microsoft/autogen · error · InvalidOperationException

Failed to create instance of {input.FullName}

Error message

Failed to create instance of {input.FullName}

What it means

Thrown by ProtobufTypeNameResolver.ResolveTypeName when Activator.CreateInstance(input) returns null for a type that implements IMessage. The resolver instantiates the message solely to read its Descriptor.FullName; failure means the type lacks a public parameterless constructor or its constructor threw.

Source

Thrown at dotnet/src/Microsoft.AutoGen/Core.Grpc/ProtobufTypeNameResolver.cs:15

// Copyright (c) Microsoft Corporation. All rights reserved.
// ProtobufTypeNameResolver.cs

using Google.Protobuf;

namespace Microsoft.AutoGen.Core.Grpc;

public class ProtobufTypeNameResolver : ITypeNameResolver
{
    public string ResolveTypeName(Type input)
    {
        if (typeof(IMessage).IsAssignableFrom(input))
        {
            // TODO: Consider changing this to avoid instantiation...
            var protoMessage = (IMessage?)Activator.CreateInstance(input) ?? throw new InvalidOperationException($"Failed to create instance of {input.FullName}");
            return protoMessage.Descriptor.FullName;
        }
        else
        {
            throw new ArgumentException("Input must be a protobuf message.");
        }
    }
}

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Ensure the message type has a public parameterless constructor (standard protobuf codegen provides one)
2. Use protobuf-generated message classes rather than custom IMessage implementations
3. Filter abstract types and interfaces out of any scanning logic that feeds ResolveTypeName

Example fix

// before
public class MyMessage : IMessage { public MyMessage(Dep dep) {...} } // no parameterless ctor

// after
public class MyMessage : IMessage { public MyMessage() {} public MyMessage(Dep dep) : this() {...} }

if (input.IsAbstract || input.GetConstructor(Type.EmptyTypes) is null) throw new ArgumentException($"{input.FullName} needs a public parameterless constructor");

static bool IsInstantiableProtobuf(Type t) => typeof(IMessage).IsAssignableFrom(t) && !t.IsAbstract && t.GetConstructor(Type.EmptyTypes) is not null;

try { return resolver.ResolveTypeName(type); } catch (InvalidOperationException ex) when (ex.Message.Contains("Failed to create instance")) { return type.FullName; }

When it happens

Trigger: Passing a protobuf message type with a non-public or parameterized-only constructor; a custom IMessage implementation whose constructor throws (e.g. missing runtime dependencies); abstract IMessage subtype passed as input.

Common situations: Custom hand-written IMessage wrappers instead of codegen'd classes; DI-injected message types with constructor parameters; abstract types picked up by assembly scanning.

Related errors

• Input must be a protobuf message.
• Could not create chat manager
• Could not create chat manager; make sure that it contains a
• Response is null.
• Unexpected message '{message}'.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/192cc80404336ce8. Report an issue: GitHub.