SharePoint Event Receiver IsFieldChanged

public static class EventReceiverExtensions
{
    static System.Text.RegularExpressions.Regex tagRegex
                = new System.Text.RegularExpressions.Regex("<[^>]*>", System.Text.RegularExpressions.RegexOptions.Compiled);

    public static bool IsFieldChanged(this SPItemEventProperties properties, string fieldName)
    {
        return properties.IsFieldChanged(properties.List.Fields.GetFieldByInternalName(fieldName));
    }

    public static bool IsFieldChanged(this SPItemEventProperties properties, SPField field)
    {
        var after = (string)properties.AfterProperties[field.InternalName];
        var before = Convert.ToString(properties.ListItem[field.Id]);


        //AfterProperties[fieldname] == null - field not changed
        if (after == null)
        {
            return false;
        }

        //AfterProperties[fieldname] == "" - field set to null
        if (after == "" && string.IsNullOrEmpty(before))
        {
            return false;
        }

        //AfterProperties[fieldname] != "", old value is null or empty - field changed
        if (string.IsNullOrEmpty(before))
        {
            return true;
        }

        var afterValue = field.GetFieldValue(after);
        var beforeValue = field.GetFieldValue(before);

        if (afterValue.Equals(beforeValue))
        {
            return false;
        }

        //Compare SPFieldLookupValue and SPFieldUserValue
        if (afterValue is SPFieldLookupValue)
        {
            return (afterValue as SPFieldLookupValue).LookupId != (beforeValue as SPFieldLookupValue).LookupId;
        }

        //Compare SPFieldMultiChoiceValue
        if (afterValue is SPFieldMultiChoiceValue)
        {
            return !EqualChoices(afterValue as SPFieldMultiChoiceValue, beforeValue as SPFieldMultiChoiceValue);
        }

        //Compare strings
        if (afterValue is string)
        {
            //normalize tags on rich edit fields
            if (field is SPFieldMultiLineText && (field as SPFieldMultiLineText).RichText)
            {
                var a = tagRegex.Replace(afterValue as string, m => m.Value.ToLower());
                var b = tagRegex.Replace(beforeValue as string, m => m.Value.ToLower());
                return !a.Equals(b);
            }
        }

        //Compare SPFieldLookupValueCollection and SPFieldUserValueCollection
        if (field is SPFieldLookup && (field as SPFieldLookup).AllowMultipleValues)
        {
            var hsa = new HashSet<int>((afterValue as SPFieldLookupValueCollection).OfType<SPFieldLookupValue>().Select(l => l.LookupId));
            var hsb = new HashSet<int>((beforeValue as SPFieldLookupValueCollection).OfType<SPFieldLookupValue>().Select(l => l.LookupId));
            return !hsa.SetEquals(hsb);
        }

        return  after != before;
    }
    private static bool EqualChoices(SPFieldMultiChoiceValue a, SPFieldMultiChoiceValue b)
    {
        if (a.Count != b.Count) return false;

        var hsa = new HashSet<string>();
        var hsb = new HashSet<string>();
        for (int i = 0; i < a.Count; i++)
        {
            hsa.Add(a[i]);
            hsb.Add(b[i]);
        }

        return hsa.SetEquals(hsb);
    }
}