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Does list.count physically iterate through the list to count it, or does it keep a pointer

I am stepping through a large list of object to do some stuff regarding said objects in the list.
During my iteration, I will remove some objects from the list depending on certain criteria.
Once all is done, I need to update the UI regarding the number of objects in my list. (List of T).
QUESTION:
When I call list.count, does .net actually iterate through the list to
count it, or does it store the count as a property/variable?
If .net physically re-iterates through the list, I may just as well keep a counter on my own iteration through the list, and save the overhead?
Thanks

It simply keeps an internal int to track the number of items. So no iteration.
The documentation says retrieving Count is an O(1) operation:
http://msdn.microsoft.com/en-us/library/27b47ht3%28v=vs.110%29.aspx
You can see for yourself:
http://referencesource.microsoft.com/#mscorlib/system/collections/generic/list.cs

List is implemented as an array list, and it keeps track of its own size, so invoking the .Count property doesn't require any iteration.
If you call the LINQ .Count() extension method, this will check whether the underlying IEnumerable<> implements ICollection (which a List<> does), and use the .Count property on that interface if possible. So this won't cause any iteration to occur either.
Incidentally, there are other problems you're going to encounter if you attempt to remove items from your list while iterating through it. It's not really clear how iteration should behave when you are removing elements out from under the iterator, so List<>s will avoid this issue entirely by throwing an exception if the list has been modified since its enumerator was created.

You can use a decompiler, such as the freely-available ILSpy, to answer these questions. If you're referring to the List<T> type, then the Count getter simply involves reading a field:
public int Count
{
get { return this._size; }
}

As stated here under the remarks tab
http://msdn.microsoft.com/en-us/library/27b47ht3(v=vs.110).aspx
Retrieving the value of this property is an O(1) operation.
Which means no iteration is occurring.

You tagged your question with both vb.net and c#, so in reply to "If .net physically re-iterates through the list, I may just as well keep a counter on my own iteration through the list, and save the overhead?"
If your iteration is with a For i = first To last then VB.NET will evaluate first and last when it enters the loop:
Dim first As Integer = 1
Dim last As Integer = 3
For i = first To last
Console.Write(i.ToString() & " ")
last = -99
Next
outputs: 1 2 3
If you do the equivalent in C#, first and last are evaluated on every iteration:
int first = 1;
int last = 1;
for (int i = first; i <= last; i++)
{
Console.Write(i.ToString() + " ");
last = -99;
}
outputs: 1
If your .Count() function/property is expensive to evaluate and/or you don't want it to be re-evaluated on each iteration (for some other reason), then in C# you could assign it to a temporary variable.

Get certain item from each Tuple from List

What is the correct way to go about creating a list of, say, the first item of each Tuple in a List of Tuples?
If I have a List<Tuple<string,string>>, how would I get a List<string> of the first string in each Tuple?

A little Linq will do the trick:
var myStringList = myTupleList.Select(t=>t.Item1).ToList();
As an explanation, since Tim posted pretty much the same answer, Select() creates a 1:1 "projection"; it takes each input element of the Enumerable, and for each of them it evaluates the lambda expression and returns the result as an element of a new Enumerable having the same number of elements. ToList() will then spin through the Enumerable produced by Select(), and add each element one at a time to a new List<T> instance.
Tim has a good point on the memory-efficiency; ToList() will create a list and add the elements one at a time, which will cause the List to keep resizing its underlying array, doubling it each time to ensure it has the proper capacity. For a big list, that could cause OutOfMemoryExceptions, and it will cause the CLR to allocate more memory than necessary to the List unless the number of elements happens to be a power of 2.

List<string> list = tuples.Select(t => t.Item1).ToList();
or, potentially less memory expensive:
List<string> list = new List<String>(tuples.Count);
list.AddRange(tuples.Select(t => t.Item1));
because it avoids the doubling algorithm of List.Add in ToList.

If you have a List<Tuple<string, string>> listoftuples, you can use the List's implementation of the Select method to take the first string from each Tuple.
It would look like this:
List<string> firstelements = listoftuples.Select(t => t.Item1).ToList();

Generialised Variant:
for selecting a particular item where the collection's tuple's length is unknown i.e 2,3,4 ...:
static IEnumerable TupleListSelectQuery<T>(IEnumerable<T> lst, int index) where T : IStructuralEquatable, IStructuralComparable, IComparable
{
return lst.Select(t => typeof(T).GetProperty("Item" + Convert.ToString(itemNumber)).GetValue(t)).ToList();
}
where index's value corresponds to the way tuples are enumerated i.e 1,2,3 ... (not 0,1,2...).

how to get Current index in stack using C#

I have inserted values into Stack in C# . I need values of mystack[0], mystack[1]. How to do it. I have tried methods in stack but please give me hint of code i will try it

You can use ElementAt() for this.
Stack<Int32> foo = new Stack<Int32>();
foo.Push(5); //element 1
foo.Push(1); //element 0
int val = foo.ElementAt(1); //This is 5
Since stacks are last on first out, if you want to get the first item you added to the stack, you can use:
int val = foo.ElementAt(foo.Count - 1);
Keep in mind, ElementAt is a LINQ extension method that will enumerate the stack as an array and return the desired index. For large stacks, or where performance is critical, you might want to consider using another data structure such as List<T>.

