In a HashTable Key cannot be null, but Value can be.

Stack st = new Stack();
st.Push(11);
st.Push(22);
st.Push(-53);
st.Push(33);
st.Push(66);

IEnumerable e;
e = st.GetEnumerator(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerator e;
e = st.GetEnumerable(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerator e;
e = st.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = Stack.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

Queue q = new Queue(); 
q.Enqueue("Sachin"); 
q.Enqueue('A'); 
q.Enqueue(false); 
q.Enqueue(38); 
q.Enqueue(5.4);

IEnumerator e;
e = q.GetEnumerator(); 
while (e.MoveNext())
Console.WriteLine(e.Current);

IEnumerable e;
e = q.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = q.GetEnumerable(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

IEnumerator e;
e = Queue.GetEnumerator(); 
while (e.MoveNext()) 
Console.WriteLine(e.Current);

1. Map
2. Stack
3. Queue
4. BitArray
5. HashTable

1. Stack
2. Queue
3. BitArray
4. ArrayList
5. HashTable

1. Method
2. Class
3. Assembly
4. Namespace
5. Enum

1. Custom attribute cannot be applied to an assembly.
2. [assembly: Tested("Sachin", testgrade.Good)]
3. [Tested("Virat", testgrade.Excellent)]
   class customer { /* .... */ }
4. Custom attribute cannot be applied to a method.
5. Custom attribute cannot be applied to a class.