string str=null; and string str= ""; the difference (net picks)

First, a string in C # is a string, but its underlying implementation is still an array of characters. And string is a reference type, and its underlying implementation is necessarily a pointer, because the underlying implementation of string is the traditional string implementation in C, which points to a pointer to a character array.
At this point, the difference between string str=null and string str= "" can be clearly seen.
The substance of string str=null is char *ps = 0;
String Str= ""; The essence is char *ps= ""; or char *ps= "/0";
Other words
String Str=null, which indicates that STR is a pointer to 0. Does not allocate any memory space.
String Str= ""; Represents the array of characters in which Str points to the first element of 0 (the string ending flag). Allocates memory space, but the first memory space is 0.