[Bug] .NET 10 SharedEventManager.DecodeSource Throws System.FormatException on NUI Callbacks #257

unknown-urge · 2026-07-25T16:05:19Z

unknown-urge
Jul 25, 2026

Summary

When triggering a client-side NUI callback (via fetch / fetchNui in NUI JS), FiveM native host dispatches the event __cfx_nui:<callbackName> with eventSource formatted as "nui:<resourceName>" (e.g. "nui:abrp" for a resource named abrp).

Inside CitizenFX.FiveM.Shared.SharedEventManager, the DecodeSource(string source) method unconditionally executes int.Parse on the substring following "nui:" (int.Parse(array[1])). Because resource names are non-numeric strings (e.g., "abrp"), int.Parse throws System.FormatException: The input string 'abrp' was not in a correct format., preventing NUI event handlers from running.

Unhandled exception while executing event handler for __cfx_nui:authenticationScript:startDiscordAuthentication (source nui:abrp): System.FormatException: The input string 'abrp' was not in a correct format.
   at System.Number.ThrowFormatException[TChar](ReadOnlySpan`1 value)
   at System.Int32.Parse(String s)
   at CitizenFX.FiveM.Shared.SharedEventManager.DecodeSource(String source)
   at CitizenFX.FiveM.Shared.SharedEventManager.InvokeOwnedEvent(String name, String source, ReadOnlyMemory`1 args)
   at CitizenFX.Core.Entrypoint.OnEvent(String eventName, String eventSource, Memory`1 args)
   at CitizenFX.Host.Host.TriggerEventScript(IntPtr scriptPtr, IntPtr eventNamePtr, IntPtr eventSourcePtr, IntPtr eventData, Int32 eventDataLength)

Issue type

Client

Repro rate

Always

Server build version

FXServer-early-access b96 win32

OS

Windows 11 24H2

CPU

i5

GPU

RTX 4060Ti

RAM

32gb DDR4

Storage type

SSD NVMe

Connection type

WiFi

ISP and bandwidth

No response

DxDiag

No response

Network graph

No response

Platform

Windows

OS version / distribution

No response

CPU

No response

RAM

No response

Using txAdmin?

None

Hosting provider

None

Machine type

None

/perf endpoint output

No response

DDoS protection

No response

ulimit -n value (Linux only)

No response

Docker Compose file (Docker only)

No response

Steps to Reproduce

ClientScript.cs:

using System.Collections.Generic;
using CitizenFX.Core;
using CitizenFX.FiveM.Client;
using CitizenFX.FiveM.Shared.FuncRef;
using CitizenFX.FiveM.Shared.Script;

public class ClientScript : Entrypoint, IScript
{
    public void Initialize()
    {
        Native.RegisterNuiCallbackType("testNuiCallback");
        Native.SetNuiFocus(true, true);
    }

    [OnEvent("__cfx_nui:testNuiCallback")]
    public void OnTestNuiCallback(IDictionary<string, object> data, FunctionReference callback)
    {
        API.Log.Info("Callback executed successfully!");
        callback?.CallVoid("ok");
    }
}

index.html:

<!DOCTYPE html>
<html>
<head><title>NUI Reproduction</title></head>
<body>
    <button onclick="triggerCallback()">Trigger NUI Callback</button>
    <script>
        function triggerCallback() {
            fetch(`https://${GetParentResourceName()}/testNuiCallback`, {
                method: 'POST',
                headers: { 'Content-Type': 'application/json' },
                body: JSON.stringify({})
            });
        }
    </script>
</body>
</html>

fxmanifest.lua:

fx_version 'cerulean'
game 'gta5'
ui_page 'index.html'
files {
    'index.html'
}
client_script 'ClientScript.dll'

Expected Behavior

No errors when handling NUI events.

Actual Behavior

System.FormatException: The input string 'abrp' was not in a correct format. exception thrown when NUI callback fires.

Evidence

No response

Additional Context

Decompiling CitizenFX.FiveM.Shared.SharedEventManager.DecodeSource:

private static ValueTuple<string, int> DecodeSource(string source)
{
    if (source.Contains(':'))
    {
        string[] array = source.Split(':', StringSplitOptions.None);
        return new ValueTuple<string, int>(array[0], int.Parse(array[1])); // <--- Root cause of FormatException
    }
    int num;
    if (int.TryParse(source, out num))
    {
        return new ValueTuple<string, int>("", num);
    }
    return new ValueTuple<string, int>(source, 0);
}

Why `int.TryParse` Resolves the Bug

int.Parse("abrp"): When an event source contains a non-numeric resource name like "nui:abrp" (array[0] = "nui", array[1] = "abrp"), int.Parse throws an unhandled System.FormatException, breaking event invocation.
int.TryParse("abrp", out handle): When given a non-numeric string, int.TryParse returns false and sets handle = 0 without throwing an exception, allowing event invocation to complete cleanly.

