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Which C# containers are contiguous and which are node-based?

In Item 1 of Effective STL, Herb Sutter makes a distinction between contiguous and node-based containers. Vectors, strings, and deques are contiguous, while linked lists and associative containers are node-based. This is useful for performance considerations (speed of insertion or deletion from the start, middle, or end, iteration, large memory allocation considerations, etc.)
In particular, I'm interested in List<T> and a list such as: BaseList : CollectionBase, ITypedList.
I heard somewhere that List<T> is more like a std::vector<T> than a linked list. So are both these C# containers contiguous? What node containers are available besides LinkedList<T>? Is there a comparison on MSDN somewhere, perhaps?

There is a table in this article that lists which containers are contiguous (scroll down):
http://geekswithblogs.net/BlackRabbitCoder/archive/2011/06/16/c.net-fundamentals-choosing-the-right-collection-class.aspx
Excerpt:
The List is a basic contiguous storage container. Some people may call this a vector or dynamic array. Essentially it is an array of items that grow once its current capacity is exceeded. Because the items are stored contiguously as an array, you can access items in the List by index very quickly. However inserting and removing in the beginning or middle of the List are very costly because you must shift all the items up or down as you delete or insert respectively. However, adding and removing at the end of a List is an amortized constant operation - O(1). Typically List is the standard go-to collection when you don't have any other constraints, and typically we favor a List even over arrays unless we are sure the size will remain absolutely fixed.

The most reliable way to check is by browsing the source code.
For instance, here's the code for List<T>, which states in a comment at the top of the file:
** Purpose: Implements a generic, dynamically sized list as an
** array.

According to MSDN,
The List class is the generic equivalent of the ArrayList class. It
implements the IList generic interface by using an array whose size
is dynamically increased as required.
So, a List<T> is contiguous.

List<long> vs long[], memory usage

Regarding the size in memory for the
List<long> ListOfLongs;
long[] ArrayOfLongs;
If each has N elements, how much memory they eat up?
I am asking that because as of my knowledge, .NET has not template (generics) specialization.

Practically the same amount of memory (technically, the List will probably consume some more because it has over-allocated so that it can grow more easily).
Generic collections in .NET do not need to box the items they hold, which would be a massive memory and performance sink.

The List<T> owns an array T[]. It uses an exponential growth strategy for this array, so a list with n elements usually has a backing array with size larger than n. Also the smaller arrays need to be garbage collected, which can be annoying if the are large enough to be on the LoH.
But you can avoid this by specifying a capacity manually, for example as a constructor parameter. Then a single array with the desired capacity will be allocated, so you avoid both of the above problems.
In addition List<T> has a small O(1) overhead for the list object itself.
But there is no per element overhead when using generics. The runtime creates a specialized version for each value type you pass in. No boxing of the elements occurs.
But you can't use C++ style template specialization, where you effectively overload the implementation for certain type parameters. All generic instantiations share the same C# code.
i.e. there is no specialized IL code, but each value type gets a specialized machine code implementation based on the same source-code.

I am asking that because as of my knowledge, .NET has not template (generics) specialization.
.Net doesn't have template specialization in the sense that you (as the programmer) can supply different code depending on the type arguments. But the compiler still can (and does) produce different code for value types than for reference type, i.e. (unlike in Java) value types are not boxed when put into a generic container. They're stored efficiently.

Using lists is more practical than using plain arrays. The key for performance and memory consumption is the Capacity of a list. By default it starts with a value of 4 and increases to 8, 16, 32, 64, ... whenever the elements of the list reach the defined capacity. Each increment is translated to an internal re-allocation and Array.Copy. So if you have a list with 1000 items and you expect 100 items in a day, you can instantiate the list with a capacity of 1200 (error margin in prediction 100%). This way you will avoid the re-allocation for 2000 items whenever you add the 10001 item, and of course the continuous re-allocations and Array.Copy to fill it with the existing 1000 items.

what is difference between string array and list of string in c#

I hear on MSDN that an array is faster than a collection.
Can you tell me how string[] is faster then List<string>.

Arrays are a lower level abstraction than collections such as lists. The CLR knows about arrays directly, so there's slightly less work involved in iterating, accessing etc.
However, this should almost never dictate which you actually use. The performance difference will be negligible in most real-world applications. I rarely find it appropriate to use arrays rather than the various generic collection classes, and indeed some consider arrays somewhat harmful. One significant downside is that there's no such thing as an immutable array (other than an empty one)... whereas you can expose read-only collections through an API relatively easily.

