Error: Cannot implicitly convert type ‘void’ to ‘System.Collections.Generic.List’

I am trying to set a property of my .ascx controls from an .aspx using that control.

So in one of my .aspx which has this control in it, I have the following code trying to set the ItemsList property of my embedded .ascx:

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:

public List<Item> ItemsList
{
   get { return this.itemsList; }
   set { this.itemsList = value; }
}

Error: Cannot implicitly convert type ‘void’ to ‘System.Collections.Generic.List’

So I don’t get where it’s getting void as part of the property?…weird.

Answers:

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Method 1

You can’t do that because the Add function returns void, not a reference to the list. You can do this:

mycontrol.ItemList = new List<Item>();
mycontrol.ItemList.Add(item);

or use a collection initializer:

mycontrol.ItemList = new List<Item> { item };

Method 2

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.

You should do this instead:

var list = new List<Item>();
list.Add(item);
ItemList.ItemList = list;

Method 3

new List<Item>().Add(item);

This line returns void.

Try:

var list = new List<Item>();
list.Add(item);
ItemListt.ItemList = list;

Method 4

ItemListt.ItemList = new List<Item>().Add(item);

Does Add method return an instance of a list based class?

EDIT: No, see this link for documentation on List<T>.Add

EDIT2: Try writing this piece of code & see what is the return value of Add method.

List<Item> items = new List<Item>();
var modifiedList = items.Add(myItem);

You will see that, the code should fail at Add because it returns void.

Method 5

So long as ItemList is instantiated on your usercontrol, you can just do UserControl.ItemList.Add(item)

Method 6

The Add method doesn’t return a reference to the list. You have to do this in two steps:

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:

List<Item> list = new List<Item>();
list.Add(item);
ItemListt.ItemList = list;

Method 7

For others who might come here from Googling the error:

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:

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.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0