“the given key was not present in the dictionary” error when using a self-defined class as key

Question

I\'ve got code like this:

if (CounterForEachRelatedTagDict.Select(x => x.Key).Contains(tag.Key))
   CounterForEachRelatedTagDict[tag.Key] += tag.Value;
         


        

Answer 1

To use your type as a dictionary key you should override two methods: GetHashCode and Equals.

By default (if you'll not override GetHashCode) every object of your type (even with the same field values) will return unique value. This means that you'll be able to find only exactly the same "reference" that you'll put into your dictionary. Consider following two types: MyType1 that not overrides GetHashCode and Equals, and MyType2 that do:

class MyType1
{
  public MyType1(int id, string name) {Id = id; Name = name;}
  public int Id {get; private set;}
  public string Name {get; private set;}
}


internal class MyType2
{
    public MyType2(int id, string name)
    {
        Id = id;
        Name = name;
    }

    public int Id { get; private set; }
    public string Name { get; private set; }

    bool Equals(MyType2 other)
    {
        return Id == other.Id && string.Equals(Name, other.Name);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != this.GetType()) return false;
        return Equals((MyType2) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return (Id*397) ^ Name.GetHashCode();
        }
    }
}

var d1 = new Dictionary();
d1[new MyType1(1, "1")] = 1;
d1[new MyType1(1, "1")]++; // will throw withKeyNotFoundException

var d2 = new Dictionary();
d1[new MyType2(1, "1")] = 1;
d1[new MyType2(1, "1")]++; // Ok, we'll find appropriate record in dictionary