SubstringBefore and SubstringAfter for .NET

C# implementation of the functions available in XPath for getting either the string before or after a given part:

static class StringEx  
{
    public static string SubstringAfter(this string source, string value)
    {
        if (string.IsNullOrEmpty(source) || string.IsNullOrEmpty(value))
            return source;
        int index = source.IndexOf(value, StringComparison.Ordinal);
        return (index >= 0) 
            ? source.Substring(index + value.Length)
            : source;
    }

    public static string SubstringBefore(this string source, string value)
    {
        if (string.IsNullOrEmpty(source) || string.IsNullOrEmpty(value))
            return source;
        int index = source.IndexOf(value, StringComparison.Ordinal);
        return (index >= 0)
            ? source.Substring(0, index)
            : source;
    }
}