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.
Example
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();
}
}
}
See Also
Tasks
How to: Use the Try/Catch Block to Catch Exceptions
How to: Use Specific Exceptions in a Catch Block