Collections & Enums
Table of contents
| Read No. | Name of chapter |
|---|---|
| 08 | Collections |
| 08 | Enums |
Collections
For many applications, you want to create and manage groups of related objects. There are two ways to group objects: by creating arrays of objects, and by creating collections of objects.
A collection is a class, so you must declare an instance of the class before you can add elements to that collection.
Using a Simple Collection
The examples in this section use the generic List
The following example creates a list of strings and then iterates through the strings by using a foreach statement.
// Create a list of strings.
var salmons = new List<string>();
salmons.Add("chinook");
salmons.Add("coho");
salmons.Add("pink");
salmons.Add("sockeye");
// Iterate through the list.
foreach (var salmon in salmons)
{
Console.Write(salmon + " ");
}
// Output: chinook coho pink sockeye
The following example removes an element from the collection by specifying the object to remove.
// Create a list of strings by using a
// collection initializer.
var salmons = new List<string> { "chinook", "coho", "pink", "sockeye" };
// Remove an element from the list by specifying
// the object.
salmons.Remove("coho");
// Iterate through the list.
foreach (var salmon in salmons)
{
Console.Write(salmon + " ");
}
// Output: chinook pink sockeye
The following example removes elements from a generic list. Instead of a foreach statement, a for statement that iterates in descending order is used. This is because the RemoveAt method causes elements after a removed element to have a lower index value.
var numbers = new List<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// Remove odd numbers.
for (var index = numbers.Count - 1; index >= 0; index--)
{
if (numbers[index] % 2 == 1)
{
// Remove the element by specifying
// the zero-based index in the list.
numbers.RemoveAt(index);
}
}
// Iterate through the list.
// A lambda expression is placed in the ForEach method
// of the List(T) object.
numbers.ForEach(
number => Console.Write(number + " "));
// Output: 0 2 4 6 8
For the type of elements in the List
private static void IterateThroughList()
{
var theGalaxies = new List<Galaxy>
{
new Galaxy() { Name="Tadpole", MegaLightYears=400},
new Galaxy() { Name="Pinwheel", MegaLightYears=25},
new Galaxy() { Name="Milky Way", MegaLightYears=0},
new Galaxy() { Name="Andromeda", MegaLightYears=3}
};
foreach (Galaxy theGalaxy in theGalaxies)
{
Console.WriteLine(theGalaxy.Name + " " + theGalaxy.MegaLightYears);
}
// Output:
// Tadpole 400
// Pinwheel 25
// Milky Way 0
// Andromeda 3
}
public class Galaxy
{
public string Name { get; set; }
public int MegaLightYears { get; set; }
}
Kinds of Collections
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Implementing a Collection of Key/Value Pairs
The Dictionary< TKey,TValue > generic collection enables you to access to elements in a collection by using the key of each element. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is fast because the Dictionary class is implemented as a hash table.
The following example creates a Dictionary collection and iterates through the dictionary by using a foreach statement.
private static void IterateThruDictionary()
{
Dictionary<string, Element> elements = BuildDictionary();
foreach (KeyValuePair<string, Element> kvp in elements)
{
Element theElement = kvp.Value;
Console.WriteLine("key: " + kvp.Key);
Console.WriteLine("values: " + theElement.Symbol + " " +
theElement.Name + " " + theElement.AtomicNumber);
}
}
private static Dictionary<string, Element> BuildDictionary()
{
var elements = new Dictionary<string, Element>();
AddToDictionary(elements, "K", "Potassium", 19);
AddToDictionary(elements, "Ca", "Calcium", 20);
AddToDictionary(elements, "Sc", "Scandium", 21);
AddToDictionary(elements, "Ti", "Titanium", 22);
return elements;
}
private static void AddToDictionary(Dictionary<string, Element> elements,
string symbol, string name, int atomicNumber)
{
Element theElement = new Element();
theElement.Symbol = symbol;
theElement.Name = name;
theElement.AtomicNumber = atomicNumber;
elements.Add(key: theElement.Symbol, value: theElement);
}
public class Element
{
public string Symbol { get; set; }
public string Name { get; set; }
public int AtomicNumber { get; set; }
}
To instead use a collection initializer to build the Dictionary collection, you can replace the BuildDictionary and AddToDictionary methods with the following method.
private static Dictionary<string, Element> BuildDictionary2()
{
return new Dictionary<string, Element>
{
{"K",
new Element() { Symbol="K", Name="Potassium", AtomicNumber=19}},
{"Ca",
new Element() { Symbol="Ca", Name="Calcium", AtomicNumber=20}},
{"Sc",
new Element() { Symbol="Sc", Name="Scandium", AtomicNumber=21}},
{"Ti",
new Element() { Symbol="Ti", Name="Titanium", AtomicNumber=22}}
};
}
The following example uses the ContainsKey method and the Item[] property of Dictionary to quickly find an item by key. The Item property enables you to access an item in the elements collection by using the elements[symbol] in C#.
