Initialization trick

You all now the cool syntax sugar for object initialization in C#:

var item = new Item { Age = 5 };

Also there are sugar for collection initialization:

var items = new List<Item> { new Item { Age = 5 } };

And, of course, we could mix these two cases:

class ItemContainer
{
    public List<Item> Items{get;set;}
} 

// ...

var itemContainer = new ItemContainer {
   Items = new List<Item> {new Item { Age = 5 }}
};

But there is one rarely used case that is even cooler. If  ItemContainer.Items is ICollection and it is created in constructor:

class ItemContainer
{
   public ItemContainer()
   {
      Items = new List<Item>();
   } 
  
   public List<Item> Items{get;set;}
} 
 

Than you can add elements to the Items collection without 'new' construction:

var itemContainer = new ItemContainer {
   Items = { new Item { Age = 5 } }
};

This code will be ILed as:

IL_0000: newobj ItemContainer..ctor
IL_0005: stloc.1 
IL_0006: ldloc.1 
IL_0007: callvirt ItemContainer.get_Items  
IL_000C: newobj Item..ctor
IL_0011: stloc.2 
IL_0012: ldloc.2 
IL_0013: ldc.i4.1 
IL_0014: callvirt Item.set_Age
IL_0019: ldloc.2 
IL_001A: callvirt System.Collections.Generic.List<Item>.Add

So there is no constructions of Items, just single method 'Add'.
The same is applicable for Expressions as well.