var test = new YourClass();
test["Item1"] = "Value1";
public class YourClass
{
private readonly IDictionary<string, string> _yourDictionary = new Dictionary<string, string>();
public string this[string key]
{
// returns value if exists
get { return _yourDictionary[key]; }
// updates if exists, adds if doesn't exist
set { _yourDictionary[key] = value; }
}
}