LINQ Group By into a Dictionary Object

Dictionary<string, List<CustomObject>> myDictionary = ListOfCustomObjects
    .GroupBy(o => o.PropertyName)
    .ToDictionary(g => g.Key, g => g.ToList());

I cannot comment on @Michael Blackburn, but I guess you got the downvote because the GroupBy is not necessary in this case.

Use it like:

var lookupOfCustomObjects = listOfCustomObjects.ToLookup(o=>o.PropertyName);
var listWithAllCustomObjectsWithPropertyName = lookupOfCustomObjects[propertyName]

Additionally, I've seen this perform way better than when using GroupBy().ToDictionary().