Convert: C # null, ", dbnull, String. Empty, convert. isdbnull ()

(1) null

The null keyword is a text value that does not reference any object's null reference. Null is the default value of the reference type variable. Then, only the referenced variable can be null. If int I = NULL, it is not possible because Int Is of the value type.

(2) dbnull

Dbnull is a separate type in DOTNET. This class can only have unique instances. dbnull. Value and dbnull have the unique role
It can represent strings, numbers, or dates in the database. Why does it mean that classes (such as datarow) that store the data in DOTNET are based on objects?
To store data. For datarow, the value returned by its row [column] is never null, or the specific value is column.
. Or dbnull. So row [column]. tostring ()
Nullreferenceexception will never occur in tostring. Dbnull implements iconvertible.
However, except that tostring is normal, other toxxx will throw an error that cannot be converted.

(3) "" And string. Empty

Both of these are empty strings, with a focus on string str1 = "" and string str2 = NULL
After this definition, str1 is an empty string, and the empty string is a special string, except that the value of this string is null and has an accurate point in the memory.
Str2 = NULL. After this definition, only a string class reference is defined. str2 does not point to any place. If it is not instantiated before use, it will be wrong.

(4) convert. isdbnull ()

Convert. isdbnull () indicates whether the specified object is of the dbnull type, that is, whether the object is dbnull. The return value is true or flase.