Extend Fiddler Classic with .NET Code

Use Fiddler's extensibility mechanisms to add to Fiddler's UI, automatically modify requests or responses, and create custom Inspectors that enable scenario-specific display and manual-modification of requests and responses.

Requirements

• Visual Studio .NET 2005+ or the free .NET Framework v2 command-line compilers
• The most recent version of Fiddler
• For Visual Studio 2010 or later: Change the project to target the .NET2.0/3.5 framework.
• If targeting .NET Framework 3.5: Ensure user has .NET Framework 3.5 installed.
• If extending 64bit Fiddler: Target AnyCPU.

See Build extension assemblies to run in both Fiddler Classic versions 2 and 4.

Debugging

• To ensure that exceptions and other extension-related errors are not silently caught: set the fiddler.debug.extensions.showerrors preference to True.
• To output logging information to the Log tab: set the fiddler.debug.extensions.verbose

Direct Fiddler Classic to load extension assemblies

• To make the extensions available to the current user, install extension assembly DLLs to: %localappdata%\Programs\Fiddler\Scripts OR %userprofile%\Documents\Fiddler2\Scripts

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

  using Fiddler;
  
  // Extension requires Fiddler Classic 2.2.8.6+ because it uses types introduced in v2.2.8...
  [assembly: Fiddler.RequiredVersion("2.2.8.6")]
  

Sample Extension: Step by Step

1. Start Visual Studio 2005 or later.

2. Create a new Project of type Visual C# Class Library.

3. Right-click the project's References folder in the Solution Explorer.

4. Click the Browse tab and select Fiddler.exe in the %localappdata%\Programs\Fiddler folder.

5. Click Ok to add the reference.

6. If your extension modifies Fiddler's UI:

   • Right-click the project's References folder in the Solution Explorer again
   • On the .NET tab, choose System.Windows.Forms.
   • Click Ok to add the reference.
   • In the Solution Explorer, right click the project. Choose Properties.

   • On the Build Events tab, add the following to the Post-build event command line:

     copy "$(TargetPath)" "%userprofile%\Documents\Fiddler2\Scripts\$(TargetFilename)"

Modify the default class1.cs (or create a new class) in your project as follows:

using System;
using System.Windows.Forms;
using Fiddler;

[assembly: Fiddler.RequiredVersion("2.3.5.0")]

public class Violin : IAutoTamper    // Ensure class is public, or Fiddler won't see it!
{
  string sUserAgent = "";

  public Violin(){
  /* NOTE: It's possible that Fiddler Classic UI isn't fully loaded yet, so don't add any UI in the constructor.

     But it's also possible that AutoTamper* methods are called before OnLoad (below), so be
     sure any needed data structures are initialized to safe values here in this constructor */

     sUserAgent = "Violin";
  }

  public void OnLoad(){ /* Load your UI here */ }
  public void OnBeforeUnload() { }

  public void AutoTamperRequestBefore(Session oSession){
    oSession.oRequest["User-Agent"] = sUserAgent;
  }
  public void AutoTamperRequestAfter(Session oSession){}
  public void AutoTamperResponseBefore(Session oSession){}
  public void AutoTamperResponseAfter(Session oSession){}
  public void OnBeforeReturningError(Session oSession){}
}