Null, dbnull. Value convert. dbnull, "" string. Empty, convert. isdbnull ()

1. null:
Null reference. It does not point to any object. Int I = NULL is not allowed.

2. dbnull. Value, convert. dbnull:
For datarow, row ["mycol"] will never return NULL, but only the value of a specific field or dbnull. value. Row ["mycol"]. tostring () is either null or the value of a specific field.
Dbnull only has a unique instance, dbnull. Value, used to determine whether the field in the database is null. Convert. dbnull works the same way as this.

3. "", String. Empty:
Same. Empty string.

4. Convert. isdbnull:
If the object in the parameter is dbnull, true or false is returned.