Define a hashset with customized key

Question

I need to define hashset which key is MyClass

public class MyClass
{
    private string m_x;
    private string m_y;
    MyClass(string x,string y)
    {
       m_x = x;
       m_y =y;
    }
}

So I have

HashSet <MyClass> myHash;
MyClass m = new MyClass("1","123");
myHash.Add(m);
bool isContained = myHash.Contains(m);

IsContained is false....What is wrong? what could be another container, which allows to save the unique keys? and return the answer if key is inside with minimum complexity?

Answer 1

a) Assuming you don't want to compare the objects by their references, you should override GetHashCode and Equals methods of MyClass

HashSet<MyClass> myHash = new HashSet<MyClass>();
MyClass m1 = new MyClass("1", "123");
MyClass m2 = new MyClass("1", "123");
myHash.Add(m1);
bool b = myHash.Contains(m2); //true

---

public class MyClass
{
    private string m_x;
    private string m_y;
    public MyClass(string x, string y)
    {
        m_x = x;
        m_y = y;
    }

    public override int GetHashCode()
    {
        return m_x.GetHashCode() ^ m_y.GetHashCode();
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(this, obj)) return true;
        if (obj == null) return false;
        var other = obj as MyClass;
        return m_x == other.m_x && m_y == other.m_y;
    }
}

b) You can also use IEqualityComparer to compare your objects, But in this case you need some public properties

public class MyClass
{
    public string m_x;
    public string m_y;
    public MyClass(string x, string y)
    {
        m_x = x;
        m_y = y;
    }
}

public class MyEqualityComparer : IEqualityComparer<MyClass>
{

    public bool Equals(MyClass x, MyClass y)
    {
        return x.m_x == y.m_x && x.m_y == y.m_y;
    }

    public int GetHashCode(MyClass obj)
    {
        return obj.m_x.GetHashCode() ^ obj.m_y.GetHashCode();
    }
}

Now, you only need to give the comparer to the HashSet's constructor

HashSet<MyClass> myHash = new HashSet<MyClass>( new MyEqualityComparer());

Answer 2

Just change your MyClass so it implements Equals and GetHashCode:

public class MyClass
{
    private string m_x;
    private string m_y;

    public MyClass(string x, string y)
    {
        m_x = x;
        m_y = y;
    }

    public override bool Equals(object obj)
    {
        var typed = obj as MyClass;
        if (typed == null) return false;
        return typed.m_x == m_x && typed.m_y ==m_y;
    }

    public override int GetHashCode()
    {
        return new {m_x, m_y}.GetHashCode();
    }
}

Now the IsContained2 also is true, even if it has not been added to the HashSet:

HashSet<MyClass> myHash =new HashSet<MyClass>();
MyClass m = new MyClass("1", "123");
MyClass m2 = new MyClass("1", "123");
myHash.Add(m);
bool isContained = myHash.Contains(m);
bool isContained2 = myHash.Contains(m2); // True!