Generic class where type must implement IEquatable

Question

I'm trying to create a generic "property" class that maintains both it's current value and the last value.

public class GenericTrackingProperty<T> where T : IEquatable<T>
{
    private T _oldValue;
    private T _currentValue;

    public T Value
    {
        get { return _currentValue; }
        set
        {
            if (value != _currentValue)
            {
                _oldValue = _currentValue;
                _currentValue = value;
            }
        }
    }
}

Despite the use of the where in the class definition to ensure the generic type is equatable the compiler complains about the comparison "if (value != _currentValue" giving an error "Operator '!=' cannot be applied to operands of type 'T' and 'T'". What am I doing wrong?

You can use a stack to pop and push the values if you want to maintain a history. — Demodave, Dec 02 '21 at 14:45
The same issue like with `IComparable` [here](https://stackoverflow.com/q/69899514) — Guru Stron, Dec 02 '21 at 15:00
Pro tip: The docs are quite good for .NET: https://learn.microsoft.com/en-us/dotnet/api/system.iequatable-1. If you look, the only member that interface has is the `Equals` method: https://learn.microsoft.com/en-us/dotnet/api/system.iequatable-1.equals — Flydog57, Dec 02 '21 at 15:20

Dmitry · Accepted Answer · 2021-12-02T14:56:03.467

1

IEquatable<T> doesn't contain operators, but it contains the Equals method.
Use it instead of the equality operator:

if (!value.Equals(_currentValue))

Or, null-aware:

if (value == null ? _currentValue == null : !value.Equals(_currentValue))

edited Dec 02 '21 at 14:56

answered Dec 02 '21 at 14:38

Dmitry

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Or `value?.Equals(_currentValue) is false`. – Johnathan Barclay Dec 02 '21 at 14:59

Dmitry Bychenko · Answer 2 · 2021-12-02T15:57:58.250

1

Let .NET compare both values for you (while checking for null etc):

if (!EqualityComparer<T>.Default.Equals(value, _currentValue)) {
  ... 
}

You can use value.Equals but in this case you have to check for null:

if (value == null && _currentValue != null || !value.Equals(_currentValue)) {
  ...
}

edited Dec 02 '21 at 15:57

answered Dec 02 '21 at 14:45

Dmitry Bychenko

180,369
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1

You don't even need the `IEquatable` constraint with this method. – D Stanley Dec 02 '21 at 15:07
1

@DStanley I would say that it is up for debate, this constraint allows to prevent passing types which don't have custom equality defined, which can be a valid use case (for example prevent _default_ by reference equality). – Guru Stron Dec 02 '21 at 15:52
2

should not it be `!EqualityComparer.Default.Equals`? – Guru Stron Dec 02 '21 at 15:53
1

@Guru Stron: thank you! You are quite right, it must have been `Default` – Dmitry Bychenko Dec 02 '21 at 15:58

Generic class where type must implement IEquatable

2 Answers2