.net框架：4.7.2
mysql服务器安装：8.0.12
mysql连接器/net:8.0.12
实体框架：6.2.0
mysql.data:8.0.12
mysql.data.entityframework:8.0.12
web.config配置

<system.data>
    <DbProviderFactories>
      <remove invariant="MySql.Data.MySqlClient" />
      <add name="MySQL Data Provider"
           invariant="MySql.Data.MySqlClient"
           description=".Net Framework Data Provider for MySQL"
           type="MySql.Data.MySqlClient.MySqlProviderServices, 
           MySql.Data.EntityFramework, Version=8.0.12.0, Culture=neutral, 
           PublicKeyToken=c5687fc88969c44d" />
    </DbProviderFactories>
</system.data>
<connectionStrings>
    <add name="DefaultConnection" 
       connectionString="server=localhost;user=root;database=app;password=root;" 
       providerName="MySql.Data.MySqlClient" />
</connectionStrings>
<providers>
      <provider invariantName="MySql.Data.MySqlClient" 
                type="MySql.Data.MySqlClient.MySqlProviderServices, 
                MySql.Data.EntityFramework, Version=8.0.12, Culture=neutral, 
                PublicKeyToken=c5687fc88969c44d"/>
</providers>

连接到db时，出现如下异常： The requested .Net Framework Data Provider's implementation does not have an Instance field of a System.Data.Common.DbProviderFactory derived type.