Question

我有以下情况：

public JsonResult ChangeFilterList(int option)
{
    var data = new[] { new { Text = "Unknown option", Value = -1 } };
    switch (option)
    {
        case 2: data = _departmentnameRepository.All.Select(x => new { Text = x.DeptName, Value = x.Id }).ToArray();
            break;
        case 3: data = Session["projectid"] == null
                ? _assetSequenceRepository.All.Select(x => new { Text = x.AssetShotName, Value = x.Id }).ToArray()
                : _assetSequenceRepository.FindBy(p => p.ProjectId == (int)Session["projectid"]).Select(x => new { Text = x.AssetShotName, Value = x.Id }).ToArray();
            break;
        default: data = _userRepository.All.Select(x => new { Text = x.DisplayName, Value = x.UserID }).ToArray();
            break;
    }            

    return Json(data, JsonRequestBehavior.AllowGet);
}

case2和default看起来不错，但在案例3（有条件）上抱怨：Cannot implicitly convert type 'AnonymousType#1[]' to 'AnonymousType#2[]'。我不应该?:能够决定类型，因为我已经为匿名提供了var data = new[] { new { Text = "Unknown option", Value = -1 } };的蓝图。

解决方案：

@Darin Dimitrov答案很棒，但我希望对匿名类型进行一些测试（简单情况总是需要它）。正如@Douglas所怀疑：我的assetSequenceRepository正在提供id long而匿名Value则支持int而不是long。由于C＃编译器没有隐式地将long强制转换为int，我得到了错误。编译片段是：

public JsonResult ChangeFilterList(int option = 3)
        {
            var data = new[] { new { Text = "Unknown option", Value = long.MaxValue } };
            switch (option)
            {
                case 2: data = _departmentnameRepository.All.Select(x => new { Text = x.DeptName, Value = (long)x.Id }).ToArray();
                    break;
                case 3: data = Session["projectid"] == null
                        ? _assetSequenceRepository.All.Select(x => new { Text = x.AssetShotName, Value = x.Id }).ToArray()
                        : _assetSequenceRepository.FindBy(p => p.ProjectId == (int)Session["projectid"]).Select(x => new { Text = x.AssetShotName, Value = x.Id }).ToArray();
                    break;
                default: data = _userRepository.All.Select(x => new { Text = x.DisplayName, Value = (long)x.UserID }).ToArray();
                    break;
            }            

            return Json(data, JsonRequestBehavior.AllowGet);
        }

Answer 1

您正在将编译器置于真实测试中。只需编写一个视图模型来结束其痛苦并使事情更明确：

public class MyViewModel
{
    public int Value { get; set; }
    public string Text { get; set; }
}

然后将LINQ查询投影到此视图模型，以避免使用条件运算符和匿名类型可能产生的任何歧义：

public ActionResult ChangeFilterList(int option)
{
    var data = new[] 
    { 
        new MyViewModel { Text = "Unknown option", Value = -1 } 
    };

    switch (option)
    {
        case 2: data = _departmentnameRepository
            .All
            .Select(x => new MyViewModel { Text = x.DeptName, Value = x.Id })
            .ToArray();
            break;
        case 3: data = Session["projectid"] == null
                ? _assetSequenceRepository
                    .All
                    .Select(x => new MyViewModel { Text = x.AssetShotName, Value = x.Id })
                    .ToArray()
                : _assetSequenceRepository
                    .FindBy(p => p.ProjectId == (int)Session["projectid"])
                    .Select(x => new MyViewModel { Text = x.AssetShotName, Value = x.Id })
                    .ToArray();
            break;
        default: data = _userRepository
            .All
            .Select(x => new MyViewModel { Text = x.DisplayName, Value = x.UserID })
            .ToArray();
            break;
    }            

    return Json(data, JsonRequestBehavior.AllowGet);
}

Answer 2

我的猜测是，您的FindBy方法会返回其属性与您期望的类型不同的对象（例如int?而不是int）。尝试指定类型转换以确保您的匿名类型具有正确的定义：

case 3: data = Session["projectid"] == null
             ? _assetSequenceRepository.All.Select(x => new { Text = x.AssetShotName, Value = x.Id }).ToArray()
             : _assetSequenceRepository.FindBy(p => p.ProjectId == (int)Session["projectid"]).Select(x => new { Text = (string)x.AssetShotName, Value = (int)x.Id }).ToArray();
        break;

关键变化是：

new { Text = (string)x.AssetShotName, Value = (int)x.Id })
                  ↖    explicit type casts    ↗

Answer 3

以下测试代码编译得很好：

public void Test(int option, string parameter)
{
    var data = new[] { new { Text = "Unknown option", Value = -1 } };

    switch(option)
    {
        case 2:
            data = Enumerable.Range(1, 4)
                             .Select(x => new { Text = x.ToString(), Value = x })
                             .ToArray();
            break;
        case 3:
            data = (new Random()).Next(2) % 2 == 1
                ? Enumerable.Range(1, 6)
                            .Select(x => new { Text = x.ToString(), Value = x })
                            .ToArray()
                : Enumerable.Range(1, 2)
                            .Select(x => new { Text = x.ToString(), Value = x })
                            .ToArray();
            break;
        default:
            data = Enumerable.Range(1, 3)
                             .Select(x => new { Text = x.ToString(), Value = x })
                             .ToArray();
            break;
    }
}

我只将您的存储库调用更改为Enumerable.Range()和Select lambda，以获取正确的string / int属性值。

我猜，你已经显示出使用的不完全是你想要编译的代码。在您的真实代码中，您有一个具有不匹配名称的属性（例如，不正确的大小写）或不正确的类型。

您可以通过将鼠标指针悬停在ToArray()电话上来尝试解决问题。工具提示上会打印a is new { string Text, int Value }。

无法将匿名类型＃1 []隐式转换为匿名类型＃2 []

3 个答案: