Question

How can I detect if a IQueryable<T> has a where filter applied?

In this code, I need to know programmatically that queryFiltered has a where applied to it and query doesn’t

IQueryable<Customer> query = Context.Customers;
IQueryable<Customer> queryFiltered = Context.Customers
                                            .Where(c=>c.Name.Contains("ABC"));

Answer 1

You will have to parse the Expression that is returned from the Expression property on the IQueryable<T> implementation.

You’ll have to query for the Queryable.Where method being called as you crawl the Expression tree.

Also note that while Queryable.Where is going to be the most common way to detect a where filter, query syntax allows for other implementations to be used (depending on what namespaces are used in the using directives); if you have something that is not using the Queryable.Where extension method then you’ll have to look for that explicitly (or use a more generic method of filtering for a Where method that takes an IQueryable<T> and returns an IQueryable<T>).

The ExpressionVisitor class (as pointed out by xanatos) provides a very easy way of crawling the Expression tree, I highly recommend using that approach as a base for processing your Expression tree.

Of note is that ExpressionVisitor class implementations are required to store and expose state on the class level. Because of that, it would be best (IMO) to create internal classes that perform the action one-time and then have a public method which creates a new instance of the ExpressionVisitor every time; this will help with dealing with mutating state, and if done properly, will allow the method to be thread-safe as well (if that is a concern of yours).