dotnet/wpf · error · ArgumentException

The parameter must implement interface

Error message

The parameter must implement interface {interfaceType}.

What it means

Verify.ImplementsInterface walks the parameter's type hierarchy (including interfaces of base types) and throws ArgumentException with the message "The parameter must implement interface {interfaceType}." if no match is found. Unlike TypeSupportsInterface it takes the object itself and works via interface list traversal.

Solutions

Make the parameter's class implement interfaceType (directly or via a base class).
Pass a different instance that already implements the required interface.
Fix the DI registration/type hierarchy so the instance actually satisfies the interface.

Example fix

// before
ctx.SetValue(new MyProvider()); // MyProvider lacks IServiceProvider
// after
class MyProvider : IServiceProvider { ... }
ctx.SetValue(new MyProvider());

Defensive patterns

Strategy: type-guard

Validate before calling

if (parameter is TRequired) { api.Call(parameter); } else { /* wrap or reject */ }

Type guard

static bool ImplementsInterfaceOf<TReq>(object o) => o is TReq;

Try / catch

try { api.Call(obj); }
catch (ArgumentException ex) when (ex.Message.Contains("must implement interface")) { /* supply an implementing instance */ }

Prevention

Program against interfaces and have concrete classes explicitly implement them.
Check DI container registrations bind the interface, not just the concrete type.
Validate object graphs at construction time with `is` checks.

When it happens

Trigger: Passing an object instance that (or whose base types) do not implement interfaceType into a WPF API that calls Verify.ImplementsInterface.

Common situations: Passing mocks, subclasses, or wrappers that dropped an interface; dependency-injection bindings registered against the concrete type instead of the interface; codegen or proxy objects that omit the interface.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0d8e047600cf5236. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Standard/Verify.cs:290

        internal static void ImplementsInterface(object parameter, Type interfaceType, string parameterName)
        {
            Assert.IsNotNull(parameter);
            Assert.IsNotNull(interfaceType);
            Assert.IsTrue(interfaceType.IsInterface);

            bool isImplemented = false;
            foreach (var ifaceType in parameter.GetType().GetInterfaces())
            {
                if (ifaceType == interfaceType)
                {
                    isImplemented = true;
                    break;
                }
            }

            if (!isImplemented)
            {
                throw new ArgumentException($"The parameter must implement interface {interfaceType}.", parameterName);
            }
        }
    }
}

View on GitHub (pinned to 81131a70a4)