Dictionary.ContainsKey return False, but a want True

Question

namespace Dic
{
public class Key
{
    string name;
    public Key(string n) { name = n; }
}

class Program
{
    static string Test()
    {
        Key a = new Key(         


        

Answer 1

If you do not have the ability to override equality operators/Equals/GetHashCode as others have mentioned (as in, you do not control the source code of the object), you can provide an IEqualityComparer<Key> implementation in the constructor of the dictionary to perform your equality checks.

class KeyComparer : IEqualityComparer<Key>
{
    public bool Equals(Key x, Key y)
    {
        return x.Name == y.Name;
    }

    public int GetHashCode(Key obj)
    {
        return obj.Name.GetHashCode();
    }
}

As it stands, your Key is a reference object, so equality is only determined on reference unless you tell the world (or the dictionary) otherwise.

Answer 2

1. Override Equals, Get Hash Code, and the '==' Operator.

The Key class must override Equals in order for the Dictionary to detect if they are the same. The default implementation is going to check references only.

Here:

        public bool Equals(Key other)
        {
            return this == other;
        }

        public override bool Equals(object obj)
        {
            if (obj == null || !(obj is Key))
            {
                return false;
            }

            return this.Equals((Key)obj);
        }

        public static bool operator ==(Key k1, Key k2)
        {
            if (object.ReferenceEquals(k1, k2))
            {
                return true;
            }

            if ((object)k1 == null || (object)k2 == null)
            {
                return false;
            }

            return k1.name == k2.name;
        }

        public static bool operator !=(Key k1, Key k2)
        {
            if (object.ReferenceEquals(k1, k2))
            {
                return false;
            }

            if ((object)k1 == null || (object)k2 == null)
            {
                return true;
            }

            return k1.name != k2.name;
        }

        public override int GetHashCode()
        {
            return this.name == null ? 0 : this.name.GetHashCode();
        }

2. If Possible, use a struct.

You should use a struct for immutable datatypes like this, since they are passed by value. This would mean that you coulnd't accidentally munge two different values into the same key.

Answer 3

In order to use your own classes as dictionary keys, you should override GetHashCode and Equals. Otherwise it will use the memory address to check for equality.

    public class Key
    {
        string name;
        public Key(string n) { name = n; }

        public override int GetHashCode()
        {
            return name.GetHashCode();
        }

        public override bool Equals(object obj)
        {
            var other = obj as Key;
            if( other == null )
                return false;

            return name == other.name;
        }
    }

Answer 4

You want true - but a and b are different objects.

You need to override GetHashCode and Equals on class Key

public class Key
{
    string name;
    public Key(string n) { name = n; }

    public override int GetHashCode()
    {
        if (name == null) return 0;
        return name.GetHashCode();
    }

    public override bool Equals(object obj)
    {
        Key other = obj as key;
        return other != null && other.name == this.name;
    }
}

Answer 5

Overriding a class' GetHashCode and Equals methods so that it will work properly in a dictionary is not a very good approach. The way a dictionary behaves should be an implementation detail of the dictionary, not of whatever class is being used as a key. You'll get into trouble when you want to use the class in different dictionaries with different behavior. Or if you don't have access to the class source code.

A better mouse trap is to give the dictionary its own comparer. For example:

using System;
using System.Collections.Generic;

class Program {
    static void Main(string[] args) {
        var d = new Dictionary<Key, int>(new MyComparer());
        d.Add(new Key("A"), 1);
        Console.WriteLine(d.ContainsKey(new Key("a")));
        Console.ReadLine();
    }
    private class MyComparer : IEqualityComparer<Key> {
        public bool Equals(Key x, Key y) {
            return string.Compare(x.Name, y.Name, true) == 0;
        }
        public int GetHashCode(Key obj) {
            return obj.Name.ToUpper().GetHashCode();
        }
    }
    public class Key {
        public string Name { get; set; }
        public Key(string name) { Name = name; }
    }
}

Answer 6

you need to override Equals and GetHashCode methods of your Key class.