cefsharp/CefSharp · error · Exception

UseAnyCpuAssemblyResolver has already been called, call

Error message

UseAnyCpuAssemblyResolver has already been called, call

What it means

SubscribeAnyCpuAssemblyResolver hooks an AppDomain.AssemblyResolve handler to load the architecture-specific CefSharp.Core.Runtime.dll at runtime. It tracks whether it has already been called via a static field; calling it a second time throws because double-subscribing would create duplicate resolve handlers and unpredictable assembly loading.

Source

Thrown at CefSharp/CefRuntime.cs:37

        /// <summary>
        /// When using AnyCPU the architecture specific version of CefSharp.Core.Runtime.dll
        /// needs to be loaded (x64/x86).
        /// This method subscribes to the <see cref="AppDomain.AssemblyResolve"/> event
        /// for <see cref="AppDomain.CurrentDomain"/> and loads the CefSharp.Core.Runtime.dll
        /// based on <see cref="Environment.Is64BitProcess"/>.
        /// This method MUST be called before you call Cef.Initialize, create your first ChromiumWebBrowser instance, basically
        /// before anything CefSharp related happens. This method is part of CefSharp.dll which is an AnyCPU library and
        /// doesn't have any references to the CefSharp.Core.Runtime.dll so it's safe to use.
        /// </summary>
        /// <param name="basePath">
        /// The path containing the x64/x86 folders which contain the CefSharp/CEF resources.
        /// If null then AppDomain.CurrentDomain.SetupInformation.ApplicationBase will be used as the path.
        /// (</param>
        public static void SubscribeAnyCpuAssemblyResolver(string basePath = null)
        {
            if(currentDomainAssemblyResolveHandler != null)
            {
                throw new Exception("UseAnyCpuAssemblyResolver has already been called, call ");
            }

            if(basePath == null)
            {
                basePath = AppDomain.CurrentDomain.SetupInformation.ApplicationBase;
            }

            currentDomainAssemblyResolveHandler = (sender, args) =>
            {
                if (args.Name.StartsWith("CefSharp.Core.Runtime"))
                {
                    string assemblyName = args.Name.Split(new[] { ',' }, 2)[0] + ".dll";
                    string archSpecificPath = Path.Combine(basePath,
                                                           Environment.Is64BitProcess ? "x64" : "x86",
                                                           assemblyName);

                    return File.Exists(archSpecificPath)
                               ? System.Reflection.Assembly.LoadFile(archSpecificPath)

View on GitHub (pinned to 16bc6e0711)

Solutions

Call SubscribeAnyCpuAssemblyResolver exactly once at application entry point.
If re-subscription is legitimately needed, call UnsubscribeAnyCpuAssemblyResolver first.
Guard the call with a check of whether CEF is already initialized or the resolver is already active.

Example fix

// before — called in two places
CefRuntime.SubscribeAnyCpuAssemblyResolver();
// ... later, in another module ...
CefRuntime.SubscribeAnyCpuAssemblyResolver(); // throws

// after — call once, or unsubscribe first
CefRuntime.UnsubscribeAnyCpuAssemblyResolver();
CefRuntime.SubscribeAnyCpuAssemblyResolver(newBasePath);

Defensive patterns

Strategy: validation

Validate before calling

// CefRuntime does not expose the internal flag publicly.
// Track it yourself:
private static int resolverSubscribed = 0;

if (Interlocked.CompareExchange(ref resolverSubscribed, 1, 0) == 1)
{
    // already subscribed, skip or unsubscribe first
    return;
}
CefRuntime.SubscribeAnyCpuAssemblyResolver(basePath);

Try / catch

try
{
    CefRuntime.SubscribeAnyCpuAssemblyResolver(basePath);
}
catch (Exception ex) when (ex.Message.Contains("already been called"))
{
    Log.Info("AnyCPU resolver already subscribed — skipping");
}

Prevention

Call SubscribeAnyCpuAssemblyResolver exactly once in Program.Main or the app entry point.
If re-subscription is needed, call UnsubscribeAnyCpuAssemblyResolver first.
Avoid calling it from library initializers that may run multiple times.

When it happens

Trigger: Calling CefRuntime.SubscribeAnyCpuAssemblyResolver() more than once in the same AppDomain without calling UnsubscribeAnyCpuAssemblyResolver in between.

Common situations: Application startup code and a DI container initializer both calling it. Test fixtures that re-run startup logic without cleanup. A library that calls it internally while the host app also calls it.

Related errors

AI-assisted analysis of cefsharp/CefSharp@16bc6e0711 (2026-08-13). Data as JSON: /api/errors/4742e2f408bfe9bd. Report an issue: GitHub.