将C＃Covert类型转换为相同类型的IEnumerable？ [英] C# Covert Type into IEnumerable of same Type?

问题描述

我有一些方法可以对数据库执行任意SQL，并将该数据集合序列化为具体类型的列表。然后将该数据序列化为JSON并存储在表的单元格中。后来，我需要返回并将这些数据反序列化为原始数据集，以便可以使用它。

I have some methods that execute arbitrary SQL against a database and serialize that data collection into a list of a concrete type. That data is then serialized into JSON and stored in a cell in a table. Later, I need to come back and deserialize that data back into its original collection so that it can be used.

我遇到了一些问题，需要弄清楚如何获取 Type 对象并创建该类型的集合以反序列化它。这是我的代码的运行方式：

I'm having some issues figuring out how to take a Type object and create a collection of that type in order to deserialize it. Here is how my code operates:

public async Task ExecuteWidget(Guid runGuid, string widgetName, Type type, string sql,
    IEnumerable<SqlParameter> parameters)
{
    var report = operationsContext.ReportRuns.FirstOrDefault(n => n.RunGuid == runGuid);

    CheckReportStatus(report);

    var param = parameters.ToList();
    var result = edwContext.Database.SqlQuery(type, sql, param.ToArray<object>());
    var query = result.GetQuery(param);
    var data = await result.ToListAsync();
    var widgetData = new ReportRunWidgetData()
    {
        ReportRunId = report?.ReportRunId ?? -1, // This should never be null.
        WidgetName = widgetName,
        WidgetData = new JavaScriptSerializer().Serialize(data),
        Query = query
    };

    operationsContext.ReportRunWidgetDatas.Add(widgetData);
    await operationsContext.SaveChangesAsync();
}

我的提取逻辑如下：

public object FetchWidgetData(Guid runGuid, string widgetName, Type dataType)
{
    var data = operationsContext.ReportRuns
        .Include("ReportRunWidgetDatas")
        .FirstOrDefault(n => n.RunGuid == runGuid)?
        .ReportRunWidgetDatas.FirstOrDefault(n => n.WidgetName == widgetName)?
        .WidgetData;

    if (data == null) return null;

    var deserialized = new JavaScriptSerializer().Deserialize(data, dataType);

    return deserialized;
}

现在，当 ExecuteWidget 方法被调用， type 参数由小部件的DTO数据类型填充。例如 HeadlineWidgetDTO 。但是，execute命令以 List< HeadlineWidgetDTO> 的形式获取数据。调用我的 FetchWidgetData 方法时，提供的 dataType 仍为 HeadlineWidgetDTO ，但实际上它的类型必须为 IEnumerable< HeadlineWidgetDTO> 才能正确反序列化。

Now when the ExecuteWidget method is called, the type parameter is populated by the widget's DTO datatype. For example HeadlineWidgetDTO. However, the execute command gets the data back as a List<HeadlineWidgetDTO>. When my FetchWidgetData method is called, the dataType supplied is still HeadlineWidgetDTO, but it actually needs to be of type IEnumerable<HeadlineWidgetDTO> to deserialize properly.

仅给出单个数据行的类型，我如何创建 Type 对象，而是一个集合

Given just the type for an individual data row, how can I create a Type object that is instead a collection of that type?

推荐答案

这主要是如何使用激活器创建通用类型的实例并将其强制转换，但是很难说。

This is mostly a duplicate of How to use Activator to create an instance of a generic Type and casting it back to that type?, however it's hard to tell.

基本上，如果您有类型对象键入theType ，则需要执行以下操作：

Basically, if you have a type object Type theType, you need to do something like:

 var listType = typeof(List<>);
 var typeParams = new [] {theType};
 var listOfTType = listType.MakeGenericType(typeParams);
 var newListOfT = Activator.CreateInstance(listOfTType);

此时，您有一个 object ，但这引用了类型 List< WhateverYourTypeIs> 的对象。假设 theType 是 typeof（int），那么您将拥有 List<的对象。 int> 。但是，将其强制转换为可用的东西是另一个问题。如果您想在列表中添加一些内容，我怀疑最好的方法是为添加获取 MethodInfo 方法和调用它。

At that point, you have a variable of type object, but that references an object of type List<WhateverYourTypeIs>. Say, theType is typeof(int), then you will have an object of List<int>. Casting it to something usuable is a whole other question though. If you want to add something to that list, I suspect the best way would be to get a MethodInfo for the Add method and Invoke it.

我想到了另一种方法，如果类型具有默认构造函数并且不是创建起来并不太昂贵。这是一个示例（创建 List< int> -但这只是我的编码方式）：

I thought of another way to do this if the type has a default constructor and isn't too expensive to create. Here's a sample (creating a List<int> - but that's just the way I have it coded):

  var type = typeof(int);
  var dummy = Activator.CreateInstance(type);
  var listOfType = new[] {dummy}.ToList();

完成后， listOfType 变量类型为 List< object> ，但引用 List< int> 。它大部分都可以使用-例如，您可以在其上调用 Add（object someObj）。您不会进行编译类型参数类型检查，但是可以使用它。

When you are finished, the listOfType variable is typed as a List<object> but refers to a List<int>. It's mostly mostly workable - for example, you can call Add(object someObj) on it. You won't get compile type parameter type checking, but you will be able to use it.

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