SortedDictionary - Comparer only for sorting

Question

I have a list of keys and values that I would like sorted by key via a custom Comparer(Of T).

I tried using a SortedDictionary, but kept getting incorrect results because it used the comparer to see if the items were the same. For example calling SortedDictionary.ContainsKey() would return false, even though it did contain the key.

When I stepped through the code after calling ContainsKey(), it would go to the comparer.Compare(x, y) function. It would then only compare against a few of the keys in the dicionary, and somehow skip the matching item (which I ensured did exist). I take it that this is some sort of optimization, where some items are skipped depending on what is returned by the comparer.Compare() function?

Is it possible to have a dictionary that only uses the comparer for sorting? Or any ideas on some way to overcome this?

Thanks a lot!

EDIT: I am using a Type object for the dictionary's key

EDIT: The comparer code: (Renderer's for a game engine that get sorted depending on their DrawOrder Property (if they have it))

Public Class RendererComparer
    Inherits Comparer(Of Type)

    Public Overrides Function Compare(ByVal x As Type, ByVal y As Type) As Integer
        If x Is y Then Return 0
        If x Is Nothing Then
            Return -1
        Else

            Dim draworderx As Integer = GetDrawOrder(x)
            Dim drawordery As Integer = GetDrawOrder(y)
            If draworderx < drawordery Then
                Return -1
            Else
                Return 1
            End If

        End If
    End Function

    Private Function GetDrawOrder(ByVal t As Type) As Integer
        Dim p As PropertyInfo = t.GetProperty("DrawOrder")

        If p Is Nothing Then Return 0

        Dim o As Object = p.GetValue(Nothing, Nothing)
        Return CInt(o)
    End Function

End Class

Answer 1

It sounds like you might need to override GetHashcode() to hash matching values onto the same hash key. Dictionary classes typically use hashcodes before equality testing first before comparing values for an exact match.

Answer 2

If you have two different items with the same GetDrawOrder then you violate the contract of comparer. As a result the sorting can break and you get undefined behavior.

In particular you violate Compare(a,b) == -Compare(b,a)

Answer 3

Your implementation of GetDrawOrder is suspect. Are you hoping to get a static property on the class and then get its value? If so you ought to use BindingFlags.Static when you get the PropertyInfo.

If you want to add a property to a type it would be much cleaner to add a custom attribute.

e.g.

[DrawOrder(5)]
public class MyType { ...}

Then, instead, just ask for the attributes on the type that are OfType DrawOrder to get the integer draw order values and return the result from comparing them.