Temporary Workaround / Bytecode Patch Details

As a temporary local workaround, I used Mono.Cecil to modify CitizenFX.FiveM.Shared.dll during build time:

var type = assembly.MainModule.GetTypes().FirstOrDefault(t => t.Name == "SharedEventManager");
if (type != null)
{
    var method = type.Methods.FirstOrDefault(m => m.Name == "DecodeSource");
    if (method != null && method.HasBody)
    {
        var tryParseMethod = assembly.MainModule.ImportReference(
            typeof(int).GetMethod("TryParse", new[] { typeof(string), typeof(int).MakeByRefType() })
        );

        var il = method.Body.GetILProcessor();
        var instructions = method.Body.Instructions;

        for (int i = 0; i < instructions.Count; i++)
        {
            var instr = instructions[i];
            if ((instr.OpCode == OpCodes.Call || instr.OpCode == OpCodes.Callvirt) &&
                instr.Operand is MethodReference mr &&
                mr.Name == "Parse" && mr.DeclaringType?.Name == "Int32")
            {
                var targetLocal = method.Body.Variables[0]; // V_0

                var ldlocaInstr = il.Create(OpCodes.Ldloca_S, targetLocal);
                var tryParseInstr = il.Create(OpCodes.Call, tryParseMethod);
                var popInstr = il.Create(OpCodes.Pop);
                var ldlocInstr = il.Create(OpCodes.Ldloc_0);

                il.InsertBefore(instr, ldlocaInstr);
                il.InsertBefore(instr, tryParseInstr);
                il.InsertBefore(instr, popInstr);
                il.InsertBefore(instr, ldlocInstr);
                il.Remove(instr);
                break;
            }
        }
    }
}

Replacing int.Parse with int.TryParse at IL_001A using Mono.Cecil allows DecodeSource to decode NUI event sources like "nui:abrp" safely without throwing exceptions.

Proposed Fix:
Replace int.Parse(array[1]) with int.TryParse(array[1], out var handle) in CitizenFX.FiveM.Shared.SharedEventManager.DecodeSource.

ESK0 · 2026-08-02T00:53:29Z

ESK0
Aug 2, 2026

This please..

0 replies

europium-cfx · 2026-08-20T13:08:05Z

europium-cfx
Aug 20, 2026
Maintainer

Hey! This issue has been fixed in the latest patch b123 and CitizenFX.FiveM NuGets 0.0.4-alpha. Please try again and let us know if the issue persists.

You can now also retrieve resource name by requesting [FromSource] string source (this will get the raw string in format of nui:resourcename) or [FromSource] CallSource source and getting source.StrSource (this will get just the resource name)

2 replies

ESK0 Aug 20, 2026

Hey! This issue has been fixed in the latest patch b123 and CitizenFX.FiveM NuGets 0.0.4-alpha. Please try again and let us know if the issue persists.

You can now also retrieve resource name by requesting [FromSource] string source (this will get the raw string in format of nui:resourcename) or [FromSource] CallSource source and getting source.StrSource (this will get just the resource name)

Would not be a better to just have a OnNuiEvent attribute? This feels kinda redundant to register callback type and event.

europium-cfx Aug 20, 2026
Maintainer

Actually that's quite fair. Will be implemented in one of the next patches.

unknown-urge · 2026-08-20T16:40:25Z

unknown-urge
Aug 20, 2026
Author

Can confirm this is fixed.

1 reply

europium-cfx Aug 20, 2026
Maintainer

Will keep this RFC open for now to keep track of implementation of the [OnNuiEvent] attributes and related APIs.

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[Bug] .NET 10 SharedEventManager.DecodeSource Throws System.FormatException on NUI Callbacks #257

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[Bug] .NET 10 SharedEventManager.DecodeSource Throws System.FormatException on NUI Callbacks #257

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unknown-urge Jul 25, 2026

Summary

Issue type

Repro rate

Server build version

OS

CPU

GPU

RAM

Storage type

Connection type

ISP and bandwidth

DxDiag

Network graph

Platform

OS version / distribution

CPU

RAM

Using txAdmin?

Hosting provider

Machine type

/perf endpoint output

DDoS protection

ulimit -n value (Linux only)

Docker Compose file (Docker only)

Steps to Reproduce

Expected Behavior

Actual Behavior

Evidence

Additional Context

Why int.TryParse Resolves the Bug

Temporary Workaround / Bytecode Patch Details

Replies: 3 comments · 3 replies

Uh oh!

ESK0 Aug 2, 2026

Uh oh!

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europium-cfx Aug 20, 2026 Maintainer

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ESK0 Aug 20, 2026

Uh oh!

europium-cfx Aug 20, 2026 Maintainer

Uh oh!

unknown-urge Aug 20, 2026 Author

Uh oh!

europium-cfx Aug 20, 2026 Maintainer

unknown-urge
Jul 25, 2026

Why `int.TryParse` Resolves the Bug

Replies: 3 comments 3 replies

ESK0
Aug 2, 2026

europium-cfx
Aug 20, 2026
Maintainer

europium-cfx Aug 20, 2026
Maintainer

unknown-urge
Aug 20, 2026
Author

europium-cfx Aug 20, 2026
Maintainer