如何结合两种通用方法

Question

I have to two extensions:

public static IQueryable<T> Existing<T>(this IQueryable<T> qry) 
    where T : class, IDeletable
{
    return qry.Where(...);
}

public static IEnumerable<T> Existing<T>(this IEnumerable<T> qry) 
    where T : class, IDeletable
{
    return qry.Where(...);
}

How do I combine those two methods into one method? I tried the following:

public static TU Existing<TU, T>(this TU qry)
    where TU : class, IQueryable<T>, IEnumerable<T>
    where T : class, IDeletable
{
    return (TU)qry.Where(...);
}

But this doesn't work because the type can't be inferred correctly (because IQueryable inherits from IEnumerable, I guess). Moreover, T isn't always IQueryable , which also seems to be a problem here (question: if I specify multiple interfaces in the where clause, aren't those matched in a disjunctive manner?). In the previous case IQuerable was used if it suited or else IEnumerable . How can I circumvent this?

Answer 1

Your attempt will always limit it to IQueryable<T> implementations. I assume you really want it to work for both in-process queries and out-of-process queries. Assuming that the "..." in the Where clause is a lambda expression, you need to be aware that even if you have the same code in both methods, the compiler will be handling them very differently - because it will already have applied overload resolution to determine the Where implementation to use.

I would strongly advise you to leave the two separate overloads as they are.

Answer 2

public static IQueryable<T> Existing<T>(this IEnumerable<T> qry) where T : class, IDeletable
{
    return qry.Where(/*...*/).AsQueryable();
}

Calling AsQueryable on something that is already queryable will do nothing. Also, IQueryable inherits from IEnumerable, so first: you can pass an IQueryable to this method, and second: the result is actually an IEnumerable as well as an IQueryable.