How to explicitly throw exceptions

You can explicitly throw an exception using the C# throw or the Visual Basic Throw statement. You can also throw a caught exception again using the throw statement. It's good coding practice to add information to an exception that's rethrown to provide more information when debugging.

The following code example uses a try/catch block to catch a possible System.IO.FileNotFoundException. Following the try block is a catch block that catches the System.IO.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 System.IO.FileNotFoundException and adds text information to the exception.

sample · csharpsamples/snippets/csharp/VS_Snippets_CLR/Exception.Throwing/CS/throw.cs1

csharp

using System;
using System.IO;

namespace Example;

public class ProcessFile
{
    public static void Main()
    {
        // <Snippet1>
        var fs = default(FileStream);
        try
        {
            // Open a text tile.
            fs = new FileStream(@"C:\temp\data.txt", FileMode.Open);
            var sr = new StreamReader(fs);

            // Read a value from the file and output to the console.
            string? line = sr.ReadLine();
            Console.WriteLine(line);
        }
        catch (FileNotFoundException e)
        {
            Console.WriteLine($"[Data File Missing] {e}");
            throw new FileNotFoundException(@"[data.txt not in c:\temp directory]", e);
        }
        finally
        {
            fs?.Close();
        }
        // </Snippet1>
    }
}

sample · vbsamples/snippets/visualbasic/VS_Snippets_CLR/Exception.Throwing/VB/throw.vb1

' Visual Basic .NET Document

' <Snippet1>
Option Strict On

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)

            ' A value is read from the file and output to the console.
            Dim line As String = sr.ReadLine()
            Console.WriteLine(line)
        Catch e As FileNotFoundException
            Console.WriteLine($"[Data File Missing] {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
' </Snippet1>

#See also

See also