title: How to resolve CS0121 Ambiguous methods or Properties?
body: <p>I have a MVC Core Web application. After a failed attempt to add in some session data, where in it asked me to include a missing reference, I suddenly get several Ambiguous reference errors with apparently no way to track pinpoint the offending references.  I have tried to solution clean and rebuild and to restore references but that didn't help.  </p>
  
  <p>I do not see a way to find the references on disk ( right clicking on the References in the Solution explorer does not offer options to delete or open location in file explorer.</p>
  
  <p>I DO NOT have the ReSharper tool.</p>
  
  <p>I am at a loss how to fix this error.
  Here are my references:</p>
  
  <p><a href="https://i.stack.imgur.com/oRs8z.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oRs8z.jpg" alt="enter image description here"></a></p>
  
  <p>The specific errors ( 3 of them ) are :</p>
  
  <pre><code>    Severity    Code    Description Project File    Line    Source
  Error   CS0121  The call is ambiguous between the following methods or properties: 'System.Security.Claims.PrincipalExtensions.FindFirstValue(System.Security.Claims.ClaimsPrincipal, string)' and 'System.Security.Claims.PrincipalExtensions.FindFirstValue(System.Security.Claims.ClaimsPrincipal, string)'  OmniPot..NETCoreApp,Version=v1.0    C:\zzWork\Omnipot\Main\Source\OmniPot\src\OmniPot\Controllers\AccountController.cs  194 IntelliSense
  </code></pre>
  
  <p>and</p>
  
  <pre><code>    Severity    Code    Description Project File    Line    Source
  Error   CS0121  The call is ambiguous between the following methods or properties: 'Microsoft.Extensions.DependencyInjection.IdentityServiceCollectionExtensions.AddIdentity&lt;TUser, TRole&gt;(Microsoft.Extensions.DependencyInjection.IServiceCollection)' and 'Microsoft.Extensions.DependencyInjection.IdentityServiceCollectionExtensions.AddIdentity&lt;TUser, TRole&gt;(Microsoft.Extensions.DependencyInjection.IServiceCollection)'  OmniPot..NETCoreApp,Version=v1.0    C:\zzWork\Omnipot\Main\Source\OmniPot\src\OmniPot\Startup.cs    82  IntelliSense
  </code></pre>
  
  <p>and</p>
  
  <pre><code>    Severity    Code    Description Project File    Line    Source
  Error   CS0121  The call is ambiguous between the following methods or properties: 'Microsoft.Extensions.DependencyInjection.OptionsServiceCollectionExtensions.Configure&lt;TOptions&gt;(Microsoft.Extensions.DependencyInjection.IServiceCollection, System.Action&lt;TOptions&gt;)' and 'Microsoft.Extensions.DependencyInjection.OptionsServiceCollectionExtensions.Configure&lt;TOptions&gt;(Microsoft.Extensions.DependencyInjection.IServiceCollection, System.Action&lt;TOptions&gt;)'  OmniPot..NETCoreApp,Version=v1.0    C:\zzWork\Omnipot\Main\Source\OmniPot\src\OmniPot\Startup.cs    105 IntelliSense
  </code></pre>