Question

I’m trying to optimise the performance of a string comparison operation on each string key of a dictionary used as a database query cache. The current code looks like:

public void Clear(string tableName)
{
    foreach (string key in cache.Keys.Where(key => key.IndexOf(tableName, StringComparison.Ordinal) >= 0).ToList())
    {
        cache.Remove(key);
    }
}

I’m new to using C# parallel features and am wondering what the best way would be to convert this into a parallel operation so that multiple string comparisons can happen ‘simultaneously’. The cache can often get quite large so maintenance on it with Clear() can get quite costly.

Answer 1

Make your cache object a ConcurrentDictionary and use TryRemove instead of Remove.

This will make your cache thread-safe; then, can invoke your current foreach loop like this:

Parallel.ForEach(cache.Keys, key =>
{
    if(key.IndexOf(tableName, StringComparison.Ordinal) >= 0)
    {
        dynamic value; // just because I don't know your dictionary.
        cache.TryRemove(key, out value);
    }
});

Hope that gives you an starting point.