Today I was surprised to find that in C# I can do:
List<int> a = new List<int> { 1, 2, 3 };
Why can I do this? What constructor is called? How can I do this with my own classes? I know that this is the way to initialize arrays but arrays are language items and Lists are simple objects ...
This is part of the collection initializer syntax in .NET. You can use this syntax on any collection you create as long as:
It implements IEnumerable
(preferably IEnumerable<T>
)
It has a method named Add(...)
What happens is the default constructor is called, and then Add(...)
is called for each member of the initializer.
Thus, these two blocks are roughly identical:
List<int> a = new List<int> { 1, 2, 3 };
And
List<int> temp = new List<int>();
temp.Add(1);
temp.Add(2);
temp.Add(3);
List<int> a = temp;
You can call an alternate constructor if you want, for example to prevent over-sizing the List<T>
during growing, etc:
// Notice, calls the List constructor that takes an int arg
// for initial capacity, then Add()'s three items.
List<int> a = new List<int>(3) { 1, 2, 3, }
Note that the Add()
method need not take a single item, for example the Add()
method for Dictionary<TKey, TValue>
takes two items:
var grades = new Dictionary<string, int>
{
{ "Suzy", 100 },
{ "David", 98 },
{ "Karen", 73 }
};
Is roughly identical to:
var temp = new Dictionary<string, int>();
temp.Add("Suzy", 100);
temp.Add("David", 98);
temp.Add("Karen", 73);
var grades = temp;
So, to add this to your own class, all you need do, as mentioned, is implement IEnumerable
(again, preferably IEnumerable<T>
) and create one or more Add()
methods:
public class SomeCollection<T> : IEnumerable<T>
{
// implement Add() methods appropriate for your collection
public void Add(T item)
{
// your add logic
}
// implement your enumerators for IEnumerable<T> (and IEnumerable)
public IEnumerator<T> GetEnumerator()
{
// your implementation
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
Then you can use it just like the BCL collections do:
public class MyProgram
{
private SomeCollection<int> _myCollection = new SomeCollection<int> { 13, 5, 7 };
// ...
}
(For more information, see the MSDN)