Filtering out values from a C# Generic Dictionary

Question

I have a C# dictionary, Dictionary<Guid, MyObject> that I need to be filtered based on a property of MyObject.

For example, I want to remove all records from the dictionary where MyObject.BooleanProperty = false. What is the best way of acheiving this?

Answer 1

If you don't care about creating a new dictionary with the desired items and throwing away the old one, simply try:

dic = dic.Where(i => i.Value.BooleanProperty)          .ToDictionary(i => i.Key, i => i.Value); 

If you can't create a new dictionary and need to alter the old one for some reason (like when it's externally referenced and you can't update all the references:

foreach (var item in dic.Where(item => !item.Value.BooleanProperty).ToList())     dic.Remove(item.Key); 

Note that ToList is necessary here since you're modifying the underlying collection. If you change the underlying collection, the enumerator working on it to query the values will be unusable and will throw an exception in the next loop iteration. ToList caches the values before altering the dictionary at all.

Answer 2

Since Dictionary implements IEnumerable<KeyValuePair<Key, Value>>, you can just use Where:

var matches = dictionary.Where(kvp => !kvp.Value.BooleanProperty); 

To recreate a new dictionary if you need it, use the ToDictionary method.