Predicates and Lists

Question

I have a generic list.

Some elements of this list belong to a parent element. I retrieved all these elements from a database and i want to recursively build a tree with them.

So, here's what i'm thinking:

Here is my predicate:

public static bool FindChildren(Int32 parentId,CategoryMapping catMapping)
{
    if (catMapping.parentId == parentId)
    {
        return true;
    }
    else
    {
        return false;
    }
}

root = list[0];
root.childrenElements = root.FindAll(FindChildren(root.id,???)

I can't figure out how this would work. How can i do this kind of predicate?

PS: I'm using VS2005 :(

@George: In case you didn't know, you can just `return catMapping.parentId == parentId`, you don't need the if/else statement. — Binary Worrier, Mar 16 '10 at 17:08

score 3 · Accepted Answer · edited Mar 16 '10 at 18:35

3

Try

root.childrenElements = 
    root
       .Where( i => i.parentId == yourCatMapping.parentId)
       .ToArray();

EDIT

In .net 2.0 I think it is

root.FindAll(
    delegate(CategoryMapping mapping)
        {
             return mapping.parentId == root.Id;
        });

edited Mar 16 '10 at 18:35

balexandre

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answered Mar 16 '10 at 14:47

Gregoire

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Asker is on .net 2.0, and therefore doesn't have linq – Binary Worrier Mar 16 '10 at 17:09
1

@Binary Worrier: the PS of the question is recent ;) – Gregoire Mar 16 '10 at 18:12

score 1 · Answer 2 · answered Mar 16 '10 at 14:47

1

You need to specify a delegate to pass to FindAll, rather than a direct function call

(assuming root is List<CategoryMapping>)

root.childrenElements = root.FindAll(c => FindChildren(root.id, c));

answered Mar 16 '10 at 14:47

thecoop

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George is using VS 2005, so a lambda expression won't work. He needs to use the `delegate` syntax. – Dan Tao Mar 16 '10 at 18:26

score 1 · Answer 3 · edited May 23 '17 at 11:47

1

You should check out this thread that I started on Forming good predicate delegates to Find() or FindAll() in a List for C# / .NET 2.0

It answers your question pretty clearly.

edited May 23 '17 at 11:47

Community

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answered Mar 16 '10 at 18:31

Pretzel

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score 0 · Answer 4 · answered Mar 16 '10 at 18:25

Gregoire's answer is the best, because it:

Doesn't use LINQ (the asker is using VS 2005)
Doesn't use a lambda expression (again, VS 2005)

That said, why not make things (slightly) easier for yourself by writing a function to generate your Predicate for you:

public static Predicate<CategoryMapping> GetIsChildOf(int parentId) {
    return delegate(CategoryMapping cm) {
        return cm.parentId == parentId;
    };
}

Then if you have a List<CategoryMapping> and you want to find all elements with a certain parentId property, you can just call:

root = list[0];
root.childrenElements = list.FindAll(GetIsChildOf(root.id));

great post dan. it cleared some things for me. Thanks all of you! SO is great. Thanks a lot guys, i was going crazy on this. — George Silva, Mar 16 '10 at 18:48

Predicates and Lists

4 Answers4