Owin.cs

public interface IApplication
{
    IResponse Respond(IRequest request);
}

public interface IRequest
{
    string Method { get; }
    string Uri { get; }

    IDictionary<string, IEnumerable<string>> Headers { get; }
    IDictionary<string, object> Items { get; }

    IEnumerable<byte[]> Body();
}

public interface IResponse
{
    int Status { get; }
    IDictionary<string, IEnumerable<string>> Headers { get; }

    IEnumerable<object> GetBody();
}