Question

I have this simple class with those 2 enum fields, I’m trying to find one item of this object in a collection (List<T>) but the Contains methods doesn’t works correctly

public class Calculator : IEqualityComparer<Calculator>
{
    public DashboardsComputationMode ComputationMode { get; set; }
    public Modes Mode { get; set; }

    public Calculator(DashboardsComputationMode dashboardsComputationMode, Modes mode)
    {
        ComputationMode = dashboardsComputationMode;
        Mode = mode;
    }

    public bool Equals(Calculator x, Calculator y)
    {
        return (x.ComputationMode.Equals(y.ComputationMode) && x.Mode.Equals(y.Mode));
    }

    public int GetHashCode(Calculator obj)
    {
        return obj.ComputationMode.GetHashCode() ^ obj.Mode.GetHashCode();
    }
}

public enum DashboardsComputationMode
{
    Weighted = 0,
    Aggregated = 1,
    PR = 2,
    CurrentValue = 3,
    EquivalentHours = 4,
    AggregatedCorrected = 5,
    PRCorrected = 6
}

public enum Modes
{
    InstantaneousMode = 0,
    DailyMode = 1,
    MonthlyMode = 2,
    YearlyMode = 5,
    Undefined = 4,
}

Why could be that this test doesn’t works

[TestMethod]
public void TestMethod1()
{
  var list = new List<Calculator>()
  {
    new Calculator(DashboardsComputationMode.PR, Modes.DailyMode),
    new Calculator(DashboardsComputationMode.CurrentValue, Modes.YearlyMode),
    new Calculator(DashboardsComputationMode.PRCorrected, Modes.MonthlyMode)
  };

  var item = new Calculator(DashboardsComputationMode.CurrentValue, Modes.YearlyMode);
  Assert.IsTrue(list[1].Equals(item));
  Assert.IsTrue(list.Contains(item));
}

The first assert works fine

Assert.IsTrue(list[1].Equals(item)) 

but the second doesn’t

Assert.IsTrue(list.Contains(item));

Answer 1

List<T>.Contains determines equality by using the default equality comparer (the one returned by the EqualityComparer<T>.Default).

Here’s the MSDN explanation on how EqualityComparer<T>.Default works:

The Default property checks whether type T implements the
System.IEquatable interface and, if so, returns an
EqualityComparer that uses that implementation. Otherwise, it
returns an EqualityComparer that uses the overrides of
Object.Equals and Object.GetHashCode provided by T.

In other words, your Calculator class should either implement the System.IEquatable (not the System.IEqualityComparer!) interface or override the Object.Equals and Object.GetHashCode methods.