CAS.FirstChanceException Event

This event is fired when a Code Access Security (CAS) exception occurs that has not been handled by the calling code. It allows you to intercept and potentially handle these exceptions, even if they are not caught by a specific `catch` block.

Syntax

Remarks

The FirstChanceException event is a diagnostic tool that can be invaluable for debugging. When an exception is thrown, the common language runtime (CLR) first checks if there is a handler for that specific exception. If no handler is found at that point, the runtime considers it an unhandled exception. The FirstChanceException event is raised at the point where the CLR first detects an exception, regardless of whether it is eventually handled or not.

This event is particularly useful for:

• Tracking down the source of exceptions that are being caught higher up the call stack, making it difficult to pinpoint the exact location of the throw.
• Monitoring security-related exceptions within the .NET CAS framework.

The event arguments, FirstChanceExceptionEventArgs, provide access to the exception itself via the Exception property.

Examples

Subscribing to the FirstChanceException Event

using System;
using System.Security.Permissions;

public class Program
{
    public static void Main(string[] args)
    {
        // Subscribe to the FirstChanceException event
        AppDomain.CurrentDomain.FirstChanceException += new EventHandler(Domain_FirstChanceException);

        Console.WriteLine("Starting operations...");

        // Example that might trigger a CAS exception (depending on your security policy)
        try
        {
            // Attempt an operation that requires specific permissions
            new FileIOPermission(PermissionState.Unrestricted).Demand();
            Console.WriteLine("File I/O permission granted.");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Caught expected exception: {ex.Message}");
        }

        // Another operation that might trigger an exception
        try
        {
            // Simulate an operation that could fail
            SimulateOperationThatMightFail();
        }
        catch(Exception)
        {
            // Exception is handled here, but FirstChanceException will still fire
        }


        Console.WriteLine("Operations finished.");
    }

    private static void Domain_FirstChanceException(object sender, FirstChanceExceptionEventArgs e)
    {
        Console.ForegroundColor = ConsoleColor.Yellow;
        Console.WriteLine($"--- First Chance Exception Detected ---");
        Console.WriteLine($"Exception Type: {e.Exception.GetType().FullName}");
        Console.WriteLine($"Message: {e.Exception.Message}");
        Console.WriteLine($"Stack Trace: {e.Exception.StackTrace}");
        Console.ResetColor();
    }

    private static void SimulateOperationThatMightFail()
    {
        // This is just a placeholder for an operation that might throw
        throw new ArgumentException("Simulated operation failure.");
    }
}

Requirements

See Also

• System.Net.Security Namespace
• FirstChanceExceptionEventArgs Class
• AppDomain.FirstChanceException Event