ finallyStatements ] ] End Try
Local variables from a Try block are not available in a Catch block because they are separate blocks. If you want to use a variable in more than one block, declare the variable outside the Try...Catch...Finally structure.
The Try block contains code where an error can occur, and the Catch block contains code to handle any error that does occur. If an error occurs in the Try block, program control is passed to the appropriate Catch statement for disposition. The exception argument is an instance of the Exception class or a class that derives from the Exception class. The Exception class instance corresponds to the error that occurred in the Try block. The instance contains information about the error. This includes, among other things, its number and message.
If a Catch statement does not specify an exception argument, it catches any kind of system or application exception. You should always use this variation as the last Catch block in the Try...Catch...Finally structure, after catching all the specific exceptions you anticipate. Control flow can never reach a Catch block that follows a Catch without an exception argument.
In partial-trust situations, such as an application hosted on a network share, Try...Catch...Finally does not catch security exceptions that occur before the method that contains the call is invoked. The following example, when you put it on a server share and run from there, produces the error "System.Security.SecurityException: Request Failed." For more information about security exceptions, see the SecurityException class.
In such a partial-trust situation, you have to put the Process.Start statement in a separate Sub. The initial call to the Sub will fail. This enables Try...Catch to catch it before the Sub that contains Process.Start is started and the security exception produced.
If a Try statement does not contain at least one Catch block, it must contain a Finally block.
The following simplified example illustrates the structure of the Try...Catch...Finally statement.
Public Sub TryExample() Dim x As Integer = 5 ' Declare variables. Dim y As Integer = 0 Try ' Set up structured error handling. x = x \ y ' Cause a "Divide by Zero" error. Catch ex As Exception When y = 0 ' Catch the error. Beep() MsgBox("You tried to divide by 0.") ' Show an explanatory message. Finally Beep() ' This line is executed no matter what. End Try End Sub