Asp.net error: "System. NullReferenceException: object reference is not set to the instance of the object"

An important knowledge point:

"System. NullReferenceException: object reference is not set to the instance of the object." The possible causes are as follows:
1. The ViewState object is Null.
2. DateSet is empty.
3. DataReader is empty due to SQL statements or Datebase.
4. When a string variable is declared without a null value, the variable is applied.
5. The new initialization object is not used.
6 ,.
7. When you assign a text value to the control, the value does not exist.
8. When Request. QueryString () is used, the obtained object does not exist, or the initial value is not assigned when the value is null.
9. When FindControl is used, the control does not exist but is not preprocessed.
10. Repeated definitions may result in an error in setting the object reference to an instance.