"Operation is not valid due to the current state of the object." Exception, when I want to retrieve Items - linq

Question

I use this method to retrieve Items for a Tree component in Blazor Serverside, In the DAL I have:
public List<TreeItem> GetTreeItems()
{
var tree = new List<TreeItem>();
TreeItem item = new TreeItem()
{
DepartmentID = 0,
CategoryID = 0,
Text = "Root",
Childs = context.Departments.OrderBy(d => d.Order).Select(d => new TreeItem()
{
DepartmentID = d.Id,
CategoryID = 0,
Text = d.Title,
Childs = d.Categories.OrderBy(c => c.Order).Select(c => new TreeItem()
{
DepartmentID = d.Id,
CategoryID = c.Id,
Text = c.Title
}).ToList()
}).ToList()
};
tree.Add(item);
return tree;
}
The TreeItem class is the following, (The model shared by the blazor Component and Dal Class):
public class TreeItem
{
public int DepartmentID { get; set; }
public int CategoryID { get; set; }
public string Text { get; set; }
public List<TreeItem> Childs { get; set; }
}
But when I was to retrieve Items for the tree in the blazor component I get the exception: Operation is not valid due to the current state of the object., admin is the DAL class I inject to Blazor component as follows:
private void GetTreeModel()
{
try
{
Items = admin.GetTreeItems();
TreeSuccess = true;
TreeMessage = "Success";
return;
}
catch (Exception ex) // Error here
{
TreeSuccess = false;
TreeMessage = "Can not load tree items";
return;
}
}
What is this error and How to solve it?

Answer 1

I solved my problem using First loading entities and then using Linq to Objects, Like this:
var tree = new List<TreeItem>();
var departments = context.Departments.OrderBy(d => d.Order).ToList();
var categories = context.Categories.OrderBy(c => c.Order).ToList();
TreeItem item = new TreeItem()
{
DepartmentID = 0,
CategoryID = 0,
Text = "Root",
Childs = departments.Select(d => new TreeItem()
{
DepartmentID = d.Id,
CategoryID = 0,
Text = d.Title,
Childs = categories.Where(c => c.DepartmentID == d.Id).OrderBy(c => c.Order).Select(c => new TreeItem()
{
DepartmentID = d.Id,
CategoryID = c.Id,
Text = c.Title
}).ToList()
}).ToList()
};
tree.Add(item);
return tree;
}