Querying with LINQ to SQL against the DBMS of your choice

Source files are missing :(

Too bad LINQ to SQL isn’t a generic ADO.NET API. Although the difficult part is SQL translation, because of the differences in syntax between DBMSs, there are other features that shouldn’t cause any trouble, like object tracking and object materializing. However, if you try for instance DataContext.Translate<T>(DbDataReader) and pass any reader except SqlDataReader you’ll get an exception.

So, what can we do besides rewriting the whole API? In my solution I take advantage of DataContext.GetCommand(IQueryable), which returns a SqlCommand, then convert that command to another DbCommand, like MySqlCommand, and finally create the objects myself.

Here is an example. Notice that data shaping is supported. Proyections are not supported.

// Since we are passing String.Empty to the base ctor Linq2Sql builds a default 
// connection string that looks like this: "Server=;Database=northwind;Integrated Security=SSPI" 
using (NorthwindDB db = new NorthwindDB()) {           

   // See the log on your Output window 
   db.Log = new DebuggerWriter();           

   // Setting our connection  
   // Because we are hiding DataContext.Connection then Linq2Sql won't use this one 
   db.Connection = new MySqlConnection("server=localhost; database=northwind; ");           

   // Disabling entity navegation, insert and updates that would use SqlConnection 
   db.ObjectTrackingEnabled = false;           

   // Immediate loading of Categories 
   var shape = new DataShape(); 
   shape.LoadWith<Product>(p => p.Category); 
   db.Shape = shape;           

   // Create our query 
   var query = from p in db.Products 
               where p.Category.CategoryName.StartsWith("B") 
               select p;           

   db.Log.WriteLine(db.Log.NewLine + "======== Connecting to SqlServer ========");           

   foreach (var p in query) { 
      db.Log.WriteLine("Product: {0}, Category: {1}", p.ProductName, p.Category.CategoryName); 

   // myQuery will now use the connection we previously set 
   var myQuery = db.TransformQuery(query);           

   db.Log.WriteLine(db.Log.NewLine + "======== Connecting to MySql ========");           

   foreach (var p in myQuery) { 
      db.Log.WriteLine("Product: {0}, Category: {1}", p.ProductName, p.Category.CategoryName); 

2007-07-31 Update

Today I realized that LINQ to SQL was connecting to SqlServer when calling DataContext.GetCommand(). The reason is because it needed to figure out which version of SqlServer it’s going to get the data from, and for that uses the DbConnection.ServerVersion property.

You can avoid this by simply specifiying the provider metadata in your mapping info, so if you are using attributes you can add the ProviderAttribute to your DataContext class and pass either typeof(Sql2000Provider) or typeof(Sql2005Provider).

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Tags: ado.net
Originally published at http://maxtoroq.wordpress.com/2007/07/24/querying-with-linq-to-sql-against-the-dbms-of-your-choice/