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 rethrown 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 FileNotFoundException and 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
Imports System
Imports System.IO
Public Class ProcessFile
Public Shared Sub Main()
Dim fs As New FileStream("data.txt", FileMode.Open)
Try
'Opens a text file.
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:\dev directory]", e)
Finally
fs.Close()
End Try
End Sub 'Main
End Class 'ProcessFile
using System;
using System.IO;
public class ProcessFile
{
public static void Main()
{
FileStream fs = null;
try
//Opens a text tile.
{
fs = new FileStream("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:\\dev directory]",e);
}
finally
{
fs.Close();
}
}
}
See Also
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