Project project = new Project("New Project.mpp");

OutlineCodeDefinition textOutline = new OutlineCodeDefinition();
textOutline.FieldId = ExtendedAttributeTask.OutlineCode7.ToString("D");
textOutline.Alias = "My Outline Code";

project.OutlineCodes.Add(textOutline);

OutlineMask mask = new OutlineMask();
mask.Type = MaskType.Characters;
textOutline.Masks.Add(mask);

OutlineValue textValue = new OutlineValue();
textValue.Value = "Text value 1";
textValue.ValueId = 1;
textValue.Type = OutlineValueType.Text;
textValue.Description = "Text value descr 1";
textOutline.Values.Add(textValue);

project.Save("MultipleOutlineValues.mpp", SaveFileFormat.MPP);