找不到包含 OwinStartupAttribute 错误的程序集

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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>

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

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I wanted to get rid of OWIN in the project:

Delete OWIN references and Nuget packages from project
Clean & Rebuild project
Run app

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

FIX:

Delete bin/ directory manually
Rebuild project

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

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

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I was missing the attribute:

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

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

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

if you have a partial class "Startup" in your Startup.Auth file, create another partial Startup class in the root of your project.
define an assembly owinstartup attribute pointing to that class
create a "Configuration" method
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://learn.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);
}
}
}

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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" />

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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.

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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

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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);
}
}
}

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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

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

<appSettings>


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

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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

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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>

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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);
}
}
}

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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.

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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>

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It needs owin startup class for my case. Add new item > OWIN Startup Class > Name Startup.cs

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I found a bug in Visual Studio 2019 V 16.10.2 that throws this error when you have a space in your project name whereby your namespace contains an underscore. To solve the issue, you have to explicitly set the startup class path in your web.config -> appSettings section:

<add key="owin:AppStartup" value="ABC_DEF.Startup"/>

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In my case, I had a main site that was with Forms Authentication. I needed to create an Web Application under that site that did a Single Sign On with a Identity Provider. The SSO application had to read a cookie that are supported by Owin, and convert it to the Form cookie to make compatible both the authentications. The problem was that it was installed under the main site the Owin Assemblies, and they, for some reason, even if they were not being used in the code or in the configuration files, were being called and trying to find the Startup class.

The solution was to uninstall all the Owin assemblies that were added by Nuget on the main site.

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My issue had a much simpler fix.

I wasn't accessing the secure site. As soon as I pointed the browser to https://... it started working.

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In my case I just deleted the bin and obj folder. Restarted the project, cleaned and build it.