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How to find element in ICollection or List using string array?

I have a string array of such type
String[] codes = { "a", "b" };
And the collection where Thing has property Code.
ICollection<Thing>
I want to take element from this collection if its code matches any element of array. How can i achieve this, because it seems that there is no method that can do such thing?

you can find the first element
var firstElement= things.FirstOrDefault(t => codes.Contains(t.Code));
or list if it can be several
var elements= things.Where(t => codes.Contains(t.Code)).ToList();

Get value from last element LinkedList C#

I have a LinkedList like this:
LinkedList<KeyValuePair<decimal, decimal>> qCompras = new LinkedList<KeyValuePair<decimal, decimal>>();
I want to obtain the value from the last object in the LinkedList, but I want to know the first of the two decimal.
How can I do this?
Thanks in advance!
Regards

Try this:
var result = qCompras.Last.Value.Key;

Why not use the LinkedList.Last Property property of the LinkedList?
For example:
var lastItem = qCompras.Last;
In there, you have the Key and Value properties.
Note that this is a LinkedListNode<T> and you need to use the LinkedListNode<T>.Value property get the underlying instance of T.

How to get IEnumerable list values?

I've an IEnumerable list named list. It keeps these sample values:
I want to access and assign to any variables these Count, Start and End values, whenever I want. How can I do this?

The IEnumerable itself doesn't have Count, Start, or End. It's elements do, so you'll need to identify the element in the collection from which you want to read those values. For example, to read the values on the first element:
var firstCount = list.First().Count;
var firstStart = list.First().Start;
var firstEnd = list.First().End;
Or if you want to get a collection of all the Count values, something like this:
var allCounts = list.Select(c => c.Count);
You're operating on a collection of elements, not a single element. So to get information from any particular element you first need to identify it from the collection. And there are lots of methods you can chain together to identify any given element or set of elements.

Use a loop ?
foreach(var item in list)
{
var count = item.Count;
}
Or use ToList and convert it to List<T> then you can access your value with index:
var myList = list.ToList();
var count = myList[0].Count;
Also if you know the type you can cast your IEnumerable to IList<T> in order to use indexer.

It's possible to access the list with Linq using the namespace
using System.Linq;
like so
var firstListElement = list.ElementAt(0);
var firstListElementCount = firstListElement.Count;
// do more stuff with my first element of the list

Try:
list.Count()
list.First() or list.FirstOrDefault()
list.Last() or list.LastOrDefault()

get list item by id

I have a list:
List<string> theList = new List<string>;
There are a few elements in the List. And now I want to get an item by the index. e.g. I want to get element number 4. How can I do it?

Just use the indexer
string item = theList[3];
Note that indexes in C# are 0 based. So if you want the 4th element in the list you need to use index 3. If you want the 5th element you would use index 4. It's unclear from your question which you intended
The indexer is a common feature on .Net collection types. For lists it's generally index based, for maps it's key based. The documentation for the type in question will tell you which and if it's available. The indexer member will be listed though as the property named Item
http://msdn.microsoft.com/en-us/library/0ebtbkkc.aspx

To get 4th item, you can use indexer:
string item = theList[3];
if you prefer to use methods, then you can use ElementAt or (ElementAtOrDefault):
string item = theList.ElementAt(3);

You can use the Indexer to get the Item at selected index
string item = theList[3];

Use the indexer:
string the4th = theList[3];
Note that this throws an exception if the list has only 3 items or less since the index is always zero-based. You might want to use Enumerable.ElementAtOrDefault then:
string the4th = theList.ElementAtOrDefault(3);
if(the4th != null)
{
// ...
}
ElementAtOrDefault returns the element at the specified index if index < list.Count and default(T) if index >= theList.Count. So for reference types(like String) it returns null and for value types their default value(e.g. 0 for int).
For collection types which implement IList<T>(arrays or lists) it uses the indexer to get the element, for other types it uses a foreach loop and a counter variable.
So you could also use the Count property to check if the list contains enough items for the index:
string the4th = null;
if(index < theList.Count)
{
the4th = theList[index];
}

use the Indexer Syntax:
var fourthItem = theList[3];

this should do it, access via array index.
theList[3]
its 3 as index's start at 0.

You can use the Indexer to get the Item at selected index
string item = theList[3];
Or if u want to get the id (if accessing from database) define a class e.g.
public class Person
{
public int PId;
public string PName;
}
and access as follow
List<Person> theList = new List<Person>();
Person p1 = new Person();
int id = theList[3].PId

How to get the data from list without using iterator in c#

i have a list contains set of strings, i want to fetch the data present in the list based on index, with out using iterator.. is there any functions like get() or getat() some sort of method using which we can fetch?

myList[index] is the way to go
List<string> myList = new List<string>();
myList.Add("string 1");
myList.Add("String 2");
Console.WriteLine(myList[0]); // string 1
Console.WriteLine(myList[1]); // String 2

List<string> myList = new List<string();
//add some elements to the list
//then get the third element
string thirdElement = myList[2];

You can just do:
item = list[i];

Use the overloaded index operator.
List<String> list; // ... initialize, populate list
String element = list[1]; // get the element at index 1

If your collection implements IList<T>, just use indexer. Otherwise, if your collection only allows forward-only access (that is, only implements IEnumerable<T>) you can use ElementAt() method, but it still uses iterator under the hood.

I don't know what kind of list you're talking about exactly, but most collections in .net have a CopyTo function, and you can access individual items with the [] operator.

List<string> list = new List<string>();
list.Add("lots of strings");
//If you want to print all the strings you can do:
foreach(string str in list)
Console.WriteLine(str);
//If you want to modify each string in the list, make each lower case for example,
// you can do. this is working by using the index of the elements in the list:
for(int i = 0; i < list.Count; i++)
list[i] = list[i].ToLower();

If you use the generic type List (or another implementation of IList) you can use the index operator to directly access items at certain positions: item = myList[3]
If you use a type that only implements IEnumerable you should use the ElementAt() function.
What's your reason to avoid the use of iterators?