C# 将 List<List<object>> 转换为 IList<IList<object>>
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convert List<List<object>> to IList<IList<object>>
提问by david
I have written a method which is
public List<List<object>> Fetch(string data),
inside I create
List<List<object>> p = new List<List<object>>();
我写了一个方法
public List<List<object>> Fetch(string data)，在里面我创建
List<List<object>> p = new List<List<object>>();
my boss now wants to return a IList<IList<object>>instead of List<List<object>>iepublic IList<IList<object>> Fetch(string data),
我的老板现在想要返回一个IList<IList<object>>而不是List<List<object>>ie public IList<IList<object>> Fetch(string data)，
so when I try do
return (IList<IList<object>>) p; //throws an exception
所以当我尝试做
return (IList<IList<object>>) p; //throws an exception
How do I convert
List<List<object>>to IList<IList<object>>and back to List<List<object>>
我如何转换
List<List<object>>为IList<IList<object>>和返回List<List<object>>
采纳答案by Jon Skeet
You can't perform that conversion via straight casting - it wouldn't be safe. Instead, you should use:
你不能通过直接转换来执行这种转换——这不安全。相反，您应该使用：
IList<IList<object>> ret = new List<IList<object>>();
Then for each "sublist" you can use:
然后对于每个“子列表”，您可以使用：
// Or whatever
ret.Add(new List<object>());
Finally, just return ret.
最后，只需 return ret。
You coulduse LINQ to perform the conversion of your existing List<List<object>>when you return it - but it would be better to just create a more appropriate type to start with, as shown above.
您可以List<List<object>>在返回时使用 LINQ 来执行现有类型的转换- 但最好先创建一个更合适的类型，如上所示。
To understand why some of the existing answers are wrong, suppose you coulddo this:
要了解为什么某些现有答案是错误的，假设您可以这样做：
IList<IList<object>> p = new List<List<object>>();
Then this would be valid:
那么这将是有效的：
List<List<object>> listOfLists = new List<List<object>>();
IList<IList<object>> p = listOfLists;
p.Add(new object[]);
List<object> list = p[0];
But p[0]is a reference to an object[], not a List<object>... our supposedly type-safe code doesn't look as safe any more...
但是p[0]是对 an 的引用object[]，而不是List<object>...我们所谓的类型安全代码看起来不再那么安全了...
Fortunately, IList<T>is invariant to prevent exactly this problem.
幸运的是，IList<T>不变式正好防止了这个问题。
回答by Aliostad
List<T>and IList<T>do not support covariance. So this does not compile:
List<T>并且IList<T>不支持协方差。所以这不会编译：
List<List<string>> list = new List<IList<string>>(); // DOES NOT COMPILE!!!!!
回答by Chris Shain
var myOriginalList = new List<List<Object>>();
var converted = ((IList<IList<Object>>)myOriginalList.Cast<IList<Object>>().ToList()); // EDIT: Added .ToList() here
You shouldn't need to convert back- you can do just about anything on IListthat you could on List.
你不应该需要转换的后台，你可以做几乎任何事情IList，你可以上List。
回答by John Ruiz
public IList<IList<object>> Fetch(string data)
{
IList<IList<object>> p = new List<IList<object>>();
// add your stuff here
return p;
}
回答by Olivier Jacot-Descombes
You would have to declare your list as
您必须将列表声明为
IList<IList<object>> list = new List<IList<object>>(); // Works!
This works, because only the outer list is created in the first place. You can then insert individual items that are compatible with IList<object>:
这是有效的，因为首先只创建了外部列表。然后，您可以插入与 兼容的单个项目IList<object>：
list.Add(new List<object>());
list.Add(new object[10]);
回答by Muhammad Hasan Khan
You need to change the declaration of the result variable from List<List<object>to IList<IList<object>>
您需要将结果变量的声明从List<List<object>更改为IList<IList<object>>
Which you can instantiate against List<IList<object>>
你可以实例化 List<IList<object>>
And each item in the result can be of type List<object>
结果中的每个项目都可以是类型 List<object>
static IList<IList<object>> Test()
{
IList<IList<object>> result = new List<IList<object>>();
var item = new List<object>() { "one", "two", "three" };
result.Add(item);
return result;
}
