LINQ Confusion

Posted by Tyler | Filed under Projects

I find myself using LINQ a lot in my C# code these days. I use collections all over the place, and there’s no doubt that LINQ makes sorting and slicing collections a lot simpler code-wise.

In my most recent weekend project, I need to randomly sort a list of cards, which are represented by an Action class. After some quick searching, I found some articles that indicated the best way to do this would be to sort the list by random GUID. This makes sense, though I certainly wouldn’t have thought of it on my own.

The examples given all worked, but not with my lists… With the following code, the compiler spits out several errors:

List<Action> cards = new List<Action>();
cards.Add( new OneCattle() );
cards.Sort( a => Guid.NewGuid() ).ToList<Action>();

Error    1    Delegate 'System.Comparison<agricola.Action>' does not take '1' arguments
Error    2    Cannot convert lambda expression to type 'System.Collections.Generic.IComparer<agricola.Action>' because it is not a delegate type
Error    3    Cannot implicitly convert type 'System.Guid' to 'int'
Error    4    Cannot convert lambda expression to delegate type 'System.Comparison<agricola.Action>' because some of the return types in the block are not implicitly convertible to the delegate return type

However, using a more explicit LINQ query without a lambda expression seems to work fine:

var q = from a in cards
        orderby Guid.NewGuid()
        select a;
List<Action> r = q.ToList<Action>();

Anybody know why this is? I haven’t wrapped my head around lambda expressions and the theory behind LINQ to understand what the root cause is…

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Permalink | March 14, 2009, 11:47 pm

2 Responses to “LINQ Confusion”

  1. Stitch Says:
    April 30th, 2009 at 1:50 pm

    First of all, I believe the second code snippet is not “a more explicit LINQ query”. It is actually the more syntactic-sugar version.

    Second of all, you calling .Sort and doing a linq orderby are completely different operators. If we you were to call cards.OrderBy(a => Guid.NewGuid()) then the two would be effectively equivelant.

    Thirdly, Sort() does not take a lambda expression. It takes an IComparer amongst other things.

    And finally, to go back to the “more explicit linq query” part: here is the “explicit” version of that linq query in the second code snippet:

    var q = cards.OrderBy(a => Guid.NewGuid()).Select(a => a);

    Cheers

  2. Tyler Says:
    May 7th, 2009 at 11:32 am

    Thanks! I realize now that I made a stupid mistake… I’m working on a follow-up post to clarify.

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