Being the good programmers that you are you like to write your code so that it is as readable as possible, so you do things like qualifying your private instance variables with Me (this in C#) even though it’s optional. Sometimes you may even use the syntax “For i = x to y…next i” just to show which loop you are currently iterating. So it’s only natural, when throwing exceptions to want to specify the exception instance that you are throwing, for example Throw ex. What you may not realize is that there is a difference between “Throw ex” and “Throw”.
The runtime generates a stack trace and maintains it from the point at which the exception is thrown. Whenever you re-throw an exception the call stack is cleared to the point at which the exception was handled and re-thrown. Therefore to keep the stack trace intact when re-throwing an exception the correct syntax is to use the “Throw” keyword without specifying an exception.
Following is some sample code to demonstrate the point. Notice that in all methods except for Sub Main() the exception is re-thrown using simply “Throw”. When you examine the output Sub1(), Sub2() and Sub3() all maintain the entire stack trace. But when Sub Main() handles and re-throws the stack trace only includes the call to Sub1 and itself.
Module Module1
Sub Main()
Try
Console.WriteLine("Beginning Main()")
Sub1()
Catch ex As Exception
Console.WriteLine()
Console.WriteLine("Handling exception in Sub Main()")
Console.WriteLine(ex.Message)
Console.WriteLine(ex.StackTrace)
End Try
Console.ReadLine()
End Sub
Sub Sub1()
Try
Sub2()
Catch ex As Exception
Console.WriteLine()
Console.WriteLine("Handling Exception in Sub1()")
Console.WriteLine(ex.Message)
Console.WriteLine(ex.StackTrace)
Throw ex
End Try
End Sub
Sub Sub2()
Try
Sub3()
Catch ex As Exception
Console.WriteLine()
Console.WriteLine("Handling Exception in Sub2()")
Console.WriteLine(ex.Message)
Console.WriteLine(ex.StackTrace)
Throw
End Try
End Sub
Sub Sub3()
Console.WriteLine()
Dim i() As Integer = {1, 2, 3}
Try
For x As Integer = 0 To 3
Console.WriteLine(i(x))
Next x
Catch Ex As Exception
Console.WriteLine()
Console.WriteLine("Handling Exception in Sub3()")
Console.WriteLine(Ex.Message)
Console.WriteLine(Ex.StackTrace)
Throw
End Try
End Sub
End Module