[英]Unable to convert List<List<int>> to return type IList<IList<int>>
For level order traversal why does this exception occur?对于层序遍历为什么会出现这个异常? Following exception occurs:出现以下异常:
Cannot implicitly convert type '
System.Collections.Generic.List<System.Collections.Generic.List<int>>
' to 'System.Collections.Generic.IList<System.Collections.Generic.IList<int>>
'.无法将类型“System.Collections.Generic.List<System.Collections.Generic.List<int>>
”隐式转换为“System.Collections.Generic.IList<System.Collections.Generic.IList<int>>
”。 An explicit conversion exists (are you missing a cast?)存在显式转换(您是否缺少演员表?)
public IList<IList<int>> LevelOrder(TreeNode root)
{
var result = new List<List<int>>();
var que = new Queue<TreeNode>();
//if(root==null) return result;
que.Enqueue(root);
while(que.Count!=0)
{
int n = que.Count;
var subList = new List<int>();
for(int i=0;i<n;i++)
{
if(que.Peek().left!=null)
que.Enqueue(que.Peek().left);
if(que.Peek().right!=null)
que.Enqueue(que.Peek().right);
subList.Add(que.Dequeue().val);
}
result.Add(subList);
}
return result;
}
Just change the declaration of your result to List<IList<int>>
.只需将结果声明更改为List<IList<int>>
。
List<T>
implements IList<T>
, but List<List<T>>
does not implement IList<IList<int>>
. List<T>
实现了IList<T>
,但List<List<T>>
没有实现IList<IList<int>>
。 Generic parameters are not covariant or contravariant unless defined that way and IList<T>
is not, so the type must match exactly.泛型参数不是协变或逆变的,除非以这种方式定义而IList<T>
不是,因此类型必须完全匹配。
public IList<IList<int>> LevelOrder(TreeNode root)
{
var result = new List<IList<int>>();
var que = new Queue<TreeNode>();
//if(root==null) return result;
que.Enqueue(root);
while (que.Count != 0)
{
int n = que.Count;
var subList = new List<int>();
for (int i = 0; i < n; i++)
{
if (que.Peek().left != null)
que.Enqueue(que.Peek().left);
if (que.Peek().right != null)
que.Enqueue(que.Peek().right);
subList.Add(que.Dequeue().val);
}
result.Add(subList);
}
return result;
}
Pretty sure that if it compiles, doing a cast is a real bad idea.可以肯定的是,如果它可以编译,那么进行强制转换是一个非常糟糕的主意。 Here's why:原因如下:
public class myStupidList : IList<int>
{
//implementation unimportant
}
private void Button_Click(object sender, RoutedEventArgs e)
{
var result = new List<List<int>>();
IList<IList<int>> imNotAListofLists = (IList<IList<int>>)result;
imNotAListofLists.Add(new myStupidList());
//result is not a very valid variable right now, is it?
}
As mentioned in my comment, these types of collection issues boil down to covariance
and contravariance
, and .NET does provide a lot of tools for dealing with them.正如我在评论中提到的,这些类型的集合问题归结为covariance
和contravariance
,.NET 确实提供了很多工具来处理它们。 (Such as various read-only collections and interfaces) (比如各种只读集合和接口)
..Which, explains why you get your error as well. ..Which 解释了为什么你也会得到你的错误。 There is no implicit cast from List<List>
to List<IList>
because that cast cannot succeed without breaking type-safety.没有从List<List>
到List<IList>
隐式转换,因为如果不破坏类型安全,转换就不会成功。 (and as @Grax mentioned, neither derives from the other) (正如@Grax 提到的,两者都不是从另一个派生的)
There should be explicit conversion and as shown below:-应该有显式转换,如下所示:-
List<IList<int>> result = new List<IList<int>>();
or要么
var result = new List<IList<int>>();
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