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I am working on a repository pattern where the API look as follows:

var visitor = repository.Find(x => x.EmailAddress == credentials.EmailAddress &&
                              x.Password == credentials.Password);

where visitor is a domain object and x represents this domain object. The method signature of the Find method on the repository is:

T Find(Func<T, bool> query);

This is all wonderful until I attempt to use this with Linq2Sql because linq2sql creates its own objects and as a result when I want to call: 

context.visitors.FirstOrDefault(query);

there is a type mismatch because linq2sql expects a function of the type it created and not the function I am passing in.

Michael Mann
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1 Answers1

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Well to start with you'll need to change your Find signature to:

T Find(Expression<Func<T, bool>> query);

LINQ to SQL needs to have the logic as an expression tree instead of a plain delegate, otherwise it can't work out how to translate it into SQL.

Beyond that, I'm afraid it's not terribly clear - it sounds like you're not using the same domain classes for your repository and LINQ to SQL. Is that right? That sounds like a potential problem; at the very least it's going to make life pretty tricky.

Jon Skeet
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  • Yes... I do not want to use the Linq2SQL classes as my domain objects because I may want to add behavior to my domain objects and I may also want to restrict the accessability of the members. – Michael Mann Aug 22 '09 at 18:18
  • Adding behaviour is easy: the generated classes are *partial classes* which means you can add your own behaviour in a separate file, but still in the same class. You can specify property accessibility in the dbml file. – Jon Skeet Aug 22 '09 at 19:22
  • That certianly seems like a much simpler approach than transforming expression trees from one type into another. I will go with that approach and restrict visibility and access through interfaces. – Michael Mann Aug 23 '09 at 14:10