Let's say that I have the following class:

public class TsvDataModel : IEquatable<TsvDataModel>
{
    public int ElementId { get; set; }

    public string Location { get; set; }
    public string RoomKey { get; set; 

    public decimal Area { get; set; }

    public bool Equals(TsvDataModel other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return ElementUid == other.ElementUid && Location == other.Location && RoomKey == other.RoomKey &&
               SubType == other.SubType;
    }

    // current equality comparison implementation
    public override bool Equals(object obj) => ReferenceEquals(this, obj) || (obj is TsvDataModel other && Equals(other));

    public override int GetHashCode() => HashCode.Combine(ElementUid, Location, RoomKey, SubType);
}

What I want is to create dynamic equality based on user-provided properties i.e. sometimes I want to compare only by ElementIds and sometimes to include/exclude other properties. I don't want to create all possible combinations. I tried using reflection to get properties via dynamic type and I can get value and type, but comparison is not working.

I would like to use either IEqualityComparer or IEquitable (I'm using this object as HashSet<T>).

CodePudding user response：

You could create a simple factory class for producing IEqualityComparer<T> for you:

public static class DynamicEqualityComparer<T>
{
    public static DynamicEqualityComparer<T, TSelector> Create<TSelector>(
        Func<T, TSelector> selector)
        => new DynamicEqualityComparer<T, TSelector>(selector);
}

public class DynamicEqualityComparer<T, TSelector> : IEqualityComparer<T>
{
    private readonly Func<T, TSelector> _Selector;
    
    public DynamicEqualityComparer(Func<T, TSelector> selector)
    {
        _Selector = selector ?? throw new ArgumentNullException(nameof(selector));
    }
    
    public int GetHashCode(T value) => _Selector(value).GetHashCode();
    public bool Equals(T a, T b) => _Selector(a).Equals(_Selector(b));
}

To create an equality comparer that compares Location and RoomKey you would do this:

DynamicEqualityComparer<TsvDataModel>.Create(dm => (dm.Location, dm.RoomKey))

and one for RoomKey and Area:

DynamicEqualityComparer<TsvDataModel>.Create(dm => (dm.RoomKey, dm.Area))

Note that there is absolutely no error checking in the code here so you would have to add it if you think about using it.

CodePudding user response：

Here's a DynamicEqualityComparer:

public class DynamicEqualityComparer<T> : IEqualityComparer<T>
{
    private readonly IEnumerable<Func<T, object>> _getPropertiesToCompare;

    public DynamicEqualityComparer(params Func<T, object>[] getPropertiesToCompare)
    {
        _getPropertiesToCompare = getPropertiesToCompare;
    }

    public bool Equals(T x, T y)
    {
       return _getPropertiesToCompare.All(getProperty => 
           object.Equals(getProperty(x), getProperty(y)));
      
    }

    public int GetHashCode(T obj)
    {
        var hashCode = new HashCode();
        foreach (var getProperty in _getPropertiesToCompare)
        {
            hashCode.Add(getProperty((T)obj));
        }

        return hashCode.ToHashCode();
    }
}

Usage:

var comparer = new DynamicEqualityComparer<TsvDataModel>(
    model => model.Location, 
    model => model.RoomKey,
    model => model.Area);

var hashset = new HashSet<TsvDataModel>(comparer);