Passing method as parameter to function

Question

Is there a way of passing in a method to a function as a parameter and then calling it via list.Sort()? I've tried this:

public static string BuildHumanSitemap(Func<TreeNode, TreeNode, int> sortMethod, params string[] classNames)
{
    //calling list sort on method passed as parameter
    nodes.sort(sortMethod);
 }

Where the functions i want to pass in all take the same params e.g.

private static int SortByDateCreated(TreeNode x, TreeNode y)
{
    DateTime xT = (DateTime)x["DocumentCreatedWhen"];
    DateTime yT = (DateTime)y["DocumentCreatedWhen"];
    return xT.CompareTo(yT);
}

I've also tried using an Action delegate type but the sort method complains when i pass it as a parameter. Can anyone offer a suggestion on how to do this?

Thankyou

Can you show us what you tried? Also, you said the sort method complains when you use Action, can you please post what the error message is? — Jetti, Dec 21 '12 at 16:29
What's the type of `nodes`? And more specifically what's the signature for the `sort` method? — Brian Rasmussen, Dec 21 '12 at 16:30
If you're talking about `List.Sort`, that method does not provide an overload that takes a delegate. — Andrew Whitaker, Dec 21 '12 at 16:30
@BrianRasmussen `nodes` is a `List`. `Sort` is just a `List.Sort()`. @Andrew so i guess what i want to achieve is not possible? — DGibbs, Dec 21 '12 at 16:33
@DGibbs: Not via any built-in library functions that I know of, you could write an extension method to do it though. — Andrew Whitaker, Dec 21 '12 at 16:33
@AndrewWhitaker - thanks. It works when i call `nodes.Sort(SortByDateCreated);` rather than passing in a parameter which is why i thought it might be possible out of the box somehow — DGibbs, Dec 21 '12 at 16:35
@DGibbs: Looks like I could be wrong--check out lazyberezovsky's answer below... — Andrew Whitaker, Dec 21 '12 at 16:35
@AndrewWhitaker Just for completeness, here's [the documentation for the `List.Sort` overload which takes in a delegate](http://msdn.microsoft.com/en-us/library/w56d4y5z.aspx). — Jeppe Stig Nielsen, Dec 21 '12 at 18:49

score 3 · Accepted Answer · answered Dec 21 '12 at 16:33

3

Create new Comparison delegate and pass it to Sort method:

nodes.Sort(new Comparison<TreeNode>(sortMethod));

answered Dec 21 '12 at 16:33

Sergey Berezovskiy

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score 2 · Answer 2 · answered Dec 21 '12 at 18:44

Maybe instead of taking in a Func<,,> delegate, you should consume a Comparison<> delegate. Because that's what List<> wants (for historical reasons; the List<>.Sort method was written for .NET 2.0, before the Func delegates were introduced).

Therefore:

public static string BuildHumanSitemap(Comparison<TreeNode> sortMethod, params string[] classNames)
{
  //calling list sort on method passed as parameter
  nodes.Sort(sortMethod);
}

Then call your method very simply like this:

BuildHumanSitemap(SortByDateCreated);

where SortByDateCreated is the "method group" from your question.

There's no need for first creating a delegate instance of type Func<TreeNode, TreeNode, int> and then create another delegate instance (of type Comparison<TreeNode>) which references the first one.

Of course you can also call your BuildHumanSitemap method with a lambda arrow as the first argument.

@DGibbs Thanks. You can even change your "accept" mark `✔` to honor this answer (but only if you feel it's better than the answer you previously accepted). — Jeppe Stig Nielsen, Jan 02 '13 at 15:40

score 0 · Answer 3 · answered Dec 21 '12 at 16:31

It works this way:

  TreeView.TreeViewNodeSorter = new CustomNodeSorter();

  private class CustomNodeSorter : IComparer
  {
     public int Compare(object x, object y)
     {
         DateTime xT = (DateTime)x["DocumentCreatedWhen"];
         DateTime yT = (DateTime)y["DocumentCreatedWhen"];
         return xT.CompareTo(yT);
     }
  }

mipe34 · Answer 4 · 2012-12-21T16:43:34.490

0

Solution with IComparer<T>.

Comparer

public class MyTreeNodeComparer : IComparer<TreeNode>
{
     public int Compare(TreeNode x, TreeNode y)
     {
         DateTime xT = (DateTime)x["DocumentCreatedWhen"];
         DateTime yT = (DateTime)y["DocumentCreatedWhen"];
         return xT.CompareTo(yT);
     }
}

Usage

list.Sort(new MyTreeNodeComparer());

edited Dec 21 '12 at 16:43

answered Dec 21 '12 at 16:32

mipe34

5,596
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Passing method as parameter to function

4 Answers4