How is this Dictionary<TKey, TValue> exception possible?

Question

Given the following stack trace:

MESSAGE: Value cannot be null.Parameter name: key
SOURCE: mscorlib
TARGETSITE: Void ThrowArgumentNullException(System.ExceptionArgument)
STACKTRACE:
   at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Collections.Generic.Dictionary'2.FindEntry(TKey key)
   at System.Collections.Generic.Dictionary'2.get_Item(TKey key)
   at MyCompany.MAF.Agent.ServiceContracts.ConvertUtils.Convert(Dictionary'2 from) in D:\Development\MAF\Agent\MyCompany.MAF.Agent\ServiceContracts\ConvertUtils.cs:line 11

I conclude that somehow the following block of code has retrieved a null from the input Dictionary's Keys collection. However, the input dictionary is an instance of Dictionary<string, string>. The implementation of Dictionary<string, string> makes that condition impossible. Upon adding an item with a null key, an exception is thrown.

  7 internal static KeyValuePair<string, string>[] Convert(IDictionary<string, string> from)
 8 {
 9     List<KeyValuePair<string, string>> ret = new List<KeyValuePair<string, string>>();
10     foreach (string key in from.Keys)
11         ret.Add(new KeyValuePair<string, string>(key, from[key]));
12     return ret.ToArray();
13 }

Answer 1

It looks more like that your IDictionary argument has an item with a Key parameter which is null.

The parameter checking for the IDictionary will probably be happening somewhere internally in the framework.

Answer 2

This exception happens if the dictionary key is null. As the built-in Dictionary class doesn't allow such keys, you might be using your own IDictionary-compatible class which allows null.

Answer 3

I've had this problem happen frequently because I made the mistake of allowing multiple threads to access the same dictionary. Make sure that this is not the case, because Dictionary is not thread-safe.

(Incidentally, your method can be greatly simplified. Dictionary<K,V> is already an IEnumerable<KeyValuePair<K,V>>. You should be able to just do ToArray on one.

Answer 4

Not sure about the null but why aren't you using:

internal static KeyValuePair<string, string>[] Convert(IDictionary<string, string> from)
{
    return from.ToArray();
}

Edit: As far as the null values are concerned. Do you have multiple threads accessing this IDictionary? Corruption is possible if you're not being thread safe. See this post for an example of corruption in the

Queue<T>

class. http://stackoverflow.com/questions/2214446/how-can-i-modify-a-queue-collection-in-a-loop/2214462#2214462

Answer 5

Could it possible be that another thread is affecting the dirctionary being passed into Convert?