The article is from 2004, that means it's about .net 1.1 and there was no generics.
Array vs collection performance actually was a problem back then because collection types caused a lot of exta boxing-unboxing operations. But since .net 2.0, where generics was introduced, difference in performance almost gone.

An array is not resizable. This means that when it is created one block of memory is allocated, large enough to hold as many elements as you specify.
A List on the other hand is implicitly resizable. Each time you Add an item, the framework may need to allocate more memory to hold the item you just added. This is an expensive operation, so we end up saying "List is slower than array".
Of course this is a very simplified explanation, but hopefully enough to paint the picture.

An array is the simplest form of collection, so it's faster than other collections. A List (and many other collections) actually uses an array internally to hold its items.
An array is of course also limited by its simplicity. Most notably you can't change the size of an array. If you want a dynamic collection you would use a List.

List<string> is class with a private member that is a string[]. The MSDN documentation states this fact in several places. The List class is basically a wrapper class around an array that gives the array other functionality.
The answer of which is faster all depends on what you are trying to do with the list/array. For accessing and assigning values to elements, the array is probably negligibly faster since the List is an abstraction of the array (as Jon Skeet has said).
If you intend on having a data structure that grows over time (gets more and more elements), performance (ave. speed) wise the List will start to shine. That is because each time you resize an array to add another element it is an O(n) operation. When you add an element to a List (and the list is already at capacity) the list will double itself in size. I won't get into the nitty gritty details, but basically this means that increasing the size of a List is on average a O(log n) operation. Of course this has drawbacks too (you could have almost twice the amount of memory allocated as you really need if you only go a couple items past its last capacity).
Edit: I got a little mixed up in the paragraph above. As Eric has said below, the number of resizes for a List is O(log n), but the actual cost associated with resizing the array is amortized to O(1).

Are stack based arrays possible in C#?

Let's say, hypothetically (read: I don't think I actually need this, but I am curious as the idea popped into my head), one wanted an array of memory set aside locally on the stack, not on the heap. For instance, something like this:
private void someFunction()
{
int[20] stackArray; //C style; I know the size and it's set in stone
}
I'm guessing the answer is no. All I've been able to find is heap based arrays. If someone were to need this, would there be any workarounds? Is there any way to set aside a certain amount of sequential memory in a "value type" way? Or are structs with named parameters the only way (like the way the Matrix struct in XNA has 16 named parameters (M11-M44))?

What you want is stackalloc; unfortunately, you can only use this in unsafe code, which means it won't run in a limited permissions context.
You could also create a struct with the necessary number of variables in it for each element type, but you would need a new type for each size of 'array' you wanted to use

The closest thing I can think of to a stack-based array would be a manually-nested structure; for an array of size N^M, the code size would be O(MN) and the access time O(M); one could scale M and N as convenient (e.g. one could handle a 4096-element array as six-deep nested 4-element structures, four-deep nested 8-element structures or three-deep nested 16-element structures, two-deep nested 64-element structures, etc.) If one wanted to do three-deep nesting of 16-element arrays (probably the most practical trade-off) one would define a 16-element structure with fields f0 through f15, and an access method using switch/case to select an element. One could then define a 16-element structure of those, a 16-element structure of those, etc.
In general, using a standard Array is apt to be better than using value-type structures to mimic arrays, but there are times when having an array-ish thing as a value type would be advantageous. The advantages of value type arrays would tend to be limited in .net, however, by some limitations in its handling of manipulating value types by reference. While it would be nice if one could simply access element 0x123 from an array described as above by writing "MyArrayishThing[1][2][3]", that would be inefficient for reading and ineffective for writing (since the subexpression MyArrayishThing[1] would make a copy of structures holding 256 elements of the array). Instead, what's necessary is to pass MyArrayishThing[1] by reference to a routine that can access element 2 of that and pass it by reference to a routine to access element 3 of that. It's possible to do that efficiently, but the code ends up looking rather nasty.

How and when to abandon the use of arrays in C#?

I've always been told that adding an element to an array happens like this:
An empty copy of the array+1element is
created and then the data from the
original array is copied into it then
the new data for the new element is
then loaded
If this is true, then using an array within a scenario that requires a lot of element activity is contra-indicated due to memory and CPU utilization, correct?
If that is the case, shouldn't you try to avoid using an array as much as possible when you will be adding a lot of elements? Should you use iStringMap instead? If so, what happens if you need more than two dimensions AND need to add a lot of element additions. Do you just take the performance hit or is there something else that should be used?

Look at the generic List<T> as a replacement for arrays. They support most of the same things arrays do, including allocating an initial storage size if you want.