private static void FindInDictionary(string symbol)
{
Dictionary<string, Element> elements = BuildDictionary();
if (elements.ContainsKey(symbol) == false)
{
Console.WriteLine(symbol + " not found");
}
else
{
Element theElement = elements[symbol];
Console.WriteLine("found: " + theElement.Name);
}
}
The following example instead uses the TryGetValue method quickly find an item by key.
private static void FindInDictionary2(string symbol)
{
Dictionary<string, Element> elements = BuildDictionary();
Element theElement = null;
if (elements.TryGetValue(symbol, out theElement) == false)
Console.WriteLine(symbol + " not found");
else
Console.WriteLine("found: " + theElement.Name);
}
Using LINQ to Access a Collection
LINQ (Language-Integrated Query) can be used to access collections. LINQ queries provide filtering, ordering, and grouping capabilities. For more information, see Getting Started with LINQ in C#.
The following example runs a LINQ query against a generic List. The LINQ query returns a different collection that contains the results.
private static void ShowLINQ()
{
List<Element> elements = BuildList();
// LINQ Query.
var subset = from theElement in elements
where theElement.AtomicNumber < 22
orderby theElement.Name
select theElement;
foreach (Element theElement in subset)
{
Console.WriteLine(theElement.Name + " " + theElement.AtomicNumber);
}
// Output:
// Calcium 20
// Potassium 19
// Scandium 21
}
private static List<Element> BuildList()
{
return new List<Element>
{
{ new Element() { Symbol="K", Name="Potassium", AtomicNumber=19}},
{ new Element() { Symbol="Ca", Name="Calcium", AtomicNumber=20}},
{ new Element() { Symbol="Sc", Name="Scandium", AtomicNumber=21}},
{ new Element() { Symbol="Ti", Name="Titanium", AtomicNumber=22}}
};
}
public class Element
{
public string Symbol { get; set; }
public string Name { get; set; }
public int AtomicNumber { get; set; }
}
Enumeration types
An enumeration type (or enum type) is a value type defined by a set of named constants of the underlying integral numeric type. To define an enumeration type, use the enum keyword and specify the names of enum members:
enum Season
{
Spring,
Summer,
Autumn,
Winter
}
By default, the associated constant values of enum members are of type int; they start with zero and increase by one following the definition text order. You can explicitly specify any other integral numeric type as an underlying type of an enumeration type. You can also explicitly specify the associated constant values, as the following example shows:
enum ErrorCode : ushort
{
None = 0,
Unknown = 1,
ConnectionLost = 100,
OutlierReading = 200
}
You use an enumeration type to represent a choice from a set of mutually exclusive values or a combination of choices. To represent a combination of choices, define an enumeration type as bit flags.
Enumeration types as bit flags
If you want an enumeration type to represent a combination of choices, define enum members for those choices such that an individual choice is a bit field. That is, the associated values of those enum members should be the powers of two. Then, you can use the bitwise logical operators | or & to combine choices or intersect combinations of choices, respectively. To indicate that an enumeration type declares bit fields, apply the Flags attribute to it. As the following example shows, you can also include some typical combinations in the definition of an enumeration type.
[Flags]
public enum Days
{
None = 0b_0000_0000, // 0
Monday = 0b_0000_0001, // 1
Tuesday = 0b_0000_0010, // 2
Wednesday = 0b_0000_0100, // 4
Thursday = 0b_0000_1000, // 8
Friday = 0b_0001_0000, // 16
Saturday = 0b_0010_0000, // 32
Sunday = 0b_0100_0000, // 64
Weekend = Saturday | Sunday
}
public class FlagsEnumExample
{
public static void Main()
{
Days meetingDays = Days.Monday | Days.Wednesday | Days.Friday;
Console.WriteLine(meetingDays);
// Output:
// Monday, Wednesday, Friday
Days workingFromHomeDays = Days.Thursday | Days.Friday;
Console.WriteLine($"Join a meeting by phone on {meetingDays & workingFromHomeDays}");
// Output:
// Join a meeting by phone on Friday
bool isMeetingOnTuesday = (meetingDays & Days.Tuesday) == Days.Tuesday;
Console.WriteLine($"Is there a meeting on Tuesday: {isMeetingOnTuesday}");
// Output:
// Is there a meeting on Tuesday: False
var a = (Days)37;
Console.WriteLine(a);
// Output:
// Monday, Wednesday, Saturday
}
}
Conversions
For any enumeration type, there exist explicit conversions between the enumeration type and its underlying integral type. If you cast an enum value to its underlying type, the result is the associated integral value of an enum member.
public enum Season
{
Spring,
Summer,
Autumn,
Winter
}
public class EnumConversionExample
{
public static void Main()
{
Season a = Season.Autumn;
Console.WriteLine($"Integral value of {a} is {(int)a}"); // output: Integral value of Autumn is 2
var b = (Season)1;
Console.WriteLine(b); // output: Summer
var c = (Season)4;
Console.WriteLine(c); // output: 4
}
}
Use the Enum.IsDefined method to determine whether an enumeration type contains an enum member with the certain associated value.
For any enumeration type, there exist boxing and unboxing conversions to and from the System.Enum type, respectively.