[c#] No assembly found containing an OwinStartupAttribute Error

This error

The following errors occurred while attempting to load the app. - No assembly found containing an OwinStartupAttribute. - The given type or method 'false' was not found. Try specifying the Assembly. To disable OWIN startup discovery, add the appSetting owin:AutomaticAppStartup with a value of "false" in your web.config. To specify the OWIN startup Assembly, Class, or Method, add the appSetting owin:AppStartup with the fully qualified startup class or configuration method name in your web.config.

appears on my screen on the most face burningly ugly error page ever created in history.

Ive tried to follow the instructions on the page by inserting the owin:AutomaticAppStartup in the config.

 <appSettings >
    <add key="owin:AppStartup" value="false"></add>
        </appSettings>

this did not fix the problem. Any suggestions?

I deleted all DLLs from the branch which wasn't working, then I copied all DDls from my branch which was working to my branch wich wasn't. This solved the issue.

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);
        }
    }
}

if you want to use signalr you haveto add startup.cs Class in your project

Right Click In You Project Then Add New Item And Select OWIN Startup Class

then inside Configuration Method Add Code Below

app.MapSignalR();

I Hope it will be useful for you

I got this error because there was an extra white space in the code

Instead of

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

It was

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

just replacing

        using (WebApp.Start(url))

with

        using (WebApp.Start<Startup>(url))

solved my problem. The class named Startup was already implemented. as mentioned above by @robthedev

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

Add below code to your web.config file then run the project...

    <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
    <dependentAssembly>
    <assemblyIdentity name="Microsoft.Owin.Security" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-3.0.1.0" newVersion="3.0.1.0"/>
    </dependentAssembly>
    <dependentAssembly>
    <assemblyIdentity name="Microsoft.Owin.Security.OAuth" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-3.0.1.0" newVersion="3.0.1.0"/>
    </dependentAssembly>
    <dependentAssembly>
    <assemblyIdentity name="Microsoft.Owin.Security.Cookies" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-3.0.1.0" newVersion="3.0.1.0"/>
    </dependentAssembly>
    <dependentAssembly>
    <assemblyIdentity name="Microsoft.Owin" publicKeyToken="31bf3856ad364e35"/>
    <bindingRedirect oldVersion="1.0.0.0-3.0.1.0" newVersion="3.0.1.0"/>
    </dependentAssembly>
    </runtime>

I know this post is old but just in case someone is looking for the same error, try adding

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

after the tag <appSettings>

and if afterwards the next error show up:

HTTP Error 401.0 - Unauthorized error message

add the next code after the tag <system.web> it can be at the beginning

<authentication mode="Forms"> <forms loginUrl="~/YourFolderName/yourFileName" timeout="1" /> </authentication>

In my case is:

<authentication mode="Forms"> <forms loginUrl="~/Login/Index" timeout="1" /> </authentication>

If deploying to Azure and you get this error. Simply delete all files on the site (backup any web.config, appsettings.json or whatver you do not want to loose) and deploy again. There are some left over dll files that should not be on the site, that makes the Azure portal think it needs to use OWIN.

Add class Startup.cs to root of project with next code:

using Microsoft.Owin;
using Owin;

[assembly: OwinStartupAttribute(typeof(ProjectName.Startup))]
namespace ProjectName
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            ConfigureAuth(app);
        }
    }
}

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);
        }
    }
}

just paste this code <add key="owin:AutomaticAppStartup" value="false" /> in Web.config Not In web.config there is two webconfig so be sure that it will been paste in Web.Config

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

Add the following key in Web.config will remove the code

<appSettings>

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

you may not have Configuration method in the class you mentioned in

<appSettings>
<add key="owin:AppStartup" value="WebApplication1.App_Start.Startup"/>

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>

Check you have the correct startup project selected. I had a web api project as startup. That generated this error.