An attribute and a IValidate class that gives the developer the options to point out an IList<ContentReference> property from a string property decorated with a SelectMany attribute so when the editor changes the selectmany property the string contentreferences are added to the IList property so we get an automatically check that a SelectMany un…

SelectManyContentReferenceValidator.cs

using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.ServiceLocation;
using EPiServer.Validation;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Web;

namespace Alloy.Business.Validators
{
    public class SelectManyContentReferenceValidator : IValidate<ContentData>
    {
        List<ValidationError> errorMessages = new List<ValidationError>();
        /// <summary>
        /// Get a list of all properties on type that are decorated with SelectManyListProperty attribute
        /// </summary>
        /// <param name="ContentInstance"></param>
        /// <returns>Dictionary with decorated property name, property name of Ilist property to be populated with contentreferences</returns>
        private static Dictionary<string, string> GetSelectManyProperties(ContentData ContentInstance)
        {
            Dictionary<string, string> _dict = new Dictionary<string, string>();

            var repo = ServiceLocator.Current.GetInstance<IContentTypeRepository>();
            var type = repo.Load(((IContent)ContentInstance).ContentTypeID);

            PropertyInfo[] props = type.ModelType.GetProperties();
            foreach (PropertyInfo prop in props)
            {
                object[] attrs = prop.GetCustomAttributes(true);
                foreach (object attr in attrs)
                {
                    SelectManyListProperty listProperty = attr as SelectManyListProperty;
                    if (listProperty != null)
                    {
                        string propName = prop.Name;
                        string listPropName = listProperty.Name;

                        _dict.Add(propName, listPropName);
                    }
                }
            }

            return _dict;
        }

        /// <summary>
        /// Validates all properties on type and checks to see if they are decorated with SelectManyListProperty attribute. If yes all string references are converted to ContentReferences and added to specified
        /// IList<ContentReference> property
        /// </summary>
        /// <param name="instance"></param>
        /// <returns>Empty list of errormessages</returns>
        public IEnumerable<ValidationError> Validate(ContentData instance)
        {
            //Get all SelectMany properties that should be validated
            var dictPropertiesToValidate = GetSelectManyProperties(instance);

            foreach (PropertyData propertyData in instance.Property)
            {

                if (dictPropertiesToValidate.ContainsKey(propertyData.Name))
                {
                    var sourcePropertyName = propertyData.Name;
                    var targetPropertyName = dictPropertiesToValidate[propertyData.Name];

                    instance.Property[targetPropertyName].Clear();

                    if (string.IsNullOrEmpty(instance.GetPropertyValue(propertyData.Name))) continue;

                    var contentRefs = propertyData.Value.ToString().Split(",".ToCharArray()).ToList().Select(x => new ContentReference(x)).ToList();

                    instance.SetValue(targetPropertyName, contentRefs);
                }
            }

            return errorMessages;
        }

    } //class
} //namepace

SelectManyListProperty.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Alloy.Business.Validators
{
    public class SelectManyListProperty : Attribute
    {
        public string Name { get; set; }
        public SelectManyListProperty(string name)
        {
            this.Name = name;
        }
    }
}

Implement the Attribute and Validator in your solution. Example code below.

SelectMany(typeof(factoryclass))
SelectManyListProperty("ListPropertyName")
public virtual string SelectManyProp { get; set; }

public virtual IList ListPropertyName {get; set;}

Blogg post:
https://world.episerver.com/blogs/Per-Nergard/Dates/2019/2/warning-before-deleting-a-page-that-is-used-in-a-property-decorated-with-a-selectone-or-selectmany-attribute/