Use .NET Assemblies in FiddlerScript

To use a .NET addon (for this example, a C# addon that modifies the user-agent string):

Add References

  1. Close Fiddler.

  2. Save the .NET file (for example, this file called UASimulator.cs):

    using System;
    using System.Windows.Forms;
    using Fiddler;
    namespace FiddlerUtility{
          public class UASimulator {
                string m_sUAString;
                public UASimulator(string s_UAString){
                      m_sUAString = s_UAString;
                public bool OverwriteUA(Session oSession){
                      oSession.oRequest["User-Agent"] = m_sUAString;
                      return true;
  3. In a VS command prompt, go to the folder where the .CS file is found.

  4. Enter the command to create a DLL in the VS command prompt. For example:

    csc /target:library /out:c:\UASim.dll UASimulator.cs /reference:"%localappdata%\Programs\Fiddler\fiddler.exe"
  5. In Fiddler, click Tools > Options.

  6. Click the Extensions tab.

  7. In the References field, enter the location of the DLL. For example:


Update Fiddler Classic Rules

Add a rule to Fiddler Classic to update your script. For example:

    import System;
    import System.Windows.Forms;
    import Fiddler;
    import FiddlerUtility;

    class Handlers{

          static var UASim = new UASimulator("Mozilla/12.0");

          static function OnBeforeRequest(oSession:Fiddler.Session){



       static function Main(){

           var today: Date = new Date();      

           FiddlerObject.StatusText = " CustomRules.js was loaded at: " + today;



See Also

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