C# 4 and .NET 4

Dictionaries ❘ 253
application by entering X . If the key is in the dictionary, it is examined with the TryGetValue() method of
the Dictionary < TKey, TValue > class. TryGetValue() returns true if the key is found and false otherwise.
If the value is found, the value associated with the key is stored in the employee variable. This value is
written to the console.
You can also use an indexer of the Dictionary < TKey, TValue > class instead of
TryGetValue() to access a value stored in the dictionary. However, if the key is not
found, the indexer throws an exception of type KeyNotFoundException .
while (true)
{
Console.Write("Enter employee id (X to exit) > ");
var userInput = Console.ReadLine();
userInput = userInput.ToUpper();
if (userInput == "X") break;
EmployeeId id;
try
{
id = new EmployeeId(userInput);
}
}
}
Employee employee;
if (!employees.TryGetValue(id, out employee))
{
Console.WriteLine("Employee with id {0} does not exist",
id);
}
else
{
Console.WriteLine(employee);
}
}
catch (EmployeeIdException ex)
{
Console.WriteLine(ex.Message);
}
Running the application produces the following output:
lookup
Enter employee id (X to exit) > C3386
C003386: Jimmie Johnson 5.024.710,00
Enter employee id (X to exit) > F3547
F003547: Carl Edwards 5.597.120,00
Enter employee id (X to exit) > X
Press any key to continue …
Dictionary < TKey, TValue > supports only one value per key. The class Lookup < TKey, TElement > resembles
a Dictionary < TKey, TValue > but maps keys to a collection of values. This class is implemented in the
assembly System.Core and defi ned with the namespace System.Linq .
Lookup < TKey, TElement > cannot be created as a normal dictionary. Instead, you have to invoke
the method ToLookup() that returns a Lookup < TKey, TElement > object. The method ToLookup()
is an extension method that is available with every class implementing IEnumerable < T > . In the
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