使用VB.net 2.0进行多重分类

Question

I'm stuck with VB.net 2.0 and I want to sort a List(Of MyObject) . I can easly do it using LINQ but as I'm back in the past with only using Framwork 2.0, I must say I don't realy know how to do this. What is the best approach to do this?

I can have many SortFields as well as none... All this because user can choose fields he wants to sort by.

I search a lot about IComparer but it only do it for one field, not many... If I understand well how it works.

What can I do?

Answer 1

Look into creating a Comparer method that does the comparisons.

For example, to create a comparer that uses two fields: (this is in C#, since my VB is somewhat rusty):

class MyCustomComparer: Comparer<MyObject>
{
    public override int Compare(MyObject obj1, MyObject obj2)
    {
        int rslt = obj1.field1.CompareTo(obj2.field1);
        if (rslt != 0)
        {
            rslt = obj1.field2.CompareTo(obj2.field2);
        }
        return rslt;
    }
}

You can then create one of those objects and pass it to the List.Sort overload that takes a comparison function.

Things get a little more complicated when the user can specify the fields and the order to sort. What I've done in the pase is to create a method for each field that can be compared, for example:

private int CompareField1(MyObject obj1, MyObject obj2)
{
    return obj1.field1.CompareTo(obj2.field1);
}

private int CompareField2(MyObject obj1, MyObject obj2)
{
    return obj1.field2.CompareTo(obj2.field2);
}

And I create a list of function references in the constructor, one for each field that the user specifies. It looks something like this:

private List<Func<MyObject, MyObject, int>> compares;

public MyCustomComparer(List<int> fieldsToCompare)
{
    compares = new List<Func<MyObject, MyObject, int>>();
    for (int i = 0; i < fieldsToCompare.Count; ++i)
    {
        switch (fieldsToCompare[i])
        {
            case 1: compares.Add(CompareField1); break;
            case 2: compares.Add(CompareField2); break;
            // other fields here
        }
    }
}

Your CompareTo method, then, loops through the compares list:

    public override int Compare(MyObject obj1, MyObject obj2)
    {
        for (int i = 0; i < compares.Count; ++i)
        {
            int rslt = compares[i](obj1, obj2);
            if (rslt != 0) return rslt;
        }
        return 0;
    }

It's not especially pretty, but it's quite effective.

Answer 2

Even .NET 2.0 had List.Sort .

In order to sort by more than one field of your data, adapt the comparer to sort by the fields consecutively: if the first comparison is equal, go by the second; if that is equal, go by the third; etc.