How to compare Strings & ints in Generic Using comparable C#

Question

I have tried to use IComparable and researched but I cannot figure out why this will not work. I want to compare strings and ints but I don't understand how to use the ICompareable interface to do compareTo on a type of Generic.

How do I implement the CompareTo to handle Generics?

public class QuickSort : IComparable
{
    public static int Partition<T>(ref T[] arr, int lo, int hi)
    {
        int i = lo + 1;
        int j = hi;
        while (j > i)
        {
            while (arr[i].) // <- Cannot apply CompareTo
                i++;
            while (arr[j] > arr[lo])
                j--;

            Swap(ref arr[i], ref arr[j]);
        }

        Swap(ref arr[lo], ref arr[j]);

        return j;

        }
}

Answer 1

You have to specify that T can be compared, the compiler can't know otherwise that type to expect. This way you can use CompareTo for both your conditions:

public class QuickSort
{
    public static int Partition<T>(T[] arr, int lo, int hi) where T : IComparable
    {
        int i = lo + 1;
        int j = hi;
        while (j > i)
        {
            while (arr[i].CompareTo(arr[lo]) == 0)
                i++;
            while (arr[j].CompareTo(arr[lo]) > 0) // means arr[j] > arr[lo]
                j--;

            Swap(ref arr[i], ref arr[j]);
        }

        Swap(ref arr[lo], ref arr[j]);

        return j;
    }
}

Also, I don't think you want to compare different QuickSort instances, so I removed its interface.

Update: Based on Hawkmooon's comment, I took a second look at your method and thought its signature can be even simpler:

    public static int Partition(IComparable[] arr, int lo, int hi)
    {
        int i = lo + 1;
        int j = hi;
        while (j > i)
        {
            while (arr[i].CompareTo(arr[lo]) == 0)
                i++;
            while (arr[j].CompareTo(arr[lo]) > 0)
                j--;

            Swap(ref arr[i], ref arr[j]);
        }

        Swap(ref arr[lo], ref arr[j]);

        return j;
    }

I think your code may be missing something. Just in case, here you have a full C# example of a working quicksort method.