How to compare two generic types?

Question

I make my class generic.The T - can be string or int.

I have this feature class:

public class Feature<T>// : IComparable
{
    public T CurrentFeatureCode { get; set; }

    public T StreetCode1 { get; set; }
    public T BuildingNumber1 { get; set; }
    public string BuildingLetter1 { get; set; }

    public T StreetCode2 { get; set; }
    public T BuildingNumber2 { get; set; }
    public string BuildingLetter2 { get; set; }

    public double CoordinateX { get; set; }
    public double CoordinateY { get; set; }

    public string Filter { get; set; }

    public string ToString(T streetCode)
    {
        return StreetCode2 == streetCode ? String.Format("{0}{1}", BuildingNumber2, BuildingLetter2) : String.Format("{0}{1}", BuildingNumber1, BuildingLetter1);
    }
}

As you can see I have ToString method inside Feature class that compares two values:

StreetCode2 == streetCode ? String.Format("{0}{1}", BuildingNumber2, BuildingLetter2) : String.Format("{0}{1}", BuildingNumber1, BuildingLetter1);

I get error on this row:

Error   11  Operator '==' cannot be applied to operands of type 'T' and 'T' .

My question is how can I compare two values of T type?

You can't compare them because T might be of different types. Naturally, `==` can't be used if every type might be on any side of it. — Zohar Peled, Nov 05 '17 at 08:21
Where is your code for how you use this class? How are you closing the generic? That is the important part and you have not shown it. What is `T`? — CodingYoshi, Nov 05 '17 at 08:22
@ZoharPeled: Well, there can be a subclass relationship, but both `Streetcode2` and `streetCode` are compatible with type `T`. There's only one `T` type here - it's not like `ToString` is generic introducing a new type parameter. — Jon Skeet, Nov 05 '17 at 08:23
@JonSkeet well it took a few reads but I get what you mean and what is my mistake. — Zohar Peled, Nov 05 '17 at 08:30

Dido · Answer 1 · 2017-11-05T08:53:52.577

3

For your type T, implement the IEquatable<T> interface. Override the Equals() method with logic that suits your case. Note that you must override GetHashCode() as well.

edited Nov 05 '17 at 08:53

answered Nov 05 '17 at 08:21

Dido

520
2
7
21

score 3 · Answer 2 · answered Nov 05 '17 at 08:36

If you are happy with the default comparer, then you can do this:

public class Feature<T>// : IComparable
{
    public T StreetCode2 { get; set; }
    public string ToString(T streetCode)
    {
        if (EqualityComparer<T>.Default.Equals(StreetCode2, streetCode))
        {
            return "Equal";
        }

        return "Not Equal";
    }
}

And here is a test:

var feature1 = new Feature<string>();
feature1.StreetCode2 = "two";
string equals = feature1.ToString("two");

score 1 · Answer 3 · answered Nov 05 '17 at 08:20

1

I believe the recommended approach would be to use the IEquatable interface with generics and the Equals() method, but I am not sure if that suits your tastes.

answered Nov 05 '17 at 08:20

Florin Toader

337
1
9

How to compare two generic types?

3 Answers3