Question

Using Visual Studio’s O/R Designer, I have created a number of classes the inherit from a base class (named SCO).

The inherited types have a discriminator property TypeId which is a string containing the name of the type. For example, the inherited class Blog would have a TypeId property of Blog.

I want to be able to create a method that returns a strongly typed collection of any of the particular inherited types. My attempt at this method is as follows:

public static IQueryable<T> GetSCOs<T>(SCODataContext dc) where T : SCO
{
    string targetType = typeof(T).Name;
    IQueryable<T> scos = from s in dc.SCOs
                         where s.TypeId == targetType
                         select s;
    return scos;
}

When I try and compile this method, I get the following error…

_Cannot implicitly convert type 'System.Linq.IQueryable<SCOs.SCO>' to 'System.Linq.IQueryable<T>'. An explicit conversion exists (are you missing a cast?)_

I (think) I understand why this is happening, but have been unable to find a solution. Any advice, guidance would be much appreciated.

Answer 1

Try this:

public static IQueryable<T> GetSCOs<T>(SCODataContext dc) where T : SCO
{
    return dc.SCOs.OfType<T>();
}

Works for LINQ-Entities, not sure if it works for LINQ-SQL. Give it a try.