This really depends on what you mean by "add."
If you mean:
T[] array;
int i;
T value;
...
if (i >= 0 && i <= array.Length)
array[i] = value;
Then, no, this does not create a new array, and is in-fact the fastest way to alter any kind of IList in .NET.
If, however, you're using something like ArrayList, List, Collection, etc. then calling the "Add" method may create a new array -- but they are smart about it, they don't just resize by 1 element, they grow geometrically, so if you're adding lots of values only every once in a while will it have to allocate a new array. Even then, you can use the "Capacity" property to force it to grow before hand, if you know how many elements you're adding (list.Capacity += numberOfAddedElements)

In general, I prefer to avoid array usage. Just use List<T>. It uses a dynamically-sized array internally, and is fast enough for most usage. If you're using multi-dimentional arrays, use List<List<List<T>>> if you have to. It's not that much worse in terms of memory, and is much simpler to add items to.
If you're in the 0.1% of usage that requires extreme speed, make sure it's your list accesses that are really the problem before you try to optimize it.

If you're going to be adding/removing elements a lot, just use a List. If it's multidimensional, you can always use a List<List<int>> or something.
On the other hand, lists are less efficient than arrays if what you're mostly doing is traversing the list, because arrays are all in one place in your CPU cache, where objects in a list are scattered all over the place.
If you want to use an array for efficient reading but you're going to be "adding" elements frequently, you have two main options:
1) Generate it as a List (or List of Lists) and then use ToArray() to turn it into an efficient array structure.
2) Allocate the array to be larger than you need, then put the objects into the pre-allocated cells. If you end up needing even more elements than you pre-allocated, you can just reallocate the array when it fills, doubling the size each time. This gives O(log n) resizing performance instead of O(n) like it would be with a reallocate-once-per-add array. Note that this is pretty much how StringBuilder works, giving you a faster way to continually append to a string.

When to abandon the use of arrays
First and foremost, when semantics of arrays dont match with your intent - Need a dynamically growing collection? A set which doesn't allow duplicates? A collection that has to remain immutable? Avoid arrays in all that cases. That's 99% of the cases. Just stating the obvious basic point.
Secondly, when you are not coding for absolute performance criticalness - That's about 95% of the cases. Arrays perform better marginally, especially in iteration. It almost always never matter.
When you're not forced by an argument with params keyword - I just wished params accepted any IEnumerable<T> or even better a language construct itself to denote a sequence (and not a framework type).
When you are not writing legacy code, or dealing with interop
In short, its very rare that you would actually need an array. I will add as to why may one avoid it?
The biggest reason to avoid arrays imo is conceptual. Arrays are closer to implementation and farther from abstraction. Arrays conveys more how it is done than what is done which is against the spirit of high level languages. That's not surprising, considering arrays are closer to the metal, they are straight out of a special type (though internally array is a class). Not to be pedagogical, but arrays really do translate to a semantic meaning very very rarely required. The most useful and frequent semantics are that of a collections with any entries, sets with distinct items, key value maps etc with any combination of addable, readonly, immutable, order-respecting variants. Think about this, you might want an addable collection, or readonly collection with predefined items with no further modification, but how often does your logic look like "I want a dynamically addable collection but only a fixed number of them and they should be modifiable too"? Very rare I would say.
Array was designed during pre-generics era and it mimics genericity with lot of run time hacks and it will show its oddities here and there. Some of the catches I found:
Broken covariance.
string[] strings = ...
object[] objects = strings;
objects[0] = 1; //compiles, but gives a runtime exception.
Arrays can give you reference to a struct!. That's unlike anywhere else. A sample:
struct Value { public int mutable; }
var array = new[] { new Value() };
array[0].mutable = 1; //<-- compiles !
//a List<Value>[0].mutable = 1; doesnt compile since editing a copy makes no sense
print array[0].mutable // 1, expected or unexpected? confusing surely
Run time implemented methods like ICollection<T>.Contains can be different for structs and classes. It's not a big deal, but if you forget to override non generic Equals correctly for reference types expecting generic collection to look for generic Equals, you will get incorrect results.
public class Class : IEquatable<Class>
{
public bool Equals(Class other)
{
Console.WriteLine("generic");
return true;
}
public override bool Equals(object obj)
{
Console.WriteLine("non generic");
return true;
}
}
public struct Struct : IEquatable<Struct>
{
public bool Equals(Struct other)
{
Console.WriteLine("generic");
return true;
}
public override bool Equals(object obj)
{
Console.WriteLine("non generic");
return true;
}
}
class[].Contains(test); //prints "non generic"
struct[].Contains(test); //prints "generic"
The Length property and [] indexer on T[] seem to be regular properties that you can access through reflection (which should involve some magic), but when it comes to expression trees you have to spit out the exact same code the compiler does. There are ArrayLength and ArrayIndex methods to do that separately. One such question here. Another example:
Expression<Func<string>> e = () => new[] { "a" }[0];
//e.Body.NodeType == ExpressionType.ArrayIndex
Expression<Func<string>> e = () => new List<string>() { "a" }[0];
//e.Body.NodeType == ExpressionType.Call;
Yet another one. string[].IsReadOnly returns false, but if you are casting, IList<string>.IsReadOnly returns true.
Type checking gone wrong: (object)new ConsoleColor[0] is int[] returns true, whereas new ConsoleColor[0] is int[] returns false. Same is true for uint[] and int[] comparisons. No such problems if you use any other collection types.
How to abandon the use of arrays.
The most commonly used substitute is List<T> which has a cleaner API. But it is a dynamically growing structure which means you can add to a List<T> at the end or insert anywhere to any capacity. There is no substitute for the exact behaviour of an array, but people mostly use arrays as readonly collection where you can't add anything to its end. A substitute is ReadOnlyCollection<T>.

