Got all drop down editors working: normal, multiple, normal publishing keys, multiple publishing keys and should be backwards compatible.

src/Umbraco.Web/PropertyEditors/DropDownMultipleWithKeysPropertyEditor.cs

@@ -0,0 +1,66 @@
+using System.Collections.Generic;
+using Umbraco.Core;
+using Umbraco.Core.Models;
+using Umbraco.Core.PropertyEditors;
+
+namespace Umbraco.Web.PropertyEditors
+{
+    /// <summary>
+    /// A property editor to allow multiple selection of pre-defined items
+    /// </summary>
+    /// <remarks>
+    /// Due to maintaining backwards compatibility this data type stores the value as a string which is a comma separated value of the 
+    /// ids of the individual items so we have logic in here to deal with that.
+    /// </remarks>
+    [PropertyEditor(Constants.PropertyEditors.DropdownlistMultiplePublishKeys, "Dropdown list multiple, publish keys", "dropdown")]
+    public class DropDownMultipleWithKeysPropertyEditor : DropDownPropertyEditor
+    {
+        protected override ValueEditor CreateValueEditor()
+        {
+            return new DropDownMultipleValueEditor(true, base.CreateValueEditor());
+        }
+
+        protected override PreValueEditor CreatePreValueEditor()
+        {
+            return new DropDownMultiplePreValueEditor();
+        }
+
+        /// <summary>
+        /// A pre-value editor for the 'drop down list multiple' property editor that ensures that 'multiple' is saved for the config in the db but is not 
+        /// rendered as a pre-value field.
+        /// </summary>
+        /// <remarks>
+        /// This is mostly to maintain backwards compatibility with old property editors. Devs can now simply use the Drop down property editor and check the multiple pre-value checkbox
+        /// </remarks>
+        internal class DropDownMultiplePreValueEditor : DropDownPreValueEditor
+        {
+            public DropDownMultiplePreValueEditor()
+            {
+                var fields = CreatePreValueFields();
+                //add the multiple field, we'll make it hidden so it is not seen in the pre-value editor
+                fields.Add(new PreValueField
+                    {
+                        Key = "multiple",
+                        Name = "multiple",
+                        View = "hidden"
+                    });
+                Fields = fields;
+            }
+
+            /// <summary>
+            /// Always
+            /// </summary>
+            /// <param name="defaultPreVals"></param>
+            /// <param name="persistedPreVals"></param>
+            /// <returns></returns>
+            public override IDictionary<string, object> FormatDataForEditor(IDictionary<string, object> defaultPreVals, PreValueCollection persistedPreVals)
+            {
+                var returnVal = base.FormatDataForEditor(defaultPreVals, persistedPreVals);
+                //always add the multiple param to true
+                returnVal["multiple"] = "1";
+                return returnVal;
+            }
+        }
+
+    }
+}