What's the difference between double quotes and single quote in C#

Question

Single quotes encode a single character (data type char), while double quotes encode a string of multiple characters. The difference is similar to the difference between a single integer and an array of integers.

char c = 'c';
string s = "s"; // String containing a single character.
System.Diagnostics.Debug.Assert(s.Length == 1);
char d = s[0];

int i = 42;
int[] a = new int[] { 42 }; // Array containing a single int.
System.Diagnostics.Debug.Assert(a.Length == 1);
int j = a[0];

when you say string s = "this string" then s[0] is a char at at specific index in that string (in this case s[0] == 't')

So to answer your question, use double quotes or single quotes, you can think of the following as meaning the same thing:

string s = " word word";

// check for space as first character using single quotes
if(s[0] == ' ') {
 // do something
}

// check for space using string notation
if(s[0] == " "[0]) {
 // do something
}

As you can see, using a single quote to determine a single char is a lot easier than trying to convert our string into a char just for testing.

if(s[0] == " "[0]) { 
 // do something
}

is really like saying:

string space = " ";
if(s[0] == space[0]) {
 // do something
}

Hopefully I did not confuse you more!

the single quote represent a single character 'A', double quote append a null terminator '\0' to the end of the string literal, " " is actually " \0" which is one byte larger than the intended size.