title: Entity Framework: "ProviderName" in connection string required- all of the sudden?
body: <p>We've been using Entity framework in a production environment for many months now, and just yesterday started to get errors on <em>some</em> machines when querying the database using our subclass of DbContext:</p>
  
  <blockquote>
    <p>"The connection string 'MyConnectionString' in the application's
    configuration file does not contain the required providerName
    attribute"</p>
  </blockquote>
  
  <p>Our problem is easily solved: I adding the <code>"providerName="System.Data.SqlClient"</code> to the connection string in the config files on all deployed servers and workstations. </p>
  
  <p>However, the mystery remains: according to <a href="http://msdn.microsoft.com/en-us/library/vstudio/htw9h4z3(v=vs.100).aspx" rel="noreferrer">the documentation</a>:</p>
  
  <blockquote>
    <p>The providerName attribute is optional, and the default is
    "System.Data.SqlClient".</p>
  </blockquote>
  
  <p>Even more mysterious is why this started happening suddenly, and apparently only on some machines. I am not aware of any recent changes in EF or .NET versions, any SQL Server version or provider changes, or anything. But I realize there has to be something I've overlooked.</p>
  
  <p>.NET 4.5
  EF 5.0</p>
  
  <p>Anyone have any hints or insights?</p>