public class Version : IComparable<Version>
{
    public int[] Parts { get; }

    public Version(string value)
    {
        if (value == null)
            throw new ArgumentNullException();
        if (!Regex.IsMatch(value, @"^[0-9]+(\.[0-9]+)*$"))
            throw new ArgumentException("Invalid format");
        var parts = value.Split('.');
        Parts = new int[parts.Length];
        for (var i = 0; i < parts.Length; i++)
            Parts[i] = int.Parse(parts[i]);
    }

    public override string ToString()
    {
        return string.Join(".", Parts);
    }

    public int CompareTo(Version that)
    {
        if (that == null) return 1;
        var thisLength = this.Parts.Length;
        var thatLength = that.Parts.Length;
        var maxLength = Math.Max(thisLength, thatLength);
        for (var i = 0; i < maxLength; i++)
        {
            var thisPart = i < thisLength ? this.Parts[i] : 0;
            var thatPart = i < thatLength ? that.Parts[i] : 0;
            if (thisPart < thatPart) return -1;
            if (thisPart > thatPart) return 1;
        }
        return 0;
    }
}

Version a, b;

a = new Version("4.2.1");
b = new Version("4.2.6");
a.CompareTo(b); // a < b : -1

a = new Version("2.8.4");
b = new Version("2.8.0");
a.CompareTo(b); // a > b : 1

a = new Version("5.2");
b = null;
a.CompareTo(b); // a > b : 1

a = new Version("3");
b = new Version("3.6");
a.CompareTo(b); // a < b : -1
        
var versions = new List<Version>
{
    new Version("2.0"),
    new Version("1.1.5"),
    new Version("3.0.10"),
    new Version("1"),
    null,
    new Version("1.0.1")
};

versions.Sort();

foreach (var version in versions)
    Console.WriteLine(version?.ToString() ?? "NULL");