Badly optimised: new List<Class1>() { instance1 }

looking closely at List initialising it seems easy to make poor choices.

Essentially if you want to init a List with 1 item you should create a list of capacity 1 and then add the first item.

Either allocate a list of size 1 and add, or combine the statement (VS always prompts you to do this) to:
list = new List<Class1>(1){instance1};

If you miss out the 1 capacity the compiler does not realise you will need capacity 1 and does a whole load of stuff - growIfNeeded, set_Capacity, Resize etc. - visible in the Profiler with extra garbage.

Could the compiler not be more clever and look at the initial input count or is this issue just common knowledge among C# speedsters?

 

If you don't need the convenience of List then just use an array.

 

I know but I often add more items, just need to start with 1.
Just have to remember to specify capacity as the compiler can not see it needs be 1 and adds a little overhead because of it, it would seem.

 

That's why the compiler can't make any assumptions about the capacity unless you explicitly tell it the capacity should be 1.

 

TBH I'm having a really hard time imagining why a List would have its capacity default to 0...

 

Ya, from what I can gather, it's zero with its default constructor, regardless of the initialized elements.
At size zero, plus an 'Add' you get a capacity of 4.

Setting the capacity is good, if you know it ahead of time.

 

Yes I think I am expecting the compiler to do more than it can. Well I will just tell it explicitly

 

It's because a collection initializer is syntactic sugar. All that's required to make use of the initializer syntax is a class that implement IEnumerable and a method called "Add" which accepts the appropriate parameters.

What's not required is a way to tell the object how many items are in the initializer, so the compiler has no way to "be smart" as you describe.

I doubt it's terribly important because hopefully you don't do too much initialization, especially in a tight loop, but it's still a good tip to be aware of. Thanks.