比较int时Linq to Entities错误？ 值

Question

我试图编写一个遍历树的递归函数。 我的实体“文件夹”具有ParentFolderID和FolderID的两个属性。 如果文件夹位于根级别，则其ParentFolderID为null。 我创建了一个递归函数，该函数接受参数ID并添加返回与ID具有相同ParentFolderID的所有文件夹实体，但是出现了“无法创建类型为System.Object”的常量的错误。 在这种情况下，仅支持原始类型或枚举类型。

这是我的功能：

    private List<Folder> GetFolderChildren(int? folderId)
    {
        var childFolders = new List<Folder>();

        //  finalList is List<> that is actually returned
        var finalList = new List<Folder>();

        //  Your looking at the root level folder
        if (folderId == null)
        {
            childFolders = DocumentEntities.Folders.Where(x => x.ParentFolderID.Equals(folderId)).ToList();
        }
        else
        {
            childFolders = DocumentEntities.Folders.Where(x => x.ParentFolderID == folderId).ToList();
        }

        //  If there are no childFolders, then you've hit the bottom of this branch.  Return this folder.
        if (!childFolders.Any())
        {
            finalList.Add(DocumentEntities.Folders.SingleOrDefault(x => x.FolderID == folderId));
        }
        else
        {
            foreach (var childFolder in childFolders)
            {
                finalList.AddRange(GetFolderChildren(childFolder.FolderID, pageName));
            }

            //  Now that you've gone through this folders children, add this folder
            finalList.Add(DocumentEntities.Folders.SingleOrDefault(x => x.FolderID == folderId));
        }

        return finalList;
    }

当folderID为null或int时，它永远不会到达foreach。 有任何想法吗？

Answer 1

如果x.ParentFolderID为null ，则您将无法通过Equals方法访问它，因为它为null。 尝试将整个值与参数进行比较，因为它的类型相同，例如

childFolders = Entities.Folders.Where(x => x.ParentFolderID == folderId).ToList();