[RESOLVED] [vs 2003] help with ArrayList.sort and IComparer

[benmartin101]

I have an arraylist and i'd like to sort them in 3 different ways. I know how to make it sort just ONE way by putting these codes in my class:

int IComparable.CompareTo(object obj)
		{
			
			FoneStruct objFone = (FoneStruct)obj;
			string str1 = this.name + this.number;
			string str2 = objFone.name + objFone.number;
			if(str1 == str2)
				return 0;
			else
				return str1.CompareTo(str2);
			
			
		}

and I just call ArrayList.sort() and it does it fine. But I would like to sort my arraylist based on different properties in my class. I noticed there is a ListArray.sort(Icomparer) function call. I'm guessing that is the key to the solution to my problem. But how do I use it?

[jmcilhinney]

The IComparer interface has a Compare method. The idea is that you declare your own class that implements IComparer and thus its Compare method. When you pass an instance of your IComparer to the ArrayList.Sort method its Compare method is called repeatedly on each pair of items to place them in the order defined by your method of comparison. Here's an example of an IComparer that sorts DateTime objects by time first, then date:

Public Class TimeAndDateComparer
    Implements IComparer
 
    Public Function Compare(ByVal x As Object, ByVal y As Object) As Integer Implements System.Collections.IComparer.Compare
        Dim dt1 As Date = CDate(x)
        Dim dt2 As Date = CDate(y)
 
        'Compare the time components.
        Dim result As Integer = dt1.TimeOfDay.CompareTo(dt2.TimeOfDay)
 
        If result = 0 Then
            'The times are the same so compare the date components.
            result = dt1.Date.CompareTo(dt2.Date)
        End If
 
        Return result
    End Function
 
End Class

I could pass an instance of that class to the Sort method of any ArrayList that contained only Date objects and it would sort them by time then date.

You can implement your Compare method in as simple or as complex a way as you like. It can compare objects of different types if you like, but it's up to you to specify what types are valid and how they are compared. You can also implement it such that it compares only one type but in different ways, depending on the properties of the objects themselves, which sounds like what you want.

If the type of the items in the ArrayList implements the IComparable interface then you can call the CompareTo method of the first object being compared as I have above.

[jmcilhinney]

B*gger! Forget I was in the C# forum. Here's a C# transaltion of the code above:

public class TimeAndDateComparer : IComparer
{
    public int Compare(object x, object y)
    {
        DateTime dt1 = (DateTime)x;
        DateTime dt2 = (DateTime)y;

        // Compare the time components.
        int result = dt1.TimeOfDay.CompareTo(dt2.TimeOfDay);

        if (result == 0)
        {
            // The times are the same so compare the date components.
            result = dt1.Date.CompareTo(dt2.Date);
        }

        return result;
    }
}

[benmartin101]

Thanks, it works.