public void AddFile(HttpFile file)
{
if (_ignoreChanges)
throw new InvalidOperationException("Cannot add files to instance HttpForm.EmptyForm.");
if(file == null)
throw new ArgumentNullException("file");
if(_files.ContainsKey(file.Name))
throw new ArgumentException("File named '" + file.Name + "' already exists!");
_files.Add(file.Name, file);
}