Question

I’ve encountered a strange issue using CompiledQuery.Compile. When trying to use a static readonly field within a query, I get the following error message:

Class member X is unmapped

If I move the field decleration out of the partial class into another class not related to LINQ-SQL, then I get the following:

Object reference not set to an instance of an object

If I pass the fields through as an argument, then I see no errors and the query works fine and generates the expected SQL.

An example is as below:

partial class Order 
{
    public static readonly string Complete = "Complete";
    public static readonly string Pending = "Pending";

    public static readonly Func<DataContext, Order, bool> IsComplete =
        CompiledQuery.Compile((DataContext context, Order o) =>
           Complete == o.Status);
}

Usage:

var test = from o in db.Orders
           select new
           {
               IsComplete = Order.IsComplete(db, o)
           };

This generates the errors mentioned. If I add a string[] as another argument to the CompiledQuery then I see no errors. Also, If I modify the strings to be const instead of static readonly this works as well, but I imagine that is due to the values being assigned at compile time.

Is there any way of getting the static readonly fields working?

Answer 1

The problem is occuring because Linq-To-Sql is trying to translate your expression into the backend SQL, because the logic sees an unmapped class member it cannot cope with converting it.

I would suggest you create a wrapping property to do the work for you

partial class Order  { 

  public static readonly string Complete = "Complete"; 
  public static readonly string Pending = "Pending"; 

  private static readonly Func<DataContext, Order, bool> _isComplete;

  public static Func<DataContext, Order, bool> IsComplete {
    get {
      if (_isComplete == null) {
        var complete=Complete; 
        _isComplete CompiledQuery.Compile((DataContext context, Order o) => 
                                                       complete == o.Status); 
      }
      return _isComplete;
    }
  }
}

}