C# 4 and .NET 4

250 ❘ ChaPTer 10 cOllectiOns
If you need to use a key type that does not implement IEquatable and override GetHashCode according
to the key values you store in the dictionary, you can create a comparer implementing the interface
IEqualityComparer. IEqualityComparer defines the methods GetHashCode() and Equals()
with an argument of the object passed, so you can offer an implementation different from the object type
itself. An overload of the Dictionary constructor allows passing an object implementing
IEqualityComparer. If such an object is assigned to the dictionary, this class is used to generate the
hash codes and compare the keys.
dictionary example
The dictionary example is a program that sets up a dictionary of employees. The dictionary is indexed by
EmployeeId objects, and each item stored in the dictionary is an Employee object that stores details of an
employee.
The struct EmployeeId is implemented to define a key to be used in a dictionary. The members of the
class are a prefix character and a number for the employee. Both of these variables are read-only and can
be initialized only in the constructor. A key within the dictionary shouldn’t change, and this way that is
guaranteed. The fields are filled within the constructor. The ToString() method is overloaded to get a
string representation of the employee ID. As required for a key type, EmployeeId implements the interface
IEquatable and overloads the method GetHashCode().
[Serializable]
public class EmployeeIdException : Exception
{
public EmployeeIdException(string message) : base(message) { }
}
[Serializable]
public struct EmployeeId : IEquatable
{
private readonly char prefix;
private readonly int number;
public EmployeeId(string id)
{
if (id == null) throw new ArgumentNullException("id");
}
prefix = (id.ToUpper())[0];
int numLength = id.Length 1;
try
{
number = int.Parse(id.Substring(1, numLength > 6 6 : numLength));
}
catch (FormatException)
{
throw new EmployeeIdException("Invalid EmployeeId format");
}
public override string ToString()
{
return prefix.ToString() + string.Format("{0,6:000000}", number);
}
public override int GetHashCode()
{
return (number ^ number