如何将datetimepicker默认设置为空？

在Load中，初始化 
this.dateTimePicker1.Format=DateTimePickerFormat.Custom; 
his.dateTimePicker1.CustomFormat=" "; 
在事件里写： 

private void dateTimePicker1_ValueChanged(object sender, System.EventArgs e) 
{ 
//this.dateTimePicker1.Format=DateTimePickerFormat.Long; 
this.dateTimePicker1.CustomFormat=null; 

} 
这样就实现了，在程序初始化时dateTimePicker显示为空