Why have a non-generic IQueryable?

Question

Why is there a non-generic IQueryable when there's a generic one? Could you please provide a use for it?
Isn't the ElementType member of the non-generic IQueryable interface redundant for the same reason?

The two IQueryable interfaces were introduced well after generics had been introduced. So, having both the generic and non-generic version does not look like an evolutionary progression as is the case with IEnumerable and its generic version.

Surely, Microsoft must have had a strong reason to keep both of the interfaces?

Related: https://stackoverflow.com/questions/56375/are-non-generic-collections-in-net-obsolete — Chris, Jan 24 '15 at 15:40
http://stackoverflow.com/questions/18535017/iqueryable-vs-iqueryablet — The One, Jan 24 '15 at 15:41
I read Matt Warren's series on building a LINQ provider and I am not sure what he means by the expression, "*dynamic query building scenarios*" when he says, "The generic IQueryable is the one you use most often in method signatures and the like. The non-generic IQueryable exist primarily to give you a weakly typed entry point primarily for dynamic query building scenarios." — Prakash, Jan 24 '15 at 15:52
I strongly disagree with @YoryeNathan comment. See my answer. — Chad Carisch, Feb 09 '15 at 13:43

Chad Carisch · Answer 1 · 2015-02-09T13:41:34.953

The main purpose of this interface is to allow access to the ElementType, Expression, and Provider without needing to know the Generic Type.

This is a fairly common practice for Microsoft when writing Generic Types. List<T> inherits from IList<T> which inherits from IList.

In other words you want to expose any property that does not require the generic type in a non generic way. In the case of IQueryable<T> there is not properties exposed. However, the generic type T allows for strongly typed extension methods, that are found in System.Linq.Queryable.

Observe the following:

void Main()
{
    var lst = new List<string>();
    lst.Add("test1");
    lst.Add("test2");
    lst.Add("test3");

    IQueryable<string> q = lst.AsQueryable();

    PrintQueryInfo(  q.Where(x=>x.Contains('1')));

}

public void PrintQueryInfo(IQueryable q){
    Console.WriteLine(q.Expression.ToString());
}

Output:

System.Collections.Generic.List`1[System.String].Where(x => x.Contains(1))

Its also worth noting that the above example could also be done with a generic method. But this is not always possible or practical.

public void PrintQueryInfo<T>(IQueryable<T> q){
    Console.WriteLine(q.Expression.ToString());
}

Why have a non-generic IQueryable?

1 Answers1