Dictionary keys don't contain a key that's already contained in keys

Question

Why is the following 'exist' boolean variable getting a value of false???

foreach (Cell existCell in this.decoratorByCell.Keys)
{   
            //this call yield the same hashcode for both cells. still exist==false
            bool exist =
                this.decoratorByCell.ContainsKey(existCell);
}

I've overridden GetHashCode() & Equals() Methods as follows:

public override int GetHashCode()
{
            string nodePath = GetNodePath();

            return nodePath.GetHashCode() + m_ownerColumn.GetHashCode();
}

public bool Equals(Cell other)
{
bool nodesEqual = (other.OwnerNode == null && this.OwnerNode == null) || (other.GetNodePath() == this.GetNodePath());
bool columnsEqual = (other.OwnerColumn == null && this.OwnerColumn == null) || (other.OwnerColumn == this.OwnerColumn);
bool treesEqual = (this.m_ownerTree == other.m_ownerTree);

return (nodesEqual && columnsEqual && treesEqual);
}

Answer 1

Your Equals and GetHashCode implementations do very different things. They should be mirroring each other.

You have no mention in GetHashCode to the m_ownerTree that you are using in your Equals implementation.

Also, adding up hashcodes is not the bast way of computing a hash. You may want to xor them (^) up.

Answer 2

A hash algorithm must have the following property:

• if two things are equal then they have the same hash

A hash algorithm should have the following properties:

• changing a mutable object does not change its hash code
• fast
• never throw an exception
• small differences between objects should cause large (ideally 50% of the bits) differences in hash code

Does your hash algorithm have the first, necessary property? It doesn't look to me like it does.