Question

Trying to build up a query to filter on data in the following manner works OK, returning users filtered by whatever filters are in the FilterNamesAndValues parameter.

GetAllUsersFiltered(..., Dictionary<string,string> FilterNamesAndValues)
{
    ....

    List<DataContracts.IUser> lstUsers = new List<DataContracts.IUser>();
    ....

    var query = from u in lstUsers select u;

    string firstName = string.Empty;
    FilterNamesAndValues.TryGetValue("FirstName", out firstName);
    query = query.Where(u => u.FirstName == firstName);

    string company = string.Empty;
    FilterNamesAndValues.TryGetValue("Company", out company);
    query = query.Where(u => u.CompanyName == company);

    ....
    return query.ToList();
}

The example below however doesn’t work and I can’t see why:

GetAllUsersFiltered(..., Dictionary<string,string> FilterNamesAndValues)
{

    ....

    List<DataContracts.IUser> lstUsers = new List<DataContracts.IUser>();

    ....

    var query = from u in lstUsers select u;

    foreach (KeyValuePair<string, string> kv in FilterNamesAndValues)
        {
            if (kv.Value != null)
            {
                switch (kv.Key)
                {
                    case "FirstName":
                        query = query.Where(u => u.FirstName == kv.Value);
                        break;

                    case "Company":
                        query = query.Where(u => u.CompanyName == kv.Value);
                        break;

                }
            }
        }

        return query.ToList();
}

After the application has hit the first switch case, I can do a query.ToList() and see a row in there. But by the time the execution has gone around the loop to hit the second filter, query.ToList() returns nothing. The query is not filtered successively the way it was in the first example and worse than that, the filter conditions have effectively been lost. There’s probably an obvious explanation for this, but right now I can’t see it.

Answer 1

The problem is that you’re closing over kv in the foreach, but the query is executed using deferred execution. This causes it to close over the wrong value. For details on what’s happening, I’d recommend Eric Lippert’s post titled “Closing over the loop variable considered harmful“.

You can solve this via a temporary:

foreach (KeyValuePair<string, string> kvOriginal in FilterNamesAndValues)
    {
        // Make a temporary in the correct scope!
        KeyValuePair<string, string> kv = kvOriginal;
        if (kv.Value != null)
        {
            switch (kv.Key)
            {
                case "FirstName":
                    query = query.Where(u => u.FirstName == kv.Value);
                    break;

                case "Company":
                    query = query.Where(u => u.CompanyName == kv.Value);
                    break;

            }
        }
    }