Question

I have a HashSet. Is there a method that can utilize the IEqualityComparer for retrieving items where you pass in an object that will satisfies the equals method defined in the IEqualityComparer?

This might explain it a bit more.

    public class Program
{
    public static void Main()
    {
        HashSet<Class1> set = new HashSet<Class1>(new Class1Comparer());
        set.Add( new Class1() { MyProperty1PK = 1, MyProperty2 = 1});
        set.Add( new Class1() { MyProperty1PK = 2, MyProperty2 = 2});

        if (set.Contains(new Class1() { MyProperty1PK = 1 }))
            Console.WriteLine("Contains the object");

        //is there a better way of doing this, using the comparer?  
        //      it clearly needs to use the comparer to determine if it's in the hash set.
        Class1 variable = set.Where(e => e.MyProperty1PK == 1).FirstOrDefault();

        if(variable != null)
            Console.WriteLine("Contains the object");
    }
}

class Class1
{
    public int MyProperty1PK { get; set; }
    public int MyProperty2 { get; set; }
}

class Class1Comparer : IEqualityComparer<Class1>
{
    public bool Equals(Class1 x, Class1 y)
    {
        return x.MyProperty1PK == y.MyProperty1PK;
    }

    public int GetHashCode(Class1 obj)
    {
        return obj.MyProperty1PK;
    }
}

Answer 1

If you want to retrieve items based on a single property, you might want to use a Dictionary<T,U> instead of a hashset. You can then place the items within the dictionary, using MyProperty1PK as the key.

Your query then becomes simple:

Class1 variable;
if (!dictionary.TryGetValue(1, out variable)
{
  // class wasn't in dictionary
}

Given that you’re already storing using a comparer which only uses this value as the uniqueness criteria, there is really no disadvantage to just using that property as the key in a dictionary instead.