Truncate String Method

Question

I have 2 Truncate string methods.

First:

public static string Truncate(string source, int length)
    {
    if (source.Length > length)
    {
        source = source.Substring(0, length);
    }
    return source;
    }

Second:

public static string Truncate2(string source, int length)
    {
    return source.PadRight(length).Substring(0, length).Trim();
    }

Which of these methods is better, faster?

Answer 1

The question is moot, as the methods give different results. It doesn't matter if a method is faster, better, simpler, nicer, fancier or cooler if it doesn't work properly.

The first method works as expected. It always truncates the string to the desired length, if the string is long enough to truncate. (Note though that this could break a combination of unicode characters used to compose a character, but that's a whole different level of expectations.)

The second method will behave badly if the string that you try to truncate contains whitespace characters in the "wrong" place.

For example, Truncate2("A B", 3) returns "A" instead of "A ".

The method will even crash in some situations where it should just return the input unchanged. To get an OutOfMemoryException you can call Truncate2("", Int32.MaxValue).

Answer 2

The first is better. It uses a straightforward logic. The second uses unintuitive, convoluted logic (pad -> substring -> trim). It should be slower for short strings due to the extra padding step. It should be slower for both due to the extra trimming step.

Also keep in mind that the trimming is an additional behavior that the first one doesn't have, and which may shorten the string below the required length, if it contains trailing spaces. So the end result of the two methods is not the same for some inputs.

The first one can be improved:

public static string Truncate(string source, int length)
{
    if (source.Length > length)
    {
        return source.Substring(0, length);
    }
    return source;
}

It's better not to reassign parameter variables. This one avoids that by returning the substring early.

The braces of the the function are also better placed.

Answer 3

janos's answer is entirely correct and I can add nothing to his reasoning about which method is superior. I was intrigued as to why you would want to try the second one at all. I suspect you want more concise code and to avoid the if control block. A more concise version of your first method can be done without the drawbacks of the second:

public static string Truncate(string source, int length)
{
    return ( source.Length > length ? source.Substring(0, length) : source);
}