尝试......抓住...最后声明

结构体：

Try
    'Your program will try to run the code in this block.  
    'If any exceptions are thrown, the code in the Catch Block will be executed,
    'without executing the lines after the one which caused the exception.
Catch ex As System.IO.IOException
    'If an exception occurs when processing the Try block, each Catch statement
    'is examined in textual order to determine which handles the exception. 
    'For example, this Catch block handles an IOException.
Catch ex As Exception
    'This catch block handles all Exception types.
    'Details of the exception, in this case, are in the "ex" variable.
    'You can show the error in a MessageBox with the below line.
    MessageBox.Show(ex.Message)
Finally
    'A finally block is always executed, regardless of if an Exception occurred.
End Try

示例代码：

Try
    Dim obj = Nothing
    Dim prop = obj.Name 'This line will throw a NullReferenceException

    Console.WriteLine("Test.") ' This line will NOT be executed
Catch ex As System.IO.IOException
    ' Code that reacts to IOException.
Catch ex As NullReferenceException
    ' Code that reacts to a NullReferenceException
    Console.WriteLine("NullReferenceException: " & ex.Message)
    Console.WriteLine("Stack Trace: " & ex.StackTrace)
Catch ex As Exception
    ' Code that reacts to any other exception.
Finally   
    ' This will always be run, regardless of if an exception is thrown.
    Console.WriteLine("Completed")
End Try