If you need the items by index, perhaps a List<T> would be a more appropriate data structure?
A stack is intended to only allow you to get the most recently inserted item. There are ways to bypass that behavior, but surely if you need the items by index this works better:
var myList = new List<Int32>();
myList.Add(100);
myList.Add(200);
myList.Add(300);
myList.Add(400);
Console.Out.WriteLine(myList[2]); // Prints "300"

Please explain System.Linq.Enumerable.Where(Func<T, int, bool> predicate)

I can't make any sense of the MSDN documentation for this overload of the Where method that accepts a predicate that has two arguments where the int, supposedly, represents the index of the source element, whatever that means (I thought an enumerable was a sequence and you couldn't see further than the next item, much less do any indexing on it).
Can someone please explain how to use this overload and specifically what that int in the Func is for and how it is used?

The int parameter represents the index of the current item within the current iteration. Each time you call one of the LINQ extension methods, you aren't in theory guaranteed to get the items returned in the same order, but you know they're all be returned once each and thus can be assigned indices. (Well, you are guaranteed if you know the query object is a List<T> or such, but not in general.)
Example:
var result1 = myEnumerable.Where((item, index) => index < 4);
var result2 = myEnumerable.Take(4);
// result1 and result2 are equivalent.

You can't index an IEnumerable<T> in the same way you can an array, but you might be able to use the index to filter the list in some way, or possibly to index some data in another collection which will be used in the condition.
EDIT: As an example, to skip every other element you could use:
var results = sequence.Where((item, idx) => idx % 2 == 0);

how properly remove item from list [duplicate]

This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Exception during iteration on collection and remove items from that collection
How to remove elements from a generic list while iterating around it?
Better way to remove matched items from a list
// tmpClientList is List<Client> type
if (txtboxClientName.Text != "")
foreach (Client cli in tmpClientList)
if (cli.Name != txtboxClientName.Text)
tmpClientList.Remove(cli);
Error: "Collection was modified; enumeration operation may not execute."
How can i remove items from the list, in some simple way, without saving indexes of these items in another list or array, and removing them in another place in the code. Tried also RemoveAt(index) but it's exactly the same situation, modifying when loop runs.

Move backwards through the list.. that way removing an item does not affect the next item.
for(var i=tmpClientList.Count-1;i>=0;i--)
{
if (tmpClientList[i].Name != txtboxClientName.Text)
tmpClientList.RemoveAt(i);
}

On a List<T>, there is a RemoveAll method that takes a delegate to indicate whether to remove the item. You can use it like this:
tmpCLientList.RemoveAll(cli => cli.Name != txtboxClientName.Text);

Either use a for/while loop, or tmpClientList.RemoveAll(a => a.Name == txtboxClientName.Text). As you didn't specify which c# version you are using, ymmw.

Don't use foreach. Use for and descend the list (i.e. start from the end), using RemoveAt.
So,
// tmpClientList is List<Client> type
if (txtboxClientName.Text != "")
foreach (int pos = tmpClientList.Length - 1; pos >= 0; pos--)
{
Client cli = tmpClientList[pos];
if (cli.Name != txtboxClientName.Text)
tmpClientList.RemoveAt(pos);
}

The problem is that you are trying the modify the list in a foreach iteration. Replace that with a for and you should be ok.
Also, since you seem to be using user input for the name, consider cleaning up the input a bit, at least with a Trim() to remove extra white spaces. If you don't, 'John ' and 'John' will be two different things.
Same for the initial != "" check.

You can create another list with the items you want to delete and iterate the new list to remove items from your "txtboxClientName" list.

Actually, foreach uses Enumerators to iterate through given Item-Collections. Going further the System.Collections.Generic.List<T> implements the IEnumarable-Interface to provide a Class, that knows how to iterate through the items of the list, i.e. the Enumerator. Now if you iterate through that list by using foreach the Enumerator keeps track of the current position, how to reach the next position and some other stuff. The internal logic could be something like storing the number of items in a variable n and then access all objects from 0 to n-1. As you may notice if any object is removed between the iteration steps we shall end in a NullReferenceException when the Enumerator tries to deliver the last object of the list. So to prevent any iteration failures, the list itself is not allowed to be modified during Enumeration.
Hope I was able to state that out at least a little bit comprehensively. :-)