Cause of "the object reference is not set to the instance of the object" Exception

1. The viewstate object is unll.
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. The session object is empty.

7: when the control is assigned a text value, 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.