Question

public class Query<T> : IQueryable<T> ... 
{
              ... 
   public IEnumerator<T> GetEnumerator() 
   {
      return((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
   }
}

Query<string> someQuery = new Query<string>();
var results1 = someQuery.Select(...).Where(...);

string[] initialSet = { };
var results2 = initialSet.Select(...).Where(...);

1. When operating on initialSet, Linq-to-object’s Where<T> returns WhereEnumerableIterator<T> and thus results2 is of type WhereEnumerableIterator<T>. But when operating on someQuery, does Where<T> operator assign to results1 an instance retrieved by calling someQuery.GetEnumerator or does it also return some custom class?

2. If the latter, when exactly is someQuery.GetEnumerator called by Where and Select operators?

Answer 1

The type of results2 is just Enumerable<T> – the type of the implementation that the value of results2 actually refers to at execution time happens to be WhereEnumerableIterator<T>, that’s all.

When operating on someQuery, it depends what you do with it – the type of the results1 variable is IQueryable<T>, so you can use more Queryable calls on it.

someQuery.GetEnumerator() may never be called – it’s up to the query provider implementation to work out exactly how to represent the query; it doesn’t need to call GetEnumerator all the way up the chain like LINQ to Objects typically does.

As for the type of object returned by Queryable.Where – again, that’s up to the query provider implementation – the difference is that whereas the knowledge is baked into Enumerable.Where and can’t be replaced, Queryable.Where will chain the call through to the query provider.