UnitySerializedReferenceUI/Editor/Core/SerializeReferenceGenericSelectionMenu.cs

#if UNITY_EDITOR
using System;
using System.Collections.Generic;
using System.Linq;
using UnityEditor;
using UnityEngine;

namespace Textus.SerializeReferenceUI.Editor
{
    public static class SerializeReferenceGenericSelectionMenu
    {
        /// Purpose.
        /// This is generic selection menu (will appear at position).
        /// Filtering. 
        /// You can add substring filter here to filter by search string.
        /// As well ass type or interface restrictions.
        /// As well as any custom restriction that is based on input type.
        /// And it will be performed on each Appropriate type found by TypeCache.
        public static void ShowContextMenuForManagedReference(this SerializedProperty property, Rect position, IEnumerable<Func<Type, bool>> filters = null)
        {
            GenericMenu context = new GenericMenu();
            FillContextMenu(filters, context, property);
            context.DropDown(position);
        }

        /// Purpose.
        /// This is generic selection menu (will appear at click position).
        /// Filtering. 
        /// You can add substring filter here to filter by search string.
        /// As well ass type or interface restrictions.
        /// As well as any custom restriction that is based on input type.
        /// And it will be performed on each Appropriate type found by TypeCache.
        public static void ShowContextMenuForManagedReference(this SerializedProperty property, IEnumerable<Func<Type, bool>> filters = null)
        {
            GenericMenu context = new GenericMenu();
            FillContextMenu(filters, context, property);
            context.ShowAsContext();
        }

        private static void FillContextMenu(IEnumerable<Func<Type, bool>> enumerableFilters, GenericMenu contextMenu, SerializedProperty property)
        {
            List<Func<Type, bool>> filters = enumerableFilters.ToList();// Prevents possible multiple enumerations

            // Adds "Make Null" menu command
            contextMenu.AddItem(new GUIContent("Null"), false, property.SetManagedReferenceToNull);

            // Collects appropriate types
            IEnumerable<Type> appropriateTypes = property.GetAppropriateTypesForAssigningToManagedReference(filters);

            // Adds appropriate types to menu
            foreach (Type appropriateType in appropriateTypes)
            {
                AddItemToContextMenu(appropriateType, contextMenu, property);
            }
        }

        private static void AddItemToContextMenu(Type type, GenericMenu genericMenuContext, SerializedProperty property)
        {
            string assemblyName = type.Assembly.ToString().Split('(', ',')[0];
            string entryName = type + "  ( " + assemblyName + " )";
            genericMenuContext.AddItem(new GUIContent(entryName), false, AssignNewInstanceCommand, new GenericMenuParameterForAssignInstanceCommand(type, property));
        }

        private static void AssignNewInstanceCommand(object objectGenericMenuParameter)
        {
            GenericMenuParameterForAssignInstanceCommand parameter = (GenericMenuParameterForAssignInstanceCommand)objectGenericMenuParameter;
            Type type = parameter.Type;
            SerializedProperty property = parameter.Property;
            property.AssignNewInstanceOfTypeToManagedReference(type);
        }

        private readonly struct GenericMenuParameterForAssignInstanceCommand
        {
            public GenericMenuParameterForAssignInstanceCommand(Type type, SerializedProperty property)
            {
                Type = type;
                Property = property;
            }

            public readonly SerializedProperty Property;
            public readonly Type Type;
        }
    }  
}
#endif