How to use DbContext in T4 template?

Question

I want to generate some code with a T4 Template using EntityFramework. I created a T4 Template in the same Assembly as my currently working EF6 DbContext:

<#@ template language="C#" hostspecific="true" debug="True" #>
<#@ assembly name="$(SolutionDir)\packages\EntityFramework.6.1.3\lib\net45\EntityFramework.dll" #>
<#@ assembly name="$(TargetPath)" #>
<#@ import namespace="Conwell.Administration.Data.Entities" #>

<#
    using (var db = new KassenautomatEntities())
    {
#>
//Hello World
<#
    }
#>

When I run it, I get the following execption:

Running transformation: System.InvalidOperationException:

The 'Instance' member of the Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' did not return an object that inherits from 'System.Data.Entity.Core.Common.DbProviderServices'. Entity Framework providers must inherit from this class and the 'Instance' member must return the singleton instance of the provider. This may be because the provider does not support Entity Framework 6 or later; see http://go.microsoft.com/fwlink/?LinkId=260882 for more information.

The same context works fine outside of T4. What I am missing?

I suppose that you will have to make a KassenautomatEntities definition in a separate assembly. Then reference to this assembly can be used in T4. — Alexander Petrov
I have just started getting this error after upgrading from VS2015 to VS2017. If I run the same script in VS2015 it works, so that is how I have been getting by but Christian did you ever discover the fix for this? And to @AlexanderPetrov my DbContext class is in a separate assembly, here I have proven that the template is complete because I am using it in VS2015, so what has changed in the context that the code is generated in VS2017 — Chris Schaller
I am afraid to say, I have used plain ado.net @ChrisSchaller — Christian Gollhardt

Mahmoud Hanafy Mahmoud Hanafy · Accepted Answer · 2017-07-23T08:59:55

I've faced the same error, and the way to get it working was:

Ensure to reference both EntityFramework, and your provider DLLs on Your T4 template; This is enough to get rid of this error.

<#@ assembly name="$(TargetDir)\EntityFramework.dll" #>
<#@ assembly name="$(TargetDir)\EntityFramework.SqlServer.dll" #>

Config files aren't read since T4 runs in a different context; hence you'll need to create a DbContext constructor accepting a connection string; then you pass it while creating the context in T4

How to use DbContext in T4 template?

1 Answers