Get a list of all Sitecore Items tagged with specific multilist items

If you wish to return all items in the content tree that have been tagged with a specific multilist item, you might be tempted to use Sitecore.Context.Database.SelectSingleItem(), passing it an XPath query. The problem with this is that performance degrades as the content tree grows. A far more efficient way is to use the GetReferrers method of the LinkDatabase class as shown below:

Database db = Sitecore.Context.ContentDatabase;

LinkDatabase linkDb = Globals.LinkDatabase;

// lookupItem is the multilist item
ItemLink[] links = linkDb.GetReferrers( lookupItem );

foreach( ItemLink link in links )
{
  Item linkedItem = db.Items[link.SourceItemID];
  // Process item...
}

Example Useful C# Extension Functions

Here are a few extension functions that I find useful:

        public static bool IsNullOrEmpty(
        this string expression )
        {
            bool result = string.IsNullOrEmpty(
            expression );
            return result;
        }

        public static string ToFormat(
        this string expression, params object[] args )
        {
            string result = String.Format(
            expression, args );
            return result;
        }

        public static bool IsValidEmailAddress(
        this string expression )
        {
            bool result = BaseRegexHelper.IsValid(
            expression, emailRegex );
            return result;
        }

        public static bool IsValidGuid(
        this string expression )
        {
            bool result = BaseRegexHelper.IsValid(
            expression, guidRegex );
            return result;
        }

The latter two functions call this method:

        public static bool IsValid(
        string input, string pattern )
        {
            bool isValid = false;

            if( input != null )
            {
                string trimmedExpression =
                input.Trim();

                if( trimmedExpression.Length > 0 )
                {
                    Regex regexGuid = new Regex(
                    pattern, RegexOptions.IgnoreCase );
                    isValid = regexGuid.IsMatch(
                    trimmedExpression );
                }
            }

            return isValid;
        }