How to: Explicitly Throw Exceptions

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.NET Framework 4

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.

Example

Visual Basic


Option Strict On

Imports System
Imports System.IO

Public Class ProcessFile

   Public Shared Sub Main()
      Dim fs As FileStream = Nothing
      Try
          '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()
          Console.WriteLine(line)
      Catch e As FileNotFoundException
          Console.WriteLine("[Data File Missing] {0}", e)
          Throw New FileNotFoundException("[data.txt not in c:\temp directory]", e)
      Finally
          If fs IsNot Nothing Then fs.Close
      End Try
   End Sub 
End Class


using System;
using System.IO;

public class ProcessFile
{
   public static void Main()
      {
      FileStream fs = null;
      try   
      {
         //Opens a text tile.
         fs = new FileStream(@"C:\temp\data.txt", FileMode.Open);
         StreamReader sr = new StreamReader(fs);
         string line;

         //A value is read from the file and output to the console.
         line = sr.ReadLine();
         Console.WriteLine(line);
      }
      catch(FileNotFoundException e)
      {
         Console.WriteLine("[Data File Missing] {0}", e);
         throw new FileNotFoundException(@"[data.txt not in c:\temp directory]",e);
      }
      finally
      {
         if (fs != null) 
            fs.Close();
      }
   }
}

Tasks

How to: Use the Try/Catch Block to Catch Exceptions

How to: Use Specific Exceptions in a Catch Block

How to: Use Finally Blocks

Other Resources

Exception Handling Fundamentals

Community Content

Stefan Hoffmann

Throwing exceptions

It is important to throw or rethrow an exception only in either the catch or the finally block. Otherwise the exception thrown in the finally block will hide the exception thrown in the catch block. E.g.:

    internal class Program
    {
        static void Main(string[] args)
        {
            try
            {
                try
                {
                    Console.WriteLine("inner try");
                    throw new Exception("inner try exception");
                }
                catch
                {
                    Console.WriteLine("inner catch");
                    throw new Exception("inner catch exception");
                }
                finally
                {
                    Console.WriteLine("inner finally");
                    // Try it twice, once without and once with throwing the exception below.
                    // throw new Exception("inner finally exception");
                }
            }
            catch (Exception exception)
            {
                Console.WriteLine("outer catch: " + exception.Message);
            }
        }
    }

Worldlifesite

Program Flow

I was hoping to find in the documetation information on program flow when you use the throw statement. Does it return to the caller or fall through to the next lines of execution?