// This goes with this blog post: http://www.traceofthought.net/2008/01/02/CustomizingTFSTeamBuildBuildNumbers.aspx 

   public class GetBuildNumber : Task
   {
      public override bool Execute()
      {
         bool result = true;

         try
         {
            ValidateProperties();

            string buildNumber = GetLastBuildNumber();
            _buildNumber = string.Format("{0}_{1}_{2}.{3}.{4}",
               BuildType,
               BaseBuildName,
               DateTime.Now.ToString("yyyyMMdd"),
               ChangesetNumber,
               buildNumber);
         }
         catch (Exception e)
         {
            result = false;

            BuildErrorEventArgs eventArgs;
            eventArgs = new BuildErrorEventArgs
                (
                    "",
                    "",
                    BuildEngine.ProjectFileOfTaskNode,
                    BuildEngine.LineNumberOfTaskNode,
                    BuildEngine.ColumnNumberOfTaskNode,
                    0, 0,
                    "GetBuildNumber failed: " + e.Message,
                    "", ""
                );

            BuildEngine.LogErrorEvent(eventArgs);

            throw;
         }

         return result;
      }

      private string GetLastBuildNumber()
      {
         string buildNumber = "0";

         string[] parts = LastBuildNumber.Split('.');
         int number = 0;

         try
         {
            // This logic is dependent on how the build number is formatted
            // up above in the Execute method.
            int changeSet = Int32.Parse(parts[parts.Length - 2]);

            // If same changeset, increment the number.
            if (changeSet == int.Parse(ChangesetNumber))
            {
               number = int.Parse(parts[parts.Length - 1]);
               number++;
               buildNumber = number.ToString();
            }
         }
         catch
         {
         }

         return buildNumber;
      }

      private void ValidateProperties()
      {
         if (String.IsNullOrEmpty(ChangesetNumber))
         {
            throw new ArgumentException("BaseBuildName is null");
         }
      }

      [Output]
      public string BuildNumber
      {
         get { return _buildNumber; }
         set { _buildNumber = value; }
      }
      private string _buildNumber = String.Empty;

      [Required]
      public string BaseBuildName
      {
         get { return _baseBuildName; }
         set { _baseBuildName = value; }
      }
      private string _baseBuildName;

      [Required]
      public string ChangesetNumber
      {
         get { return _changesetNumber; }
         set { _changesetNumber = value; }
      }
      private string _changesetNumber;

      [Required]
      public string BuildType
      {
         get { return _buildType; }
         set { _buildType = value; }
      }
      private string _buildType;

      public string LastBuildNumber
      {
         get { return _lastBuildNumber; }
         set { _lastBuildNumber = value; }
      }

      private string _lastBuildNumber;
   }