C＃list.Sort和CompareTo抛出IndexOutOfRangeException

Question

I use c#, .net4.0 and Winform

I have implemented a sort method for a list of custom object. The main class has a list of objects (lpHTC), that will be sorted

public class PeriodeHTList
{
    private List<PeriodeHTConflit> _lpHTC;

    public PeriodeHTList()
    {
        _lpHTC = new List<PeriodeHTConflit>();
    }

    public void AddPeriodeHTConflit(PeriodeHT pHT)
    {
        PeriodeHTConflit pHTC = new PeriodeHTConflit();
        pHTC.pHT = pHT;
        _lpHTC.Add(pHTC);
    }
    public void Sort()
    {
        _lpHTC.Sort();
    }
}

The second class is more complex in reality. The pHT variable will be use in the sort

public class PeriodeHTConflit : IComparable<PeriodeHTConflit>
{
    public PeriodeHT pHT;

    public PeriodeHTConflit()
    {
}

    public int CompareTo(PeriodeHTConflit pHTC)
    {
        if(pHTC==null)
            return 1;

        if(pHT.date<=pHTC.pHT.date)
        {
            return -1;
        }
        else
        {
            return 1;
        }
    }
}

The last class

public class PeriodeHT
{
    public DateTime date;

    public PeriodeHT()
    {
    }
}

I create a PeriodeHTList, then populate it with AddPeriodeHTConflit(). Then I call periodeHTListVar.Sort().

try
{
   periodeHTListVar.Sort();
}
catch (Exception ex)
{
   //some error code
}

And I get a First Chance IndexOutOfRangeException. The catch is not executed, and the program continues. But the sort is not completed.

I put a debug point in the CompareTo() method, and I get a strange behavior : After a few correct call to CompareTo, all the other calls use two same object (the PeriodeHTConflit object and the pHTC parameter), and always the last of the unsorted list.

I use Icomparable and CompareTo() for a lot of things without any problem, except here (and another similar objet).

Can someone help me ?

Answer 1

If you wish to compare objects by comparing their parts, then delegate comparison to parts:

public int CompareTo(PeriodeHTConflit pHTC)
{
    if(pHTC==null)
        return 1;

    if(pHT.date<=pHTC.pHT.date)
    {
        return -1;
    }
    else
    {
        return 1;
    }
}

This piece of code is only partially correct - it will not return 0 if two dates are equal, and then that may confuse the sorting algorithm by giving it false information.

Better solution is to rely on comparison of dates:

public int CompareTo(PeriodeHTConflit pHTC)
{
    if(pHTC==null)
        return 1;
    return pHT.date.CompareTo(pHTC.pHT.date);
}

Answer 2

I think in you function

 public int CompareTo(PeriodeHTConflit pHTC)

When two dates are equal, you should return 0.

See CompareTo document :

Notes to Implementers: For objects A, B and C, the following must be true: A.CompareTo(A) must return zero. If A.CompareTo(B) returns zero, then B.CompareTo(A) must return zero. If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) must return zero. If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) must return a value of the opposite sign. If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) must return a value of the same sign as x and y.