As the others have said, the '+' operator is used for simple string
concatenation and as Sunasara has demonstrated the '@' string literal enables you to use a more freeform style in your strings.
You may have encountered problems with the '\' character in your own attempts to solve this. Sunasara'a solution will work (apart from the fact that he shows a slash rather than a backslash), or you could use
string c = a + "\\" + b;
Note the doubled backslash. The first is called an
escape character
and enables you to use characters in your strings that would otherwise be interpreted as keywords or control characters, such as the quote mark itself. Have a search for
c# escape character for more information.