从特定索引处的可观察集合中检索项目

Question

I am trying to retrieve items at specific index locations from an ObservableCollection. According to MSDN , there are properties Item and Items .

private ObservableCollection<string> _strings = new ObservableCollection<string>();
string item1;
item1 _strings.Item[0];
item1 _strings.Items[0];

When I use Item , I get:

'System.Collections.ObjectModel.ObservableCollection' does not contain a definition for 'Item' and no extension method 'Item' accepting a first argument of type 'System.Collections.ObjectModel.ObservableCollection' could be found (are you missing a using directive or an assembly reference?)

And when I use Items , I get:

Cannot access protected member 'System.Collections.ObjectModel.Collection.Items' via a qualifier of type 'System.Collections.ObjectModel.ObservableCollection'; the qualifier must be of type 'Model.MyStringCollection' (or derived from it)

I just cannot see what I am doing wrong here, right now.

Answer 1

You just call it like:

private ObservableCollection<string> _strings = new ObservableCollection<string>();
...

string item1 = _strings[0];

But you need to add to the collection first!

Answer 2

When any class has an indexed property it gets labeled as and Item property. This allows referencing the indexer property with object[index] syntax, not object.Item[index] syntax. This is due to the fact that the property needs a name, and is a property defined as

public T this[int index]
{
    get 
    {
        //...
    }
    set        
    {
        //...
    }
}

EDIT: See this article on indexer properties for further reading

Answer 3

The right syntax is this:

_strings.Item(0);

not this

_strings.Item[0];

The call to Item() or Items() is actually a method call. The indexer operator is not needed.