Confusion over how IComparer works

[xtech1005]

This is probably a newbie question but I'm really confused on how the IComparer works and I've looked for information online and offline. The code works, I just don't understand how.

I'm sorting an array that has only one element but the comparer has two arguments:

public int Compare(object a, object b)
{
yada yada
}

I've stepped through the code and I can see that both object a and object b have values from the single-element array but I don't understand how that happens. Can anyone enlighten me?

Here is the associated code:

// files is simply a list of files returned by DirectoryInfo

sortByDateTime fCompare = new sortByDateTime();
Array.Sort(files, fCompare)
 
private class sortByDateTime : IComparer<object>
        {
            public int Compare(object a, object b)
            {
                FileInfo f1;
                FileInfo f2;
                f1 = (FileInfo)a;
                f2 = (FileInfo)b;
                return f2.LastWriteTime.CompareTo(f1.LastWriteTime);
            }
        }

thanks!

[tlhintoq]

Realize you aren't sending to the Compare you wrote. You are calling the .Sort method of the Array class using the Compare as an argument you are sending.

The sort method in the Array class is then doing things you don't see. It is receiving two things: Your array and the compare method you wrote. It then loops through the array elements taking element 0 & 1 and sending them to the compareing, then 1&2, then 2&3 and so on... until it is done sending everything and every comparrison returns that it is in correct order.

Lines 10 and 11 presume that objects a and b are FileInfo objects and typecasts them in FileInfo objects F1 and F2.

Line 6 shows this method with return an int.
Line 12 compares the two LastWriteTime properties and returns the result of the comparison as an int. If I remember right that compare is something like
-1 means first item is greater than second item
0 means the two items are equal
+1 means the second item is greater than the first time
but look it up or confirm it through your own testing to be sure I don't have that backwards.

[GaryTexmo]

I'm not 100% sure since I've never implemented IComparer before, but if it's anything like operator overloading (and I hear it is), then the other value would be what it's compared too...

if (objA == objB) { ... }

Then right there are your a and b objects in your Compare method. A comparison (equality) always has two objects. The Array.Sort method takes a IComprarer implementation so it knows how to sort the list... that is, so it knows if something is equal (I would imagine you'd need greater and less than as well), and can thus sort your list.

It's the Array.Sort method that goes through the elements in the array and makes the comparisons, so that's what ends up as a and b in your method.

[xtech1005]

Thanks to both of you for your replies.

tlhintoq, you really explained things well and you're right, I didn't know what was going on behind the scenes. Makes a lot more sense now. How did you know that it takes element 0 & 1 then 1 & 2, etc. ? I can't seem to find information on it....

[cloud255]

How did you know that it takes element 0 & 1 then 1 & 2, etc. ?

The Array.Sort method uses the Quicksort Algorithm.

But one handy thing to remember when using IComparer is that you can customize exactly how the Compare method works so that you can compare any property (or several properties) of an object:

int IComparer.Compare(Object x, Object y)
        {
            FileInfo FIx = (FileInfo) x;
            FileInfo FIy = (FileInfo) y;
 
            if ((FIx.CreationTime > (FIy.CreationTime)) && (FIx.LastAccessTime > FIy.LastAccessTime))
                return -1;
            else if ((FIx.CreationTime < (FIy.CreationTime)) && (FIx.LastAccessTime < FIy.LastAccessTime))
                return 1;
            else return 0;
        }
 

The example above is not really one that you would use, but is rather just to illustrate the extensibility of the Compare method.

[xtech1005]

cloud255....thanks!

After all the replies I am much more familiar with using IComparer. Thanks to everyone for your input.