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How to: Explicitly Throw Exceptions

You can explicitly throw an exception using the throw statement. You can also throw a caught exception again using the throw statement. It is good coding practice to add information to an exception that is re-thrown to provide more information when debugging.

The following code example uses a try/catch block to catch a possible FileNotFoundException. Following the try block is a catch block that catches the FileNotFoundExceptionand writes a message to the console if the data file is not found. The next statement is the throw statement that throws a new FileNotFoundException and adds text information to the exception.

Option Strict On 

Imports System
Imports System.IO

Public Class ProcessFile

   Public Shared Sub Main()
      Dim fs As FileStream = Nothing 
          'Opens a text file.
          fs = New FileStream("c:\temp\data.txt", FileMode.Open)
          Dim sr As New StreamReader(fs)
          Dim line As String 

          'A value is read from the file and output to the console.
          line = sr.ReadLine()
      Catch e As FileNotFoundException
          Console.WriteLine("[Data File Missing] {0}", e)
          Throw New FileNotFoundException("[data.txt not in c:\temp directory]", e)
          If fs IsNot Nothing Then fs.Close
      End Try 
   End Sub  
End Class