string sqrt = "\u221A";      // √
string emoji = "\U0001F601"; // 😁
string text = "\u0022Hello World\u0022"; // "Hello World"
string variableWidth = "\x22Hello World\x22"; // "Hello World"

Apostrophes

char apostrophe = '\'';

Backslash

char oneBackslash = '\\';

Backslash

// The filename will be c:\myfile.txt in both cases
string filename = "c:\\myfile.txt";
string filename = @"c:\myfile.txt";

The second example uses a verbatim string literal, which doesn't treat the backslash as an escape character.

Quotes

string text = "\"Hello World!\", said the quick brown fox.";
string verbatimText = @"""Hello World!"", said the quick brown fox.";

Both variables will contain the same text.

"Hello World!", said the quick brown fox.

Newlines

Verbatim string literals can contain newlines:

string text = "Hello\r\nWorld!";
string verbatimText = @"Hello
World!";

Both variables will contain the same text.

The following examples will not compile:

string s = "\c";
char c = '\c';

Instead, they will produce the error Unrecognized escape sequence at compile time.

Escape sequences are not restricted to string and char literals.

Suppose you need to override a third-party method:

protected abstract IEnumerable<Texte> ObtenirŒuvres();

and suppose the character Œ is not available in the character encoding you use for your C# source files. You are lucky, it is permitted to use escapes of the type \u#### or \U######## in identifiers in the code. So it is legal to write:

protected override IEnumerable<Texte> Obtenir\u0152uvres()
{
    // ...
}

and the C# compiler will know Œ and \u0152 are the same character.

(However, it might be a good idea to switch to UTF-8 or a similar encoding that can handle all characters.)