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Array with two classes

Hi im doing a school assignment, and I need to convert this JAVA code to C#
private Map<ItemID, ProductDescription> descriptions = new HashMap()<ItemID, ProductDescription>;
Is it possible to make a straight conversion?
I've already decided to make ItemID into an int, and ProductDescription is a class.

Yes, of course you can.
Please look into following examples:
IDictionary<int, string> h = new Dictionary<int, string>();
h.Add(1, "a");
h.Add(2, "b");
h.Add(3, "c");
SortedList<int, string> s = new SortedList<int, string>();
s.Add(1, "a");
s.Add(2, "b");
I think this is what you are looking for.

You could use a Dictionary<int, ProductDescription> instead.
Dictionary<TKey, TValue> Class
Represents a collection of keys and values. The key must be unique.

private Dictionary<ItemID, ProductDescription> descriptions = new Dictionary<ItemID, ProductDescription>();
The hasmap indeed allows for one null key entry. In the (rare?) case you would need this I'd simply create a special ItemID and use that for the null key.
You could ofcourse make a dictionary descendant with null key support, but that would be overdoing it imho ;-)

Yes, just replace HashMap with Dictionary. You might want to type the variable as an IDictionary (in the same spirit as the Java code), but that's not strictly necessary.

Yes, You can do the conversion using a Dictionary instead of HashMap. And of course it is more effective to get the idea of each code segment and convert. Trying to convert line by line is not recommended since you may miss a better way that can be used to resolve the problem.

There are many options.
There is an
Hashtable in C#
KeyValuePair So it can be List<KeyValuePair<T,U>>
Dictionary //Preferred

This is a good match but,
private IDictionary<ItemID, ProductDescription> descriptions
= new Dictionary<ItemID, ProductDescription>();
Note
HashMap will accept null key values, where as Dictionary will not.
If you really want to support null key values, I'd like to see you reasoning before attempting a perfect .Net HashMap implementation.

Alternative to Dictionary with enum keys?

In my solution i depend heavily on Dictionaries with an enum as a key. I find it is easy to understand and to read this construction.
One significant obstacle to the above is that it is not possible to serialize this. See Problems with Json Serialize Dictionary<Enum, Int32> for more info on this.
My question is:
Is there a equally readable and intuitive pattern for replacing the Dictionary<enunm,object> that is json serializable with the built in json serializer?
Today I have replaced a.Instance.QueryableFields[Fields.Title] with a.Instance.QueryableFields[ Fields.Title.ToString()] . Not very elegant, and it is opening up for errors.

When serializing it, just select the string value. It's not very neat, but it works.
a.Instance.QueryableFields.ToDictionary(x => x.Key.ToString(), x => x.Value)

You can easily replace a dictionary with an array here, as in C# Enums are internally saved as integers:
public enum MyEnum { Zero=0, One, Two };
object[] dictionary = new object[3];
dictionary[(int)MyEnum.Zero] = 4;
EDIT: (see comments)
You can also replace Dictionary<Enum, Object> with Dictionary<Int, Object>.

IDictionary, Dictionary

I have:
IDictionary<string, IDictionary<string, IList<long>>> OldDic1;
(just for illustration purposes, it is instantiated and has values - somewhere else)
Why can I do this: ?
Dictionary<string, IDictionary<string, IList<long>>> dic1 =
OldDic1 as Dictionary<string, IDictionary<string, IList<long>>>;
Basically dic1 after executing this line has all the values from OldDic1; works.
However when I do this:
Dictionary<string, Dictionary<string, List<long>>> dic1 =
OldDic1 as Dictionary<string, Dictionary<string, List<long>>>;
I get null, it is the same as casting except it doesn't crash and instead it returns null. So the question is why I can't cast it from the interfaces to types? is there solution, other then changing how it is stored in the first place?

You can only re-cast the outermost interface/class name, not the generic parameters. The reason your second cast doesn't work is the same reason you can't cast from one array type to another even if you can cast the "contained" objects. Here's why:
List<object> objects;
List<string> strings;
objects = strings as List<object>;
// Uh oh, that's not a string!
objects.Add(42);
objects = new List<object> { "The", "answer", "is", 42 };
// Uh oh, now strings contains an integer?
strings = objects as List<string>;
The same thing would happen in your second case. You could add some kind of IDictionary to OldDic1 which is not actually a Dictionary, and then dic1 would blow up. It would have a non-Dictionary value. Ruh roh!
So, when you have containers you can change from IList<X> to List<X> and back as long as X is identical for each.

