How to URL Encode a string in C#

urlencode.cs

        private string urlencode(string input_url)
        {
            StringBuilder b = new StringBuilder(100);
            foreach (char c in input_url)
            {
                b.Append(replace(c));
            }
            return b.ToString();
        }

        private string replace(char c)
        {
            switch (c)
            {
                case '~':
                        return "%7E";
                case '!':
	                return "%21";
                case '@':
	                return "%40";
                case '#':
	                return "%23";
                case '$':
	                return "%24";
                case '%':
	                return "%25";
                case '^':
	                return "%5E";
                case '&':
	                return "%26";
                case '*':
	                return "%2A";
                case '(':
	                return "%28";
                case ')':
	                return "%29";
                case '{':
	                return "%7B";
                case '}':
	                return "%7D";
                case '[':
	                return "%5B";
                case ']':
	                return "%5D";
                case '=':
	                return "%3D";
                case ':':
	                return "%3A";
                case '/':
	                return "/";
                case ',':
	                return "%2C";
                case ';':
	                return "%3B";
                case '?':
	                return "%3F";
                case '+':
	                return "%2B";
                case '\'':
	                return "%27";
                case '"':
	                return "%22";
                case '\\':
	                return "%5C";
                default:
                    return c.ToString();
            }

        }