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

By Steve Smith and Rick Anderson

ASP.NET Core:

• Supports the Open Web Interface for .NET (OWIN).
• Has 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: http://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.
        };
    });
});

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)

Request data (OWIN v1.1.0)

Response data (OWIN v1.0.0)

Other data (OWIN v1.0.0)

Common keys

SendFiles v0.3.0

Opaque v0.3.0

WebSocket v0.3.0

Additional resources

• See the source on GitHub for OWIN keys supported in the translation layer.
• Middleware
• Servers