IComparer does not implement interface member - Error CS0738

Question

I am trying to sort two objects by one of their properties (.Transaction.topLeftX, an integer) using the following code to create a comparer to use in a Sort method:

public class RespComp : IComparer<Kairos.Net.RecognizeImage>
{     
        public Kairos.Net.RecognizeImage Compare(Kairos.Net.RecognizeImage x, Kairos.Net.RecognizeImage y)
        {
            if (x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX) <= 0) return x;
               else return y;                      
        }
}

However, I get the error message Error CS0738 'RecogniseFacesKairos.RespComp' does not implement interface member 'IComparer.Compare(RecognizeImage, RecognizeImage)'. 'RecogniseFacesKairos.RespComp.Compare(RecognizeImage, RecognizeImage)' cannot implement 'IComparer.Compare(RecognizeImage, RecognizeImage)' because it does not have the matching return type of 'int'.

Does the comparer used in the Sort method need to have return type int?

"because it does not have the matching return type of 'int'". Yes, your method must return "int". — gunr2171, Oct 08 '20 at 02:28
`because it does not have the matching return type of 'int'.` Yes - that is what you need to do. — mjwills, Oct 08 '20 at 05:23

score 1 · Answer 1 · answered Oct 08 '20 at 02:51

The IComparer<T> interface is supposed to implement a method that returns an int comparison. -1 for less than, 0 for equal and 1 for greater than.

Look at your code, if you're just comparing the top left, you can probably just do the following:

public int Compare(FooImage x, FooImage y) {
    return x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX);
}

score 0 · Answer 2 · answered Oct 08 '20 at 03:13

The desired outcome of sorting objects by one of their parameters was achieved by the following code:

...
Kairos.Net.KairosClient client = new Kairos.Net.KairosClient();
client.ApplicationID = appId;
client.ApplicationKey = appKey;
Kairos.Net.RecognizeResponse resp = client.Recognize(...);

RespComp SortImages = new RespComp();
resp.Images.Sort(SortImages);
...

 public class RespComp : IComparer<Kairos.Net.RecognizeImage>
    {
        public int Compare(Kairos.Net.RecognizeImage x, Kairos.Net.RecognizeImage y)
        {
            return x.Transaction.topLeftX.CompareTo(y.Transaction.topLeftX);
        }
    }

IComparer does not implement interface member - Error CS0738

2 Answers2