C# 错误：无法将类型“void”隐式转换为“System.Collections.Generic.List”
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Error: Cannot implicitly convert type 'void' to 'System.Collections.Generic.List'
提问by PositiveGuy
I am trying to set a property of my .ascx controls from an .aspx using that control.
我正在尝试使用该控件从 .aspx 设置我的 .ascx 控件的属性。
So in one of my .aspxwhich has this control in it, I have the following code trying to set the ItemsList property of my embedded .ascx:
因此，在其中包含此控件的.aspx之一中，我有以下代码尝试设置嵌入的 .ascx 的 ItemsList 属性：
Item item = GetItem(itemID);
myUsercontrol.ItemList = new List<Item>().Add(item);
The property in the .ascx that I'm trying to set looks like this:
我试图设置的 .ascx 中的属性如下所示：
public List<Item> ItemsList
{
get { return this.itemsList; }
set { this.itemsList = value; }
}
Error: Cannot implicitly convert type 'void' to 'System.Collections.Generic.List'
错误：无法将类型“void”隐式转换为“System.Collections.Generic.List”
So I don't get where it's getting void as part of the property?...weird.
所以我不明白它作为财产的一部分在哪里变得无效？......奇怪。
采纳答案by Mark Byers
You can't do that because the Add function returns void, not a reference to the list. You can do this:
您不能这样做，因为 Add 函数返回 void，而不是对列表的引用。你可以这样做：
mycontrol.ItemList = new List<Item>();
mycontrol.ItemList.Add(item);
or use a collection initializer:
或使用集合初始值设定项：
mycontrol.ItemList = new List<Item> { item };
回答by shahkalpesh
ItemListt.ItemList = new List<Item>().Add(item);
Does Addmethod return an instance of a list based class?
Add方法是否返回基于列表的类的实例？
EDIT: No, see this linkfor documentation on List<T>.Add
编辑：不，有关文档，请参阅此链接List<T>.Add
EDIT2: Try writing this piece of code & see what is the return value of Addmethod.
EDIT2：尝试编写这段代码并查看Add方法的返回值是什么。
List<Item> items = new List<Item>();
var modifiedList = items.Add(myItem);
You will see that, the code should fail at Addbecause it returns void.
您将看到，代码应该失败，Add因为它返回void.
回答by Lee
After creating the List<Item>you're immediately calling Add on it, which is a method returning void. This cannot be converted to the type of ItemList.ItemList.
创建之后，List<Item>您立即对其调用 Add，这是一个返回 void 的方法。这不能转换为 ItemList.ItemList 的类型。
You should do this instead:
你应该这样做：
var list = new List<Item>();
list.Add(item);
ItemList.ItemList = list;
回答by Scott Anderson
So long as ItemListis instantiated on your usercontrol, you can just do UserControl.ItemList.Add(item)
只要ItemList在您的用户控件上实例化，您就可以UserControl.ItemList.Add(item)
回答by Guffa
The Add method doesn't return a reference to the list. You have to do this in two steps:
Add 方法不返回对列表的引用。您必须分两步执行此操作：
ItemListt.ItemList = new List<Item>();
ItemListt.ItemList.Add(item);
Alternatively use a local variable to hold the reference before you put it in the property:
或者，在将引用放入属性之前，使用局部变量来保存引用：
List<Item> list = new List<Item>();
list.Add(item);
ItemListt.ItemList = list;
回答by mYsZa
new List<Item>().Add(item);
This line returns void.
此行返回无效。
Try:
尝试：
var list = new List<Item>();
list.Add(item);
ItemListt.ItemList = list;
回答by jeromej
For others who might come here from Googling the error:
对于可能通过谷歌搜索错误来到这里的其他人：
Sometimes it's because you didn't finish writing a line but it compiles anyway and the error only shows up on the next line which can be confusing/misleading if you don't pay attention. Eg:
有时是因为你没有写完一行，但它仍然编译，错误只出现在下一行，如果你不注意，这可能会造成混淆/误导。例如：
var foo = 33;
var bar =
// Note that I never finished the assignation (got distracted, saved and it compiled)
//
// Some unrelated code returning "void" (another example would be List.Add)
await SomethingAsync(); // -> Error shows up here instead
Might save some of you all some headache trying to figure why "awaiting on my function" raises a "Cannot convert void to X", when really it's not the actual issue.
可能会让你们中的一些人头疼，试图弄清楚为什么“等待我的函数”会引发“无法将 void 转换为 X”，而实际上这不是实际问题。
