为什么我的 SelectList 不喜欢 IList 的内容？

问题描述

这是我的 Razor 页面中的相关代码：我收到一条错误消息，指出对象（在选择元素行上）未设置为对象的实例。

<select asp-for="EmployeeLocation.EmployeeID" asp-items="@Model.EmployeeSelectList">

这是我的 PageModel 类中的相关代码：

public EmployeeLocation EmployeeLocation { get; set; }
public Employee Employee { get; set; }

public class JoinResult
{
    public int EmployeeID;
    public string LastName;
}
private IQueryable<JoinResult> JoinResultIQueryable;
private IList<JoinResult> JoinResultIList;
public SelectList EmployeeSelectList;

public async Task OnGetAsync()
{

    // Get the employee's locations.
    List<int> EmployeeLocationsList = HttpContext.Session.Getobject<List<int>>("EmployeeLocationsList");

    // Populate the employee select list.

    JoinResultIQueryable = (
        from e in IDDSContext.Employee
        join p in IDDSContext.Position on e.PositionID equals p.PositionID
        join el in IDDSContext.EmployeeLocation on e.EmployeeID equals el.EmployeeID
        where e.Active == true
            && e.PositionID != 1 // Do not display the super administrator's data.
            && EmployeeLocationsList.Contains(el.LocationID)
        select new JoinResult
        {
            EmployeeID = e.EmployeeID,LastName = e.LastName
        });

    JoinResultIList = await JoinResultIQueryable
        .distinct()
        .OrderBy(jr => jr.LastName)
        .ToListAsync();

    // display the results in the Output window. Just done to show contents of JoinResultIList (see below).
    foreach (var item in JoinResultIList)
    {
        Debug.WriteLine("[" + item.EmployeeID + "][" + item.LastName + "]");
    }

    // ***** The problem seem to occur here. Evidently,the SelectList doesn't like contents of the IList. Why is this happening?
    EmployeeSelectList = new SelectList(JoinResultIList,"EmployeeID","LastName");

}

以下是 JoinResultIList 的内容（在输出窗口中）：

[4][Anderson (OH)]
[30][Becon (OH)]
[26][Smith (OH)]
[25][Stevens (OH)]

解决方法

好的。所以，这是我想出的解决方案。

// Get the employee's locations.
List<int> EmployeeLocationsList = HttpContext.Session.GetObject<List<int>>("EmployeeLocationsList");

// Populate the employee select list.
IQueryable JoinResultIQueryable = (
    from e in IDDSContext.Employee
    join p in IDDSContext.Position on e.PositionID equals p.PositionID
    join el in IDDSContext.EmployeeLocation on e.EmployeeID equals el.EmployeeID
    where e.Active == true
        && e.PositionID != 1 // Do not display the super administrator's data.
        && EmployeeLocationsList.Contains(el.LocationID)
    orderby e.LastName,e.FirstName
    select new
    {
        EmployeeID = e.EmployeeID,Employee = $"{e.LastName},{e.FirstName} ({p.Position1})",});
EmployeeSelectList = new SelectList(JoinResultIQueryable,"EmployeeID","Employee");