How to get the IQueryable<T> extension methods instead of the IEnumerable<T> extensions?

Question

Probably really simple so please excuse my ignorance...

To my knowledge, there are a couple flavours of the Where() extension method:

Queryable.Where<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

Enumerable.Where<TSource> Method (IEnumerable<TSource>, Func<TSource, Boolean>)

Both of the above extensions are housed in the System.Linq namespace, so that I can do Where() at all is hopefully proof enough that I've imported the correct namespace - or is there another namespace I need for IQueryable extensions?

I understand that IQueryable<T> inherits from IEnumerable<T> but why can't I get the IQueryable<T> extensions?

class Test
{
    IQueryable<Test> SomeMethod(Func<T, bool> criteria)
    {
        return new List<Test> { new Test() }.AsQueryable().Where(criteria); // compiler error converting IEnumerable<T> to IQueryable<T>
    }

}

As shown above, there should be an extension method available that returns IQueryable? Why is it resolving to the IEnumerable Extensions?

Answer 1

The Queryable.Where Extension Method expects an Expression<Func<T, bool>> (ie an expression tree representing a lambda), not a Func<T, bool> (a lambda itself).

This works:

IQueryable<Test> ApplyCriteria(IQueryable<Test> queryable,
                               Expression<Func<Test, bool>> criteria)
{   //                              ↑
    return queryable.Where(criteria);
}

Answer 2

IQueryable<T>.Where()仅适用于Expression<Func<T, bool>>作为谓词参数。