Null type in C #

　　1: a null type

　　Nullable type is a value type System.Nullable <T> of the struct

　　Nullable types in addition to the correct value which represents the underlying data type (i.e., T) range, can also be expressed null

　　1.1: the following, bool value True and may be a type False, but can not be Null

　　　　　　　 Nullable <bool> In addition to an outer True and False, may also be Null

　　1.2: Nullable to write too much trouble, there are simple wording, only you need to add after the type, as follows?

2: Null, empty, empty string

            string name = "John Doe"; // normal string, there is non-null 

            string name1 = null; // value is null 

            String NAME2 = ""; // string is an empty 

            string name3 = ""; // whitespace string, is the space between the double quotes or Tab key

　　2.1: How to determine Null, empty, empty string

　　　　2.1.1: judge Null

string name = null; 

if (name == null)
{
    //...
}

　　　　2.1.2: Analyzing empty, that had nothing between double quotes following the method also comprises the case of string Null

string name2  = ""; 

if (string.IsNullOrEmpty(name2))
{
    //...
}

　　　　2.1.3: Analyzing empty string, it may be an empty string, the string may be Null, use the following

string name3 = "    ";

if (string.IsNullOrWhiteSpace(name3))
{
    //...
}

3: Nullable <T> common attributes and methods

　　.HasValue // If the value is null, the result is false; otherwise: true 　　

　　.Value // Value underlying value type, if Nullable <T> is the value Null, will be reported abnormal

　　.GetValueOrDefault () // value of the underlying value type or the default value of that type, meaning that if your value is not Null, the value will return back; if it is Null, then return the default value of this value, int default value is 0, so the second return 0 FIG.

　　.GetValueOrDefault (default) // Default value or the bottom value of the specified type, meaning it can specify default values, not the underlying returns Null value type; if it returns a default value Null