Error passing existing connections to DbContext constructor when using Database-First

Question

I'm trying to create a class to perform work on the database and have the need (or preference) to use a combination of DbContext and good old fashioned ADO. Why, well EF is great for simplifying a great deal of code but ADO still has many uses for more complex methods that EF cannot yet handle.

This link on MSDN states that I can pass an existing SqlConnection to my context as follows:

using (var conn = new SqlConnection("..."))
{ 
    conn.Open();
    using (var context = new SampleContext(conn, contextOwnsConnection: false))
    {
    // Do Something
    }
}

Now I'm using Database-First so this constructor doesn't appear as standard. I therefore created a new Partial Class file and created the appropriate constructor as follows:

public partial class MyEntities : DbContext
{
    public MyEntities(System.Data.Common.DbConnection conn, bool contextOwnsConnection = false)
        : base(existingConnection: conn, contextOwnsConnection: contextOwnsConnection)
    {

    }
}

However, when I run the code the moment it hits a call to the new DbContext constructor, I get the following UnintentionalCodeFirstException() error thrown by OnModelCreating in my EDMX file:

"Code generated using the T4 templates for Database First and Model First development may not work correctly if used in Code First mode. To continue using Database First or Model First ensure that the Entity Framework connection string is specified in the config file of executing application. To use these classes, that were generated from Database First or Model First, with Code First add any additional configuration using attributes or the DbModelBuilder API and then remove the code that throws this exception."

Am I missing something obvious here, or can it just not be done with Database-First?

Clearly, I could just use two connections, one for my SqlConnection object, and another for my DbContext object but if I can, naturally I'd prefer to use a single connection if possible.

Any and all help greatly appreciated. For full disclosure, I'm using SQL-Server 2012, .NET 4.5.1, C# and EF6.0.2.

Answer 1

Connection strings used by the designer are not regular connection strings. Rather they are EntityConnection strings. The difference is that entity connection strings contain additional information about where to find metadata describing the model which is in form of the edmx at design time - read more here. Code First uses just regular connection strings since it builds the model on the fly based on the code. So, the UnintentionalCodeFirstException is preventing the user from using CodeFirst functionality with edmx models because the model is specified in the edmx and not in the code and if it was allowed you would effectively end up using two different models (one from edmx and one built from the code) which very likely won't be in sync which would result in weird behavior or even could lead to data corruption and crashes/exceptions. Since the EntityConnection is derived from DbConnection and just wraps regular connection you can use it in places where you would use the provider connection. Alternatively you can access the wrapped provider connection using the StoreConnection provider on the EntityConnection.