Razor 页面模型绑定错误上的 SelectList

Question

我正在尝试按照此处的示例代码进行操作，但我一定遗漏了一些明显的内容。我没有从数据源中读取我的选择列表选项，而是尝试在构造函数中加载它们。但我不断收到错误消息。

InvalidOperationException: Could not create an instance of type 'Microsoft.AspNetCore.Mvc.Rendering.SelectList'. Model bound complex types must not be abstract, or value types and must have a parameterless constructor. Alternatively, set the 'SearchOptions' property to a non-null value.

这是CS代码：

public class TestSelectModel : PageModel
{
    private List<SelectListItem> _searchoptions;
    [BindProperty(SupportsGet = true)]
    public SelectList SearchOptions { get; set; }
    public TestSelectModel()
    {
        _searchoptions = new List<SelectListItem>();
        _searchoptions.Add(new SelectListItem("By Email", "By Email", true));
        _searchoptions.Add(new SelectListItem("By Request Name", "By Request Name", false));
    }
    public void OnGet()
    {
        SearchOptions = new SelectList(_searchoptions, "Key", "Value", "By Email");

    }
}

这是 cshtml 代码：

<h2>TestSelect</h2>
<form>
    <div>
        <select asp-items="Model.SearchOptions">
            <option value="">Choose an search method</option>
        </select>
        <input type="submit" value="Filter" />
    </div>
</form>

我有一种感觉，这很容易，但我就是看不到它。

编辑

每个评论还尝试了下面的代码，它确实运行了 OnGet() 函数，但它抛出NullReferenceException: Object reference not set to an instance of an object.

private List<SelectListItem> _searchoptions;
public SelectList SearchOptions { get; set; }

[BindProperty(SupportsGet = true)]
public string BoundSearchField { get; set; }

public TestSelectModel()
{
    _searchoptions = new List<SelectListItem>();
    _searchoptions.Add(new SelectListItem("By Email", "By Email", true));
    _searchoptions.Add(new SelectListItem("By Request Name", "By Request Name", false));
}
public void OnGet()
{
    SearchOptions = new SelectList(_searchoptions, "Key", "Value", "By Email");
}


<form>
    <div>
        <select name="BoundSearchField" asp-items="Model.SearchOptions">
            <option value="">Choose an search method</option>
        </select>
        <input type="submit" value="Filter" />
    </div>
</form>

Answer 1

原始问题的答案

为什么我收到这个错误 InvalidOperationException: Could not create an instance of type 'Microsoft.AspNetCore.Mvc.Rendering.SelectList'. Model bound complex types must not be abstract or value types and must have a parameterless constructor. Alternatively, set the 'SearchOptions' property to a non-null value

BindProperty在您的属性上具有该属性SearchOptions将尝试将标头数据绑定到您的SelectList. 从您的选择列表中删除 BindPropery 属性并创建一个属性来接收所选值。

您编辑中问题的答案

现在我收到错误 NullReferenceException: Object reference not set to an instance of an object

是因为"Key"应该是"Text"

更改 SearchOptions = new SelectList(_searchoptions, "Key", "Value", "By Email");
为SearchOptions = new SelectList(_searchoptions, "Text", "Value", "By Email");

完整代码：

private List<SelectListItem> _searchoptions;

// [BindProperty(SupportsGet = true)] // Remove this here
public SelectList SearchOptions { get; set; }

// Add a property to receive your selected value
[BindProperty(SupportsGet = true)]
public string BoundSearchField { get; set; }

public TestSelectModel()
{
    _searchoptions = new List<SelectListItem>();
    _searchoptions.Add(new SelectListItem("By Email", "By Email", true));
    _searchoptions.Add(new SelectListItem("By Request Name", "By Request Name", false));
}
public void OnGet()
{
    // Change "Key" to "Text"
    SearchOptions = new SelectList(_searchoptions, "Text", "Value", "By Email");
}

然后添加name="BoundSearchField"到您的 .cshtml

<form>
    <div>
        <select name="BoundSearchField" asp-items="Model.SearchOptions">
            <option value="">Choose an search method</option>
        </select>
        <input type="submit" value="Filter" />
    </div>
</form>