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 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.

var fs = default(FileStream);
try
{
    // Opens a text tile.
    fs = new FileStream(@"C:\temp\data.txt", FileMode.Open);
    var sr = new StreamReader(fs);

    // A value is read 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
{
    if (fs != null)
        fs.Close();
}

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

See also

• Exceptions