Retrieved Dictionary Key Not Found

Question

I have a SortedDictionary declared like such:

SortedDictionary<MyObject,IMyInterface> dict = new SortedDictionary<MyObject,IMyInterface>();

When its populated with values, if I grab any key from the dictionary and then try to reference it immediately after, I get a KeyNotFoundException:

MyObject myObj = dict.Keys.First();
var value = dict[myObj];     // This line throws a KeyNotFoundException

As I hover over the dictionary (after the error) with the debugger, I can clearly see the same key that I tried to reference is in fact contained in the dictionary. I'm populating the dictionary using a ReadOnlyCollection of MyObjects. Perhaps something odd is happening there? I tried overriding the == operator and Equals methods to get the explicit comparison I wanted, but no such luck. That really shouldn't matter since I'm actually getting a key directly from the Dictionary then querying the Dictionary using that same key. I can't figure out what's causing this. Has anyone ever seen this behavior?

EDIT 1

In overriding Equals I also overloaded (as MS recommends) GetHashCode as well. Here's the implementation of MyObject for anyone interested:

public class MyObject
{
public string UserName { get; set;}
public UInt64 UserID  { get; set;}

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

        // Return true if the fields match:
        return this.Equals((MyObject)obj);
    }

    public bool Equals(MyObject other)
    {
        // Return true if the fields match
        return this.UserID == other.UserID;
    }

    public override int GetHashCode()
    {
        return (int)this.UserID;
    }


public static bool operator ==( MyObject a, MyObject b)
{
    // If both are null, or both are same instance, return true.
    if (System.Object.ReferenceEquals(a, b))
    {
        return true;
    }

    // If one is null, but not both, return false.
    if (((object)a == null) || ((object)b == null))
    {
        return false;
    }

    // Return true if the fields match:
    return a.UserID == b.UserID
}

public static bool operator !=( MyObject a, MyObject b)
{
    return !(a == b);
}
}

What I noticed from debugging is that if I add a quick watch (after the KeyNotFoundException is thrown) for the expression:

dict.ElementAt(0).Key == value;

it returns true. How can this be?

EDIT 2 So the problem ended up being because SortedDictionary (and Dictionary as well) are not thread-safe. There was a background thread that was performing some operations on the dictionary which seem to be triggering a resort of the collection (adding items to the collection would do this). At the same time, when the dictionary iterated through the values to find my key, the collection was being changed and it was not finding my key even though it was there.

Sorry for all of you who asked for code on this one, I'm currently debugging an application that I inherited and I didn't realize this was going on on a timed, background thread. As such, I thought I copied and pasted all the relevant code, but I didn't realize there was another thread running behind everything manipulating the collection.

Answer 1

It appears that the problem ended up being because SortedDictionary is not thread-safe. There was a background thread that was performing some operations on the dictionary (adding items to the collection) which seems to be triggering a resort of the collection. At the same time, when the dictionary was attempting to iterate through the values to find my key, the collection was being changed and resorted, rendering the enumerator invalid, and it was not finding my key even though it was there.