string str = "This is a test";
str = str.Replace(" ", String.Empty);
// Output: Thisisatest
string str = "C Sharp";
str = Regex.Replace(str, @"\s", "");
using System.Linq;
...
string source = "abc \t def\r\n789";
string result = string.Concat(source.Where(c => !char.IsWhiteSpace(c)));
Console.WriteLine(result);