You have not given much details about where this line of code is written. COnsidering it is used where EventTextBox is already initialized, only other point I can see is initialization of StringCollection
Have you done initialization something like below?
StringCollection myCollection = new StringCollection();
Also, note that name of the property is different as
DateEventNames at definition vs.
DatesEventNames when you adding. Just check.
Hope that helps
Milind