FromCache now supports local collections

Posted by: Pete Montgomery on: January 3, 2011

• In: Uncategorized
• 12 Comments

A commenter recently discovered that the technique I use to generate a cache key for any IQueryable was not working properly for one of his queries. The query contained a call to List.Contains, and the contents of the list was not getting put into the cache key.

The problem was that the query cache didn’t specifically know about local collection values.

Local collection values can be supplied as parameters to query operators such as Contains and Any, if the query provider supports them.

var ids = new List<int> { 1, 3, 5 };

var q = from c in db.Customers
        where ids.Contains(c.CustomerId)
        select c;

As far as FromCache was concerned, ids is just a constant expression, and treated it no differently to any other constant when evaluating its cache key.

Unfortunately, the ToString implementation of Lists (and of Arrays, and IEnumerables in general) is not suitable for use as a cache key, because it outputs the type name and not (for example) a representation of every element in the collection.

I couldn’t understand how I’d overlooked this. But then I remembered that Entity Framework v1, which was the main provider I was using at the time, didn’t support local collections either!

I’d actually written my own WhereAny and WhereAll methods, which build up expressions from local collections and produce queries that Entity Framework v1 could understand. But that post never made it out of my WordPress drafts.

LINQ to SQL has always supported the Contains method, and Entity Framework now supports both Any and Contains overloads with local collections. So, I’ve updated the original source code and the blog post to support local collections.

This involves a pass over the expression tree to expand appropriate local collection values which might be used in methods like Any or Contains. Each method call in the expression tree is examined, and the argument to any parameters of type IEnumerable<> or List<> is wrapped in an object with an implementation of ToString which prints every element in the collection.

I’ve also extracted the method IQueryable.GetCacheKey and made it public, which could help emphasise that the meat of the idea is to automatically generate a cache key for a query (and that you can do whatever you like with it, such as make a FromCache extension method, or managing the cache manually).

If you’re using the FromCache extension method, please update use the latest version of the source code!

.NET 4 methods are now included in the source for those of you using 3.5.