A Safe String.SubString Extension Method

The String.Substring method in .net is useful, but has one slight quirk: If you try to read too much (i.e. "123".Substring(1,5)), you get an ArgumentOutOfRangeException saying that "Index and length must refer to a location within the string.". Here is an extension method that will not throw an exception, but instead return a truncated string, even it it is shorter than desired. That way, you can use it as a way to always Truncate longer strings while leaving shorter ones unchanged.

public static class StringExtensions
{
    public static string SafeSubstring(this string input, int startIndex, int length)
    {
        // Todo: Check that startIndex + length does not cause an arithmetic overflow
        if (input.Length >= (startIndex + length))
        {
            return input.Substring(startIndex, length);
        }
        else
        {
            if (input.Length > startIndex)
            {
                return input.Substring(startIndex);
            }
            else
            {
                return string.Empty;
            }
        }
    }
}

This was inspired by a Question on StackOverflow, and since I needed this so many times now, I've decided to write it.

Note that this requires the .net 3.5 Compiler.

February 27th, 2009 in Development

Comments (5)

software developersAugust 11th, 2009 at 18:38

Cool,

split can be used in this example as well

Thanks

Andrei RineaOctober 3rd, 2009 at 22:55

A small correction : the .NET 3.5 compiler

mstumOctober 4th, 2009 at 15:03

Indeed. It's the C# 3.0 Compiler included in .net 3.5. Confusing Version numbers 🙂

Andrei RineaOctober 5th, 2009 at 15:17

Got bitten by these confusing numbers myself too. Ah well, these confusions will end in 4.0 🙂

AaronNovember 13th, 2012 at 19:01

Good simple technique to solve this common problem. I did the same but a slight modification to reduce the lines of code and make it a little cleaner.

public static string SafeSubstring(this string source, int startIndex, int length)
{
if (startIndex > source.Length-1)
{
throw new ArgumentException("Start index cannot be greater than string length.", "startIndex");
}

return startIndex + length > source.Length ? source.Substring(startIndex) : source.Substring(startIndex, length);
}