Can someone tell me why this lambda sort expression doesn't work?

Question

I'm new to lambda expressions, and just ran into something I don't understand.

I have an object like so:

class MyListItem
{
   string date; //date in the format "2010-12-05"
   int Hour;    //hour of day as an int

}

I have a list of these objects, representing some dates and hours.

I want to sort this list by date and hour, so I try this:

List<MyListItem> myList = new List<MyListItem>();

myList = getsomedata(); //populate list

myList.Sort((a, b) => (a.date + a.Hour.ToString()).CompareTo(b.date + b.Hour.ToString()));

and that works, sort of. The issues is that the hour is an int, so it's sometimes not 2 digits, resulting in a sort like so:

2010-12-05 1
2010-12-05 10
2010-12-05 11
2010-12-05 12
2010-12-05 13
2010-12-05 2
2010-12-05 21
2010-12-05 22

I want it to be like:

2010-12-05 1
2010-12-05 2
2010-12-05 10
2010-12-05 11
2010-12-05 12
2010-12-05 13
2010-12-05 21
2010-12-05 22

so I try formatting the string to add a zero before I parse together in the lambda:

ret.Sort((a, b) => (a.date + a.Hour.ToString("00")).CompareTo(b.date + b.Hour.ToString("00")));

But it won't compile. It tells me:

Cannot convert lambda expression to type 'Systems.Collections.Generic.IComparer<MyListItem>' because it is not a delegate type. 

Huh? What is different between the plain .ToString() (with no format string) and .ToString("00") in this situation?

Also, any suggestions as to how to get this working?

Answer 1

I'm not at a PC so I can't explain the first, but I'd sidestep it:

ret.Sort((a,b) => {
    int result = string.Compare(a.date,b.date);
    if(result==0) result = a.hour.CompareTo(b.hour);
    return result;
});

Less string creations, no parsing overheads, etc ;)