LINQ Gotcha

After a long hiatus, I was trying to figure out something cool or at least interesting to blog about.  I was having trouble figuring out what I wanted to write, but then one after another, I got a ton of ideas.

One thing I wanted to do for my current project was finding out if a specific ConnectionString was present in the <ConnectionStrings> setting of a .config file. This is a big oversimplification but it's close enough to explain. So I wrote the following:

 foreach (var Mine in myVar)
 {
       Console.WriteLine(myVar);
 }

I knew the second I tried to write a where clause that there was a problem b/c intellisense wasn't showing anything I expected. When I tried to display some of the properties in the foreach loop, nothing was visible. I knew something was wrong. So I tried to compile and got the following error:

Could not find an implementation of the query pattern for source type 'System.Configuration.ConnectionStringSettingsCollection'.  'Select' not found.  Consider explicitly specifying the type of the range variable 'cs'. 

Well, that was helpful b/c I knew at least the problem was with the  range variable .  I wasn't sure what type of collection the ConfigurationManager.ConnectionStrings  property was so I decided to look it up just to be sure. (I also just wanted to make sure it was in fact some type of collection or another. I couldn't imagine that it wouldn't have been, but just wanted to make sure for purely superstitious reasons) Ok, so it's a ConnectionStringsSettingsCollection , nothing surprising there. Digging deeper I saw that ConnectionStringsSettingsCollection inherits from ConfigurationElementCollection . At that point, it all became crystal clear for I saw the following in the class definition:

public abstract class ConfigurationElementCollection : ConfigurationElement, ICollection, 
    IEnumerable

Do you see the problem? There's an IEnumerable but no IEnumerable<T>   . You see, the collection implements the non-Generic IEnumerable but not the Generic IEnumerable. Hence, a explicit range variable is in order to make this work.  I know I know, that's exactly what the error message recommended, but I didn't understand why at first and wanted to dig deeper into it. So bascially, here's what was needed to make it work.  Simply use the Cast<T>   extension method on the collection and well, that's it.

ConnectionStringSettingsCollection cfg = ConfigurationManager.ConnectionStrings;

var myvar = from cs in cfg.Cast<ConnectionStringSettings>()
                  select cs;
foreach (var mine in myvar)
{
    Console.WriteLine(mine);
}


Of course, you could just cut out some of the bloat and address the collection directly

var myvar = from cs in ConfigurationManager.ConnectionStrings.Cast<ConnectionStringSettings>()
            select cs;
foreach (var mine in myvar)
{
  Console.WriteLine(mine);
}

Whenever Intellisense doesn't do what you're expecting it to, that's the first tipoff something is wrong in your query.  Although my days of 2 day work weeks have come to an end temporarily, I'll try to find some time to blog the rest of what I was working on - there's so much you can do with LINQ that I often overlook.  And this is the first time I've come across the explicit range variable issue but I'm guessing it'll come up again, particularly if you use LINQ regularly.