dotnet/wpf · error · ProxyAssemblyNotLoadedException

SR.CouldNotFindType0InAssembly1 (formatted with typeName…

Error message

SR.CouldNotFindType0InAssembly1 (formatted with typeName, assemblyName)

What it means

RegisterProxyAssembly looks for the conventional type '<AssemblyName>.UIAutomationClientSideProviders' inside the loaded proxy assembly; if Assembly.GetType returns null it throws ProxyAssemblyNotLoadedException with SR.CouldNotFindType0InAssembly1. A valid proxy assembly must expose this well-known provider-description type.

Solutions

Define/rename the class so its full name equals '<AssemblyName>.UIAutomationClientSideProviders' (e.g., assembly 'MyProxies' needs type 'MyProxies.UIAutomationClientSideProviders').
Make sure the type is public and compiled into the referenced assembly (not a different assembly or internal class).
Verify the assembly root name (assemblyName.Name) matches the namespace you used for the provider class.

Example fix

// before
namespace MyCompany.Proxies { class UIAutomationClientSideProviders { ... } } // type not found
// after
namespace MyProxies { public class UIAutomationClientSideProviders { public static ClientSideProviderDescription[] ClientSideProviderDescriptionTable = ...; } }

Defensive patterns

Strategy: validation

Validate before calling

var asm = Assembly.LoadFrom(proxyDllPath);
string expected = asm.GetName().Name + ".UIAutomationClientSideProviders";
bool ok = asm.GetType(expected, throwOnError: false) != null;

Type guard

static bool HasProviderType(Assembly asm) => asm.GetType(asm.GetName().Name + ".UIAutomationClientSideProviders", false) != null;

Try / catch

try { ClientSettings.RegisterClientSideProviders(table); }
catch (ProxyAssemblyNotLoadedException ex) { log.Error($"Invalid proxy assembly layout: {ex.Message}"); }

Prevention

Name the provider class exactly '<AssemblyName>.UIAutomationClientSideProviders'.
Make the type public and compile it into the proxy assembly itself.
Add a unit test that loads the proxy assembly and checks the expected type exists.

When it happens

Trigger: Registering a proxy assembly that lacks a public type named '<assembly root namespace>.UIAutomationClientSideProviders' — the default name derived from assemblyName.Name + ".UIAutomationClientSideProviders".

Common situations: Custom proxy assemblies where the provider class was renamed or placed in a namespace not matching the assembly name; building proxies from templates that changed; typo in the class name.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/bfb2229803afb503. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClient/MS/Internal/Automation/ProxyManager.cs:64

        // load proxies from specified assembly
        internal static void RegisterProxyAssembly ( AssemblyName assemblyName )
        {
            Assembly a = null;
            try
            {
                a = Assembly.Load( assemblyName );
            }
            catch(System.IO.FileNotFoundException)
            {
                throw new ProxyAssemblyNotLoadedException(SR.Format(SR.Assembly0NotFound,assemblyName));
            } 
            
            string typeName = assemblyName.Name + ".UIAutomationClientSideProviders";
            Type t = a.GetType( typeName );
            if( t == null )
            {
                throw new ProxyAssemblyNotLoadedException(SR.Format(SR.CouldNotFindType0InAssembly1, typeName, assemblyName));
            }

            FieldInfo fi = t.GetField("ClientSideProviderDescriptionTable", BindingFlags.Static | BindingFlags.Public);
            if (fi == null || fi.FieldType !=  typeof(ClientSideProviderDescription[]))
            {
                throw new ProxyAssemblyNotLoadedException(SR.Format(SR.CouldNotFindRegisterMethodOnType0InAssembly1, typeName, assemblyName));
            }

            ClientSideProviderDescription[] table = fi.GetValue(null) as ClientSideProviderDescription[];
            if (table != null)
            {
                ClientSettings.RegisterClientSideProviders(table);
            }
        } 
        
        // register specified proxies
        internal static void RegisterWindowHandlers(ClientSideProviderDescription[] proxyInfo)
        {

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