title: Identity user manager find by phonenumber
body: <p>In aspnet core identity2.0, Is there anyway to find a user by its phone number? The <a href="https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.identity.usermanager-1?view=aspnetcore-2.0#Methods" rel="nofollow noreferrer"><code>UserManager</code></a> can find a user by its username and email, but there isn't a find by phone or even better, a generic <code>find(Func&lt;TUser, bool&gt;)</code> function provided. When user register with phone, need to check if a given phone number already existed, because a phone cannot be used by two users.</p>
  
  <p>edit</p>
  
  <p>start auth code:</p>
  
  <pre><code>services.AddIdentity&lt;AccountUser, IdentityRole&gt;()
              .AddEntityFrameworkStores&lt;AccountDbContext&gt;()
              .AddDefaultTokenProviders();
  services.AddAuthentication().AddJwtBearer(...);
  </code></pre>