Use .NET Assemblies in FiddlerScript
Updated on Nov 6, 2025
To use a .NET addon (for this example, a C# addon that modifies the user-agent string):
Add References
-
Close Fiddler.
-
Save the .NET file (for example, this file called UASimulator.cs):
c#
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;
}
}
}-
In a VS command prompt, go to the folder where the .CS file is found.
-
Enter the command to create a DLL in the VS command prompt. For example:
bash
csc /target:library /out:c:\UASim.dll UASimulator.cs /reference:"%localappdata%\Programs\Fiddler\fiddler.exe"-
In Fiddler, click Tools > Options.
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Click the Extensions tab.
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In the References field, enter the location of the DLL. For example:
bash
C:\UASim.dllUpdate Fiddler Classic Rules
Add a rule to Fiddler Classic to update your script. For example:
c#
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){
UASim.OverwriteUA(oSession);
}
static function Main(){
var today: Date = new Date();
FiddlerObject.StatusText = " CustomRules.js was loaded at: " + today;
}
}