When the array is resized, a new array must be allocated, and the contents copied. If you are only modifying the contents of the array, it is just a memory assignment.
So, you should not use arrays when you don't know the size of the array, or the size is likely to change. However, if you have a fixed length array, they are an easy way of retrieving elements by index.

ArrayList and List grow the array by more than one when needed (I think it's by doubling the size, but I haven't checked the source). They are generally the best choice when you are building a dynamically sized array.
When your benchmarks indicate that array resize is seriously slowing down your application (remember - premature optimization is the root of all evil), you can evaluate writing a custom array class with tweaked resizing behavior.

Generally, if you must have the BEST indexed lookup performance it's best to build a List first and then turn it into a array thus paying a small penalty at first but avoiding any later. If the issue is that you will be continually adding new data and removing old data then you may want to use a ArrayList or List for convenience but keep in mind that they are just special case Arrays. When they "grow" they allocate a completely new array and copy everything into it which is extremely slow.
ArrayList is just an Array which grows when needed.
Add is amortized O(1), just be careful to make sure the resize won't happen at a bad time.
Insert is O(n) all items to the right must be moved over.
Remove is O(n) all items to the right must be moved over.
Also important to keep in mind that List is not a linked list. It's just a typed ArrayList. The List documentation does note that it performs better in most cases but does not say why.
The best thing to do is to pick a data structure which is appropriate to your problem. This depends one a LOT of things and so you may want to browse the System.Collections.Generic Namespace.
In this particular case I would say that if you can come up with a good key value Dictionary would be your best bet. It has insert and remove that approaches O(1). However, even with a Dictionary you have to be careful not to let it resize it's internal array (an O(n) operation). It's best to give them a lot of room by specifying a larger-then-you-expect-to-use initial capacity in the constructor.
-Rick

A standard array should be defined with a length, which reserves all of the memory that it needs in a contiguous block. Adding an item to the array would put it inside of the block of already reserved memory.

Arrays are great for few writes and many reads, particularly those of an iterative nature - for anything else, use one of the many other data structures.

You are correct an array is great for look ups. However modifications to the size of the array are costly.
You should use a container that supports incremental size adjustments in the scenario where you're modifying the size of the array. You could use an ArrayList which allows you to set the initial size, and you could continually check the size versus the capacity and then increment the capacity by a large chunk to limit the number of resizes.
Or you could just use a linked list. Then however look ups are slow...

If I think I'm going to be adding items to the collection a lot over its lifetime, than I'll use a List. If I know for sure what the size of the collection will be when its declared, then I'll use an array.
Another time I generally use an array over a List is when I need to return a collection as a property of an object - I don't want callers adding items that collection via List's Add methods, but instead want them to add items to the collection via my object's interface. In that case, I'll take the internal List and call ToArray and return an array.

If you are going to be doing a lot of adding, and you will not be doing random access (such as myArray[i]). You could consider using a linked list (LinkedList<T>), because it will never have to "grow" like the List<T> implementation. Keep in mind, though, that you can only really access items in a LinkedList<T> implementation using the IEnumerable<T> interface.

The best thing you can do is to allocate as much memory as you need upfront if possible. This will prevent .NET from having to make additional calls to get memory on the heap. Failing that then it makes sense to allocate in chunks of five or whatever number makes sense for your application.
This is a rule you can apply to anything really.