Question

I have this generic method in my repository:

public T GetFirstOrDefault(Expression<Func<T, bool>> where, Expression<Func<T, object>> keySelector, bool ascending)
{
    if (ascending)
    {
        return dbset.Where(where).OrderBy(keySelector).FirstOrDefault<T>();
    }
    else
    {
        return dbset.Where(where).OrderByDescending(keySelector).FirstOrDefault<T>();
    }
}

And I would like to be able to use it like this, where DateCreated is of type DateTime:

someRepository.GetFirstOrDefault(m => m.myprop == someValue, m => m.DateCreated, true);

and like this, where Id is of type Int:

someRepository.GetFirstOrDefault(m => m.myprop == someValue, m => m.Id, true);

and like this, where Name is of type string:

someRepository.GetFirstOrDefault(m => m.myprop == someValue, m => m.Name, true)

However, it gives me the following error when I use it:

Unable to cast the type ‘System.DateTime’ to type ‘System.Object’. LINQ to Entities only supports casting Entity Data Model primitive types.

After doing some research, I found out that maybe I have to use a keySelector of type Expression<Func<TSource, TKey>> or something like that, and this is where I am getting confused. If I declare my method like this, I am not sure where TKey should be coming from:

public T GetFirstOrDefault(Expression<Func<T, bool>> where, Expression<Func<TSource, TKey>> keySelector, bool ascending) { }

Am I on the right path? How should my OrderBy clause be declared?

Answer 1

It seems this method is part of a generic class, which is where the type parameter T is coming from. If you need another type parameter just for this method, you have to make the method itself generic:

public T GetFirstOrDefault<TKey>(
    Expression<Func<T, bool>> where,
    Expression<Func<T, TKey>> keySelector,
    bool ascending)

You can then call it and specify the parameter explicitly:

someRepository.GetFirstOrDefault<string>(m => m.myprop == someValue, m => m.Name, true)

Or let the compiler infer it:

someRepository.GetFirstOrDefault(m => m.myprop == someValue, m => m.Name, true)

(Note that you certainly don’t want assignment = in the first lambda, you want ==. Fortunately, your code wouldn’t even compile.)