Get All Values From NewForm.aspx using Event Receiver Item Adding

HI All,

         I have conditions to check before the insertion of "Calendar Event".For this I am using Item Adding Event Receiver ,When Click on Save button I need to get all the values of Items filled in NewForm.aspx and check the condition,If condition satisfies make them insert else show alert info  as"Not Valid",Below is code and error message

public override void ItemAdding(SPItemEventProperties properties)
        {
            base.ItemAdding(properties);
           string StartTime = properties.AfterProperties["Start Time"].ToString();

           string EndTime = properties.AfterProperties["End Time"].ToString();

}
Error
Object reference not set to an instance of an object.
Use the New "Keyword to create an object instance

Can any one help me how can I do it.

Thanks,

Quality Communication Provides

Quality Work.

http://siddiq-sharepoint2010.blogspot.in/

Siddiqali Mohammad .