No assembly found containing an OwinStartupAttribute Error

Solution 1:

Add this code in web.config under the <configuration> tag as shown in image below. Your error should then be gone.

<configuration>
  <appSettings>
    <add key="owin:AutomaticAppStartup" value="false" />
  </appSettings>
  ...
</configuration>

Solution 2:

I wanted to get rid of OWIN in the project:

1. Delete OWIN references and Nuget packages from project
2. Clean & Rebuild project
3. Run app

Then I got OWIN error. These steps didn't work, because OWIN.dll was still in bin/ directory.

FIX:

1. Delete bin/ directory manually
2. Rebuild project

Solution 3:

For those who do want owin to start, <add key="owin:AutomaticAppStartup" value="false" /> won't work, but the following worked for me.

1. if you have a partial class "Startup" in your Startup.Auth file, create another partial Startup class in the root of your project.

2. define an assembly owinstartup attribute pointing to that class

3. create a "Configuration" method

4. rebuild your application

You could also create the "Configuration" method, and add the assembly attribute to the Startup.Auth, but doing it this way allows you to keep your Startup class separated by leveraging C# class definition splitting. Read more here: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/partial-classes-and-methods

This is what my Startup.cs file looked like:

using Microsoft.Owin;
using Owin;

[assembly: OwinStartupAttribute(typeof(ProjectNameSpace.Startup))]

namespace ProjectNameSpace
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            ConfigureAuth(app);
        }
    }
}

Solution 4:

I was missing the attribute:

[assembly: OwinStartupAttribute(typeof(projectname.Startup))]

Which specifies the startup class. More details: https://docs.microsoft.com/en-us/aspnet/aspnet/overview/owin-and-katana/owin-startup-class-detection

Solution 5:

Check if you have the Startup class created in your project. This is an example:

using Microsoft.Owin;
using Owin;

[assembly: OwinStartupAttribute(typeof({project_name}.Startup))]

namespace AuctionPortal
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            ConfigureAuth(app);
        }
    }
}