Question

I have written the following C# code:

_locationsByRegion = new Dictionary<string, IEnumerable<string>>();
foreach (string regionId in regionIds)
{
    IEnumerable<string> locationIds = Locations
        .Where(location => location.regionId.ToUpper() == regionId.ToUpper())
        .Select(location => location.LocationId); //If I cast to an array here, it works.
    _locationsByRegion.Add(regionId, LocationIdsIds);
}

This code is meant to create a a dictionary with my “region ids” as keys and lists of “location ids” as values.

However, what actually happens is that I get a dictionary with the “region ids” as keys, but the value for each key is identical: it is the list of locations for the last region id in regionIds!

It looks like this is a product of how lambda expressions are evaluated. I can get the correct result by casting the list of location ids to an array, but this feels like a kludge.

What is a good practice for handling this situation?

Answer 1

You’re using LINQ. You need to perform an eager operation to make it perform the .Select. ToList() is a good operator to do that. List is generic it can be assigned to IEnumberable directly.

In the case where you’re using LINQ it does lazy evaluation by default. ToList/eager operations force the select to occur. Before you use one of these operators the action is not performed. It is like executing SQL in ADO.NET kind of. If you have the statement “Select * from users” that doesn’t actually perform the query until you do extra stuff. The ToList makes the select execute.