【Unity】UnityEditor でドラッグアンドドロップ処理

using UnityEditor;
using UnityEngine;
using Object = UnityEngine.Object;
 
public static class DragAndDropUtility
{
    /// <summary>
    /// D&Dを開始する
    /// </summary>
    public static void Begin(Object[] objs, object data)
    {
        DragAndDrop.PrepareStartDrag();
        DragAndDrop.paths = null;
        DragAndDrop.objectReferences = objs;
        DragAndDrop.SetGenericData("data", data);
        DragAndDrop.StartDrag(objs.Length == 1 ? objs[0].name : "<Multiple>");
    }
    
    /// <summary>
    /// D&Dを受け付ける
    /// </summary>
    /// <param name="window"></param>
    /// <param name="rect"></param>
    /// <returns></returns>
    public static bool Check(EditorWindow window, Rect? rect = null)
    {
        var ev = Event.current;
        // エリアが指定されていれば範囲内か確認
        if (rect.HasValue && !rect.Value.Contains(ev.mousePosition))
            return false;
 
        if (ev.type != EventType.DragUpdated && ev.type != EventType.DragPerform)
            return false;
 
        var paths = DragAndDrop.paths;
        if (EditorWindow.mouseOverWindow != window || paths.Length <= 0)
            return false;
                
        DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
        if (ev.type == EventType.DragPerform)
        {
            DragAndDrop.activeControlID = 0;
            DragAndDrop.AcceptDrag();
            return true;
        }
 
        DragAndDrop.activeControlID = GUIUtility.GetControlID(FocusType.Passive);
        Event.current.Use();
 
        return false;
    }
}

サンプル

using UnityEditor;
using UnityEngine;
 
public class SampleEditorWindow : EditorWindow
{
    private void OnGUI()
    {
        if (DragAndDropUtility.Check(this))
        {
            foreach (var path in DragAndDrop.paths)
            {
                Debug.Log(path);
            }
        }
    }
}