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"$" is a string interpolation operation method that is new feature of C# 6. String interpolation syntax allows strings to directly replace the literal string indexes by "holes" with the expressions that correspond to the values. It is a advance form of string format(string.format)

like

string url=String.Format("Hello my url is {0} and path is {1}",baseUrl,path);

You can achivev same thing uisng interpolation operator in some simple steps like

string url = $"Hello My Url is {baseUrl} and path is {path}";

That is more simpler way to format a string compare to previous method