IApplication.cs

public interface IApplication {
    // either may throw exception
    IAsyncResult BeginInvoke(IRequest request, AsyncCallback callback, object state);
    IResponse EndInvoke(IAsyncResult result);
}

IRequest.cs

public interface IRequest {
    // a bucket into which the host, app, middleware, or whomever can throw things.
    IDictionary<string, object> Items { get; }
    
    string Method { get; }
    string Uri { get; }
    IDictionary<string, IEnumerable<string>> Headers { get; }
    
    // application calls pair until EndReadBody returns 0. either may throw exception.
    IAsyncResult BeginReadBody(byte[] buffer, int offset, int count, AsyncCallback callback, object state);
    int EndReadBody(IAsyncResult result);
}

IResponse.cs

public interface IResponse {
    string Status { get; } // e.g., "200 OK"
    IDictionary<string, IEnumerable<string>> Headers {get;}
    
    // can contain string, byte[], ArraySegment<byte>, FileInfo, 
    // Task<string>, Task<byte[]>, Task<ArraySegment<byte>>, Task<FileInfo>
    //
    // exceptions: 
    // - GetBody() could throw
    // - GetBody().GetEnumerator() could throw
    // - GetBody().GetEnumerator().MoveNext() could throw
    // - GetBody().GetEnumerator().Current could throw
    IEnumerable GetBody();
}