The underlying provider failed on Open

Question

I was using .mdf for connect to DB and entityClient. Now i want to change connection string that trehe will be no .mdf

Is this conectionstring correct

<connectionStrings>
<!--<add name="conString" connectionString="metadata=res://*/conString.csdl|res://*/conString.ssdl|res://*/conString.msl;provider=System.Data.SqlClient;provider connection string=&quot;Data Source=.\SQL2008;AttachDbFilename=|DataDirectory|\NData.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True;MultipleActiveResultSets=True&quot;" providerName="System.Data.EntityClient" />-->
<add name="conString" connectionString="metadata=res://*/conString.csdl|res://*/conString.ssdl|res://*/conString.msl;provider=System.Data.SqlClient;provider connection string=&quot;Data Source=.\SQL2008;Initial Catalog=NData;Integrated Security=True;Connect Timeout=30;User Instance=True;MultipleActiveResultSets=True&quot;" providerName="System.Data.EntityClient" />

becouse i always get error !The underlying provider failed on Open!

Answer 1

I had this error and found a few solutions:

Looking at your connection string, it looks valid. I found this blog post, the problem here is that they were using Integrated Security. If you are running on IIS, your IIS user needs access to the database.

If you are using Entity Framework with Transactions, Entity Framework automatically opens and closes a connection with each database call. So when using transactions, you are attempting to spread a transaction out over multiple connections. This elevates to MSDTC.

(See this reference and this one for more information.)

Changing my code to the following fixed it:

using (DatabaseEntities context = new DatabaseEntities())
{
    context.Connection.Open();
    // the rest
}

Answer 2

context.Connection.Open() didn't help solving my problem so I tried enabling "Allow Remote Clients" in DTC config, no more error.