It can be a null type.

 

  1                //  Compilation error. You cannot convert null to "int" because it is a type that cannot be null.
  2     //  Because the capacity of the value type is sufficient to indicate a value suitable for this type, it cannot be blank; the value type does not indicate the additional capacity required for the null value.  
  3        Private    Int  One  =     Null  ;
  4   
  5   
  6     //  Compiled, T? The syntax is short for nullable <t>, where T is the value type. These two forms can be exchanged. The nullable <t> structure supports only one value type as a type that can be null.  
  7       Private     Int  ?  Two  =     Null  ;
  8   
  9     Public    Void Test ()
  10   {
  11     //  If the two variable contains a value, the hasvalue attribute returns true; or if the value of the two variable is null, false is returned.  
  12     If  (Two. hasvalue)
  13   {
  14    //  If a value has been assigned, the Value Attribute returns this value. Otherwise, system. invalidoperationexception is thrown.  
  15   One  =  Two. value;
  16   }
  17     //  Getvalueordefault () Return Value: If the hasvalue attribute is true, it is the value of the Value Attribute; otherwise, it is the default value of the current nullable <t> object.
  18    //  The default type is the type parameter of the current nullable <t> object, and the default value only contains binary zero.  
  19   One  =  Two. getvalueordefault ();
  20   
  21     //  Use ?? Assign a default value to the operator. If you assign a null value to a non-null type, the default value is applied.  
 22     Int  Four  =  Two  ??     1  ;
  23   }
  24   
  25     // Compilation error. The type "string" must be of a type that cannot be null value to be used as the parameter "T" in the generic type or method "system. nullable <t>"
  26     //  You cannot create a type that can be null based on the reference type. (The reference type supports null values .)  
  27     Private  System. nullable  <  String     >  Three =     Null  ;
  28   
  29     /*  *
  30   * Application: based on the specific environment, use the type that can be null to indicate existence or non-existence of things.
  31   * An empty column in a database table may exist in a row of the table, but not in another row.
  32  * This column can be represented as a field in the class and can be defined as a value type. This field can contain all valid values of the column,
  33   * However, it cannot provide an additional value to indicate that the column does not exist. In this case, the field should be defined as a type that can be null, rather than a value type.
  34     */ 

It mainly selects whether to use the value type or the type that can be null when the object model is created.