How to remove new line characters from a string?

You want to use String.Replace to remove a character.

s = s.Replace("\n", String.Empty);
s = s.Replace("\r", String.Empty);
s = s.Replace("\t", String.Empty);

Note that String.Trim(params char[] trimChars) only removes leading and trailing characters in trimChars from the instance invoked on.

You could make an extension method, which avoids the performance problems of the above of making lots of temporary strings:

static string RemoveChars(this string s, params char[] removeChars) {
    Contract.Requires<ArgumentNullException>(s != null);
    Contract.Requires<ArgumentNullException>(removeChars != null);
    var sb = new StringBuilder(s.Length);
    foreach(char c in s) { 
        if(!removeChars.Contains(c)) {
            sb.Append(c);
        }
    }
    return sb.ToString();
}

I like to use regular expressions. In this case you could do:

string replacement = Regex.Replace(s, @"\t|\n|\r", "");

Regular expressions aren't as popular in the .NET world as they are in the dynamic languages, but they provide a lot of power to manipulate strings.