Using Generics with LINQ Inheritance

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.

Please mark the answer as accepted if it solves the problem, this prevents other users from adding more answers that might be wrong and also helps others looking for answers to find your question instead of asking their own. — David Mårtensson
– David Mårtensson, Commented Dec 22, 2010 at 9:47
The thing people forget when using generics is that they need to be known at compile time otherwise the compiler can't guarantee type safety (which is the whole point of using generics). LINQ queries on the other hand are run JIT so the method you are using creates some problems for the compiler, hence the IQueryable<T> in your error message. — Jonas Van der Aa
– Jonas Van der Aa, Commented Dec 22, 2010 at 11:25

RPM1984 · Accepted Answer · 2010-12-22 09:34:43Z

7

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.

answered Dec 22, 2010 at 9:34

RPM1984

73.3k62 gold badges241 silver badges364 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Evren Kuzucuoglu Over a year ago

This is awesome. I was going to answer with "dc.SCOs.Where(s => s is T).Cast<T>();" but this is better!

RPM1984 Over a year ago

Sweet - i had a feeling it would work, since the method is from the LINQ-Objects API. Don't forget to tick this the correct answer.

Collectives™ on Stack Overflow

Using Generics with LINQ Inheritance

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related