Case insensitive key dictionaries and MongoDb C# serializers

First of all, every C# programmer should know that Dictionary<Tkey, Tvalue> class (as well as other collections) have a special constructor that can be used to specify the serializer used to compare keys in the dictionary. The most obvious situation is where you have a string key and you want the dictionary to be case insensitive during key search.

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  public SortedDictionary<string, StringProperty> StringProperties { get; private set; } 
    = new SortedDictionary<string, StringProperty>(StringComparer.OrdinalIgnoreCase);

The above code is inside a class where I need to keep a dictionary of StringProperty class, using a Sorted dictionary where the key must be case insensitive. This allows me to write code like this

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  myclass.StringProperties["BLAH"] = new StringProperty....
  ...
  myclass.StringProperties["blah"]

If you omit the StringComparer.OrdinalIgnoreCase in dictionary constructor in the third line of your code you will have an exception because there is no element in dictionary with the key blah. Everything is ok until you save the object in MongoDb, because when you deserialize the object, all Dictionary properties are deserialized with a dictionary with default constructor, and the above code does not works anymore. This is annoying because you have a class that is not the same after MongoDb deserialization.

Whenever you encounter such a problem in MongoDb, the solution is to create a Custom serializer, but for complex stuff is not an easy task. Luckily enough if you look ad MongoDb C# driver source, you can find that there is a nice base helper class that can solve all of your problem. Thanks to that base class you can write a custom serializer with few lines of code.

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public class CaseInsensitiveSortedKeyDictionarySerializer<TKey, TValue> :
        DictionarySerializerBase<SortedDictionary<TKey, TValue>, TKey, TValue>
{
    protected override ICollection<KeyValuePair<TKey, TValue>> CreateAccumulator()
    {
        return Activator.CreateInstance(
            typeof(SortedDictionary<TKey, TValue>),
            new object[] { StringComparer.OrdinalIgnoreCase }) as SortedDictionary<TKey, TValue>;
    }
}

This class is super simple because DictionarySerializerBase class allows you to override only how the underling dictionary is created, solving all of my problems. You can create other serializer for basic Dictionary<TKey, TValue> not only for SortedDictionary, and you can use the most suitable constructor for the dictionary type you need.

You can easily use that serializer with a simple attribute in class property

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  [BsonSerializer(typeof(CaseInsensitiveSortedKeyDictionarySerializer<string, StringProperty>))]
  public SortedDictionary<string, StringProperty> StringProperties

Now even after deserialization my class still have case insensitive key based dictionary property.

Gian Maria.