title: EF Core: Method 'ProcessModelFinalized' does not have implementation
body: <p>I'm facing an error while trying to obtain information of the DbSets in a DbContext object by using Entity Framework core. </p>
  
  <p>My DbContext object looks this way:</p>
  
  <pre><code>public class CatalogueContext : DbContext
  {
      public DbSet&lt;ConnectorCatalogueItemConv&gt; CatalogueItemConvs { get; set; }
  
      public CatalogueContext(DbContextOptions&lt;CatalogueContext&gt; options)
          : base(options)
      {
  
      }
  
      public CatalogueContext()
      {
  
      }
  }
  </code></pre>
  
  <p>I'm trying to instante the context by calling a method which receives a generic type T which might be childs of DbContext this way:</p>
  
  <pre><code>public T GetContext&lt;T&gt;() where T: DbContext, new()
  {
      var optionsBuilder = new DbContextOptionsBuilder&lt;T&gt;();
      var connectionString = Configuration.GetConnectionString(ExternalTablesKey);
      optionsBuilder.UseSqlServer(connectionString);
      return Activator.CreateInstance(typeof(T), optionsBuilder.Options) as T;
  }
  </code></pre>
  
  <p>The connection string is obtained properly by using Microsoft.Extensions.Configuration so the problem it's not there. </p>
  
  <p>Finally, I invoke this method and try to obtain any record on the DbSets declared as follows:</p>
  
  <pre><code> public void SomeMethod()
   {
       using (var db = this.GetContext&lt;CatalogueContext&gt;())
       {
           var val = db.CatalogueItemConvs.ToList(); //Here is where I get the error below.
       }
   }
  </code></pre>
  
  <p>The error shown says:</p>
  
  <blockquote>
    <p>Method 'ProcessModelFinalized' in type
    'Microsoft.EntityFrameworkCore.Metadata.Conventions.SqlServerValueGenerationStrategyConvention'
    from assembly 'Microsoft.EntityFrameworkCore.SqlServer,
    Version=3.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'
    does not have an implementation.'</p>
  </blockquote>
  
  <p>I searched everywhere, but seems to be that there's very few information about this error. Any thoughts?</p>
  
  <p><strong>EDIT 1:</strong>
  My solution includes Microsoft.EntityFrameworkCore.SqlServer in version 3.0.0.0</p>
  
  <p><strong>EDIT 2:</strong>
  As requested, I have edited the code to include only one entity and its class declaration. There's no mapping at this point.</p>
  
  <pre><code>public class ConnectorCatalogueItemConv
  {
      public string CodConnector { get; set; }
      public string CodCatalogue { get; set; }
      public string CodItemCia { get; set; }
      public string CodItemInbk { get; set; }
      public bool Defaultvalue { get; set; }
  }
  </code></pre>
  
  <p><strong>EDIT 3:</strong> <a href="https://stackoverflow.com/questions/50788272/how-to-instantiate-a-dbcontext-in-ef-core">How to instantiate a DbContext in EF Core</a></p>