DropAllowedHandler.cs

using System.Collections;
using System.Windows;
using Vereinsmeisterschaften.Core.Models;
using GongSolutions.Wpf.DragDrop;
 
namespace Vereinsmeisterschaften.ViewModels
{
    public class DropAllowedHandler : DefaultDropHandler
    {
        #region Singleton
 
        private static readonly DropAllowedHandler instance = new DropAllowedHandler();
 
        // Explicit static constructor to tell C# compiler not to mark type as beforefieldinit
        static DropAllowedHandler()
        {
        }
 
        private DropAllowedHandler()
        {
        }

        public static DropAllowedHandler Instance => instance;
 
        #endregion
 
        // ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

        public int MaxItemsInTargetCollection { get; set; } = 3;

        public override void DragOver(IDropInfo dropInfo)
        {
            dropInfo.DropTargetHintAdorner = DropTargetAdorners.Hint;
 
            if (dropAllowed(dropInfo))
            {
                dropInfo.DropTargetHintState = DropHintState.Active;
                dropInfo.DropTargetAdorner = DropTargetAdorners.Insert;
                dropInfo.Effects = DragDropEffects.Move;
            }
            else
            {
                dropInfo.DropTargetHintState = DropHintState.Error;
                dropInfo.DropHintText = Properties.Resources.PersonStartDragDropErrorString;
                dropInfo.Effects = DragDropEffects.None;
            }
        }

        public override void Drop(IDropInfo dropInfo)
        {
            if (dropAllowed(dropInfo))
            {
                base.Drop(dropInfo);
            }
        }
 
        // only allow drag and drop if the source and destination items have the same swimming style and distance and limit the number of items in the target collection to MaxItemsInTargetCollection
        private bool dropAllowed(IDropInfo dropInfo)
        {
            ICollection sourceCollection = dropInfo.DragInfo.SourceCollection as ICollection;
            ICollection targetCollection = dropInfo.TargetCollection as ICollection;
 
            PersonStart dragItem = dropInfo.DragInfo.SourceItem as PersonStart;
            PersonStart dropItem = dropInfo.TargetItem as PersonStart;
 
            bool dropAllowed = (targetCollection == sourceCollection ||
                                targetCollection != sourceCollection && targetCollection.Count + 1 <= MaxItemsInTargetCollection);
            if (!CanAcceptData(dropInfo))
            {
                return false;
            }
            else if (dragItem != null && dropItem != null)
            {
                return dropAllowed && 
                       dragItem.Style == dropItem.Style &&
                       dragItem.CompetitionObj?.Distance == dropItem.CompetitionObj?.Distance;
            }
            else if (dragItem != null && dropItem == null && targetCollection.Count == 0)
            {
                return dropAllowed;
            }
            else if (dragItem != null && dropItem == null && targetCollection.Count > 0)
            {
                PersonStart firstStart = (targetCollection as IList)?.Cast<PersonStart>().FirstOrDefault();
 
                return dropAllowed &&
                       dragItem.Style == firstStart?.Style &&
                       dragItem.CompetitionObj?.Distance == firstStart?.CompetitionObj?.Distance;
            }
            return false;
        }
 
    }
}