The behavior is related to the as keyword in C#. A Dictionary is not the same thing as an IDictionary.
If you were casting the other way, you may be able to get it to work in the next version of .NET which has increased support for covariance and contravariance.
The solution you might want to determine why you need to cast to the concrete Dictionary/List, and if it's required, then change the storage of the type.

How can I retrieve first n elements from Dictionary<string, int>?

Is there a way to retrieve first n elements from a Dictionary in C#?

Dictionaries are not ordered per se, you can't rely on the "first" actually meaning that. From MSDN: "For enumeration... The order in which the items are returned is undefined."
You may be able to use an OrderedDictionary depending on your platform version, and it's not a particularly complex thing to create as a custom descendant class of Dictionary.

Oftentimes omitting the cast to dictionary won't work:
dictionary = dictionary.Take(n);
And neither will a simple case like this:
dictionary = dictionary.Take(n).ToDictionary();
The surest method is an explicit cast:
dictionary = dictionary.Take(n).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

Note that there's no explicit ordering for a Dictionary, so although the following code will return n items, there's no guarantee as to how the framework will determine which n items to return.
using System.Linq;
yourDictionary.Take(n);
The above code returns an IEnumerable<KeyValuePair<TKey,TValue>> containing n items. You can easily convert this to a Dictionary<TKey,TValue> like so:
yourDictionary.Take(n).ToDictionary();

You can't really take the first N elements from a Dictionary<TKey,TValue> because it is not an ordered collection. So it really has no concept of First, Last, etc ... But as others have pointed out, if you just want to take N elements regardless of order the LINQ take function works fine
var map = GetTheDictionary();
var firstFive = map.Take(5);

Could use Linq for example?
var dictionary = new Dictionary<string, int>();
/// Add items to dictionary
foreach(var item in dictionary.Take(5))
{
// Do something with the first 5 pairs in the dictionary
}

How do I get all the values of a Dictionary<TKey, TValue> as an IList<TValue>?

I have a the following dictionary:
IDictionary<int, IList<MyClass>> myDictionary
and I am wanting to get all the values in the dictionary as an IList....
Just to add a bit of a background as to how I've gotten into this situation....
I have a method that gets me a list of MyClass. I then have another method that converts that list into a dictionary where they key is the id for MyClass. Later on...and without access to that original list...I'm needing to obtain the original ungrouped list of MyClass.
When I pass myDictionary.Values.ToList() to a method that takes an IList I get a compile error that says that it can't convert from
System.Collections.Generic.List<System.Collections.Generic.IList<MyClass>>
to:
System.Collections.Generic.IList<MyClass>
Now, I can understand that its gone and added each of the groups of IList to the new list as separate elements of the list....but in this instance its not really what I'm after. I just want a list of all the values in the entire dictionary.
How then can I get what I'm after without looping through each of the key values in the dictionary and creating the list I want?

Noticed a lot of answer were quite old.
This will also work:
using System.Linq;
dict.Values.ToList();

Because of how a dictionary (or hash table) is maintained this is what you would do. Internally the implementation contains keys, buckets (for collision handling) and values. You might be able to retrieve the internal value list but you're better of with something like this:
IDictionary<int, IList<MyClass>> dict;
var flattenList = dict.SelectMany( x => x.Value );
It should do the trick ;) SelectMany flattens the result which means that every list gets concatenated into one long sequence (IEnumerable`1).

A variation on John's suggestion:
var flattenedValues = dict.Values.SelectMany(x => x);
If you need them in a list, you can of course call ToList:
var flattenedList = dict.Values.SelectMany(x => x).ToList();

dictionary.values.toList();
if You want to get Sum just do
myDictionary.values.sum();

Values gets a ICollection containing the values of your dictionary. As implied by the definition of your dictionary, it can be defined as a ICollection<IList<MyClass>> collection. So if you really want a IList<IList<MyClass>>, use spacedog's solution.
If what you really want is a flat `IList', then there is no other solution than looping through each value :
IList<MyClass> l=new List<MyClass>();
foreach (IList<MyClass> v in myDictionary.Values)
l.AddRange(v);
Note that this is so grossly inefficient that you should think again about using a dictionary for what you are trying to achieve.