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Open Web Interface for .NET (OWIN) with ASP.NET Core

  • Article

By Steve Smith and Rick Anderson

ASP.NET Core:

  • Supports the Open Web Interface for .NET (OWIN).
  • Has .NET Core compatible replacements for the Microsoft.Owin.* (Katana) libraries.

OWIN allows web apps to be decoupled from web servers. It defines a standard way for middleware to be used in a pipeline to handle requests and associated responses. ASP.NET Core applications and middleware can interoperate with OWIN-based applications, servers, and middleware.

OWIN provides a decoupling layer that allows two frameworks with disparate object models to be used together. The Microsoft.AspNetCore.Owin package provides two adapter implementations:

  • ASP.NET Core to OWIN
  • OWIN to ASP.NET Core

This allows ASP.NET Core to be hosted on top of an OWIN compatible server/host or for other OWIN compatible components to be run on top of ASP.NET Core.

Note

Using these adapters comes with a performance cost. Apps using only ASP.NET Core components shouldn't use the Microsoft.AspNetCore.Owin package or adapters.

View or download sample code (how to download)

Running OWIN middleware in the ASP.NET Core pipeline

ASP.NET Core's OWIN support is deployed as part of the Microsoft.AspNetCore.Owin package. You can import OWIN support into your project by installing this package.

OWIN middleware conforms to the OWIN specification, which requires a Func<IDictionary<string, object>, Task> interface, and specific keys be set (such as owin.ResponseBody). The following simple OWIN middleware displays "Hello World":

public Task OwinHello(IDictionary<string, object> environment)
{
    string responseText = "Hello World via OWIN";
    byte[] responseBytes = Encoding.UTF8.GetBytes(responseText);

    // OWIN Environment Keys: https://owin.org/spec/spec/owin-1.0.0.html
    var responseStream = (Stream)environment["owin.ResponseBody"];
    var responseHeaders = (IDictionary<string, string[]>)environment["owin.ResponseHeaders"];

    responseHeaders["Content-Length"] = new string[] { responseBytes.Length.ToString(CultureInfo.InvariantCulture) };
    responseHeaders["Content-Type"] = new string[] { "text/plain" };

    return responseStream.WriteAsync(responseBytes, 0, responseBytes.Length);
}

The sample signature returns a Task and accepts an IDictionary<string, object> as required by OWIN.

The following code shows how to add the OwinHello middleware (shown above) to the ASP.NET Core pipeline with the UseOwin extension method.

public void Configure(IApplicationBuilder app)
{
    app.UseOwin(pipeline =>
    {
        pipeline(next => OwinHello);
    });
}

You can configure other actions to take place within the OWIN pipeline.

Note

Response headers should only be modified prior to the first write to the response stream.

Note

Multiple calls to UseOwin is discouraged for performance reasons. OWIN components will operate best if grouped together.

app.UseOwin(pipeline =>
{
    pipeline(next =>
    {
        return async environment =>
        {
            // Do something before.
            await next(environment);
            // Do something after.
        };
    });
});

Run ASP.NET Core on an OWIN-based server and use its WebSockets support

Another example of how OWIN-based servers' features can be leveraged by ASP.NET Core is access to features like WebSockets. The .NET OWIN web server used in the previous example has support for WebSockets built in, which can be leveraged by an ASP.NET Core application. The example below shows a simple web app that supports WebSockets and echoes back everything sent to the server through WebSockets.

public class Startup
{
    public void Configure(IApplicationBuilder app)
    {
        app.Use(async (context, next) =>
        {
            if (context.WebSockets.IsWebSocketRequest)
            {
                WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
                await EchoWebSocket(webSocket);
            }
            else
            {
                await next();
            }
        });

        app.Run(context =>
        {
            return context.Response.WriteAsync("Hello World");
        });
    }

    private async Task EchoWebSocket(WebSocket webSocket)
    {
        byte[] buffer = new byte[1024];
        WebSocketReceiveResult received = await webSocket.ReceiveAsync(
            new ArraySegment<byte>(buffer), CancellationToken.None);

        while (!webSocket.CloseStatus.HasValue)
        {
            // Echo anything we receive
            await webSocket.SendAsync(new ArraySegment<byte>(buffer, 0, received.Count), 
                received.MessageType, received.EndOfMessage, CancellationToken.None);

            received = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), 
                CancellationToken.None);
        }

        await webSocket.CloseAsync(webSocket.CloseStatus.Value, 
            webSocket.CloseStatusDescription, CancellationToken.None);
    }
}

OWIN environment

You can construct an OWIN environment using the HttpContext.


   var environment = new OwinEnvironment(HttpContext);
   var features = new OwinFeatureCollection(environment);

OWIN keys

OWIN depends on an IDictionary<string,object> object to communicate information throughout an HTTP Request/Response exchange. ASP.NET Core implements the keys listed below. See the primary specification, extensions, and OWIN Key Guidelines and Common Keys.

Request data (OWIN v1.0.0)

Key Value (type) Description
owin.RequestScheme String
owin.RequestMethod String
owin.RequestPathBase String
owin.RequestPath String
owin.RequestQueryString String
owin.RequestProtocol String
owin.RequestHeaders IDictionary<string,string[]>
owin.RequestBody Stream

Request data (OWIN v1.1.0)

Key Value (type) Description
owin.RequestId String Optional

Response data (OWIN v1.0.0)

Key Value (type) Description
owin.ResponseStatusCode int Optional
owin.ResponseReasonPhrase String Optional
owin.ResponseHeaders IDictionary<string,string[]>
owin.ResponseBody Stream

Other data (OWIN v1.0.0)

Key Value (type) Description
owin.CallCancelled CancellationToken
owin.Version String

Common keys

Key Value (type) Description
ssl.ClientCertificate X509Certificate
ssl.LoadClientCertAsync Func<Task>
server.RemoteIpAddress String
server.RemotePort String
server.LocalIpAddress String
server.LocalPort String
server.OnSendingHeaders Action<Action<object>,object>

SendFiles v0.3.0

Key Value (type) Description
sendfile.SendAsync See delegate signature Per Request

Opaque v0.3.0

Key Value (type) Description
opaque.Version String
opaque.Upgrade OpaqueUpgrade See delegate signature
opaque.Stream Stream
opaque.CallCancelled CancellationToken

WebSocket v0.3.0

Key Value (type) Description
websocket.Version String
websocket.Accept WebSocketAccept See delegate signature
websocket.AcceptAlt Non-spec
websocket.SubProtocol String See RFC6455 Section 4.2.2 Step 5.5
websocket.SendAsync WebSocketSendAsync See delegate signature
websocket.ReceiveAsync WebSocketReceiveAsync See delegate signature
websocket.CloseAsync WebSocketCloseAsync See delegate signature
websocket.CallCancelled CancellationToken
websocket.ClientCloseStatus int Optional
websocket.ClientCloseDescription String Optional

Additional resources