Please note: The FiddlerAPI is still under construction, and future releases may require changes to your code.  Thanks for your patience.

Extending Fiddler with .NET Code

Introduction

Fiddler2 is a highly versatile platform that offers extensibility via both script and .NET code.  Using the extensibility mechanisms, you can add to Fiddler's UI, automatically modify requests or responses, and custom Inspectors that enable scenario-specific display and manual-modification of requests and responses.

Prerequisites

Writing extensions for Fiddler2 requires Visual Studio .NET 2005 or the free .NET Framework v2 command-line compilers. 

You will also need to have the most recent version of Fiddler2 installed in order to develop Fiddler extensions.

If you've previously developed Fiddler extensions, you may want to read the article Updating Fiddler Extensions for v2.1.

Direct Fiddler2 to load your extension assemblies

Fiddler loads extension assembly DLLs from the %Program Files%\Fiddler2\Scripts and %USERPROFILE%\My Documents\Fiddler2\Scripts folders.  Install to the %Program Files% location to make your extensions available to all users on the machine, or the %UserProfile% folder to make the extension available only to the current user.

In addition to placing your extension DLLs in the appropriate folder, you must also mark your assembly to indicate the minimum version of Fiddler required for your Assembly to load correctly. 

Set the Fiddler.RequiredVersion attribute in your AssemblyInfo.cs file (or elsewhere in your code), as follows:

using Fiddler;

// This Assembly requires Fiddler v2.1.0.1 or later because it uses types introduced in v2.1.
[assembly: Fiddler.RequiredVersion("2.1.0.1")]

If Fiddler finds a RequiredVersion attribute that indicates a later version of Fiddler is required, the user will be notified that a later version of Fiddler is required to use your extension.  Assemblies which do not contain a RequiredVersion attribute are silently ignored.

The IFiddlerExtension Interface

Public classes in your assembly that implement the IFiddlerExtension interface will be loaded by Fiddler during startup.

public interface IFiddlerExtension
{
  // Called when Fiddler User Interface is fully available
  void OnLoad();

  // Called when Fiddler is shutting down
  void OnBeforeUnload();
}

The OnLoad function will be called when Fiddler has finished loading and its UI is fully available.  At this point, you can safely add menu items, tabbed pages, or other elements to the Fiddler UI.

The OnBeforeUnload function will be called when Fiddler is shutting down and unloading all extensions.

The IAutoTamper Interface

Extensions that implement the IAutoTamper interface (which descends from the IFiddlerExtension interface) are called for each HTTP/HTTPS request and response, enabling modifications, logging, or other operations. 

public interface IAutoTamper : IFiddlerExtension
{
  // Called before the user can edit a request using the Fiddler Inspectors
  void AutoTamperRequestBefore(Session oSession);

  // Called after the user has had the chance to edit the request using the Fiddler Inspectors, but before the request is sent
  void AutoTamperRequestAfter(Session oSession);

  // Called before the user can edit a response using the Fiddler Inspectors
  void AutoTamperResponseBefore(Session oSession);

  // Called before the user can edit a response using the Fiddler Inspectors
  void AutoTamperResponseAfter(Session oSession);

  // Called Fiddler returns a self-generated HTTP error (for instance DNS lookup failed, etc)
  void OnBeforeReturningError(Session oSession);
}

The IHandleExecAction Interface

Extensions that implement the IHandleExecAction interface are called when the user has entered a command into the QuickExec box.  If your extension would like to react to the command (and prevent further processing by other extensions and Fiddler itself) return true from this method.

public interface IHandleExecAction
{
  // return TRUE if handled.
  bool OnExecAction(string sCommand);
}

Note that the Fiddler.Utilities class includes a helper function which you may find useful when interpreting the sCommand parameter.

[CodeDescription("Tokenize a string into tokens. Delimits on whitespace; Quotation marks are dropped unless preceded by a \ character.")]
public static string[] Parameterize(string sCommand)

Step-by-step Sample Extension