In this article, we are going to learn about Indexers in C#, how to declare them in a class, struct, or interface, and overload them. We’re also going to learn what the differences between indexers and properties are.<\/p>\n
Let’s start.<\/p>\n
We’re all used to dealing with arrays<\/a>. The array is easy to use and most importantly its elements are indexed, i.e. we can access any element by its index<\/strong>. But what if we want to add indexing to our class or struct?<\/p>\n This is precisely what the Indexers achieve for us.<\/p>\n Indexer allows instances of a class or struct to have indexes.<\/strong> It enables us to do things that arrays alone cannot, and we’re going to see what exactly these things are going forward.<\/p>\n Let’s start by checking out the Indexers syntax. We can define Indexer by using the access modifier: <\/strong>Can be public, private, protected, or internal.<\/p>\n return type:<\/strong> Can be any type we need, it certainly can’t be void<\/strong>.<\/p>\n parameters:<\/strong> Can be any type and not just int type as in arrays. We should define one parameter at least.<\/p>\n get accessor<\/strong>: Returns a value.<\/p>\n set accessor:<\/strong> Assigns a value.<\/p>\n We should define either When the body of the Beginning with C# 7.0, we can also use this expression with the When the indexer has only one parameter, this is similar to one-dimensional arrays:<\/p>\n We create a\u00a0 In the constructor, we split the sentence into words to fill the Now, we can use the So far this looks pretty similar to what we can do with arrays, so what is the difference between indexers and arrays exactly here?<\/p>\n One of the advantages of Indexers is that we can add validation to the input:<\/p>\n In the constructor, we add a condition to check if In the As we’ve mentioned, we can pass more than one parameter to Indexers, similarly to how we do it with multi-dimensional arrays.<\/p>\n A good example of this is the Tic-Tac-Toe<\/strong> game.<\/p>\n First, let’s create an enum that contains our Then, let’s create Similar to the previous example, here we have two parameters representing row number and column number, respectively.<\/p>\n We can use the Indexers can also be overloaded, i.e we can define more than one indexer in our class or our struct.<\/p>\n Let’s apply this to the previous example, we want access to the cell by one integer index representing a cell number.<\/p>\n Cell numbering begins with 1, and ends with 9, where cell(0,0) becomes cell(1), cell(0,1) becomes cell(2), etc ..<\/p>\n We will add a simple method to archive a conversion process:<\/p>\n And we let’s add another Indexer with one parameter:<\/p>\n We just convert the cell number to a row and a column and then call the other indexer.\u00a0<\/p>\n Now, we can use If we need to, we can also declare indexers in interfaces<\/a> the same way we do in classes, except interface accessors don’t have modifiers:<\/p>\n We notice that a Let’s add another indexer with a default implementation:<\/p>\n This indexer does not need to be implemented in Indexers resemble properties but there are some differences between them:<\/p>\n\n<\/a>How to Implement Indexers in C#<\/h2>\n
this<\/code> keyword:<\/p>\n[acccess modifier] [return type] this([parameters])\r\n{\r\n get \r\n { \r\n \/\/return a value \r\n }\r\n set \r\n { \r\n \/\/set a value\r\n }\r\n}<\/pre>\nset<\/code> or get<\/code> accessor at least.<\/p>\n<\/a>Expression Body Definitions<\/h3>\n
get<\/code> accessor is a single statement, we can use lambda (=><\/code>) expression<\/a>.<\/p>\nset<\/code> accessor:<\/p>\n[acccess\u00a0modifier]\u00a0[return\u00a0type]\u00a0this([parameters])\r\n{\r\n get => \/\/ single statment\r\n set => \/\/ single statment\r\n}\r\n<\/pre>\n<\/a>Create One-Dimensional Arrays With Indexers<\/h2>\n
public class Sentence\r\n{\r\n private string[] _words;\r\n public\u00a0int\u00a0Length\u00a0=>\u00a0_words.Length;\u00a0\r\n \r\n public Sentence(string sentence)\r\n {\r\n _words = sentence.Split(' ');\r\n }\r\n\r\n public string this[int wordNumber]\r\n {\r\n get\r\n {\r\n return _words[wordNumber];\r\n }\r\n set\r\n {\r\n _words[wordNumber] = value;\r\n }\r\n }\r\n}<\/pre>\nSentence<\/code> class, it contains a _words<\/code> array and the Indexer takes an int <\/strong>type as a parameter, its modifier is public<\/strong> and the return type is string<\/strong>.<\/p>\n_words<\/code> array.<\/p>\nSentence<\/code> class as an array:<\/p>\nvar sentence = new Sentence(\"Hello from Code Maze\");\r\nfor (int i = 0; i < sentence.Length; i++)\r\n{\r\n Console.WriteLine(\"Word {0} : {1}\", i + 1, sentence[i]);\r\n}\r\n\r\n\/\/ The output is:\r\n\/\/ Word 1 : Hello\r\n\/\/ Word 2 : from \r\n\/\/ Word 3 : Code \r\n\/\/ Word 4 : Maze<\/pre>\npublic class Sentence\r\n{\r\n private string[] _words;\r\n public int Length => _words.Length;\r\n\r\n public Sentence(string sentence)\r\n {\r\n if (sentence == null)\r\n throw new ArgumentNullException(nameof(sentence));\r\n\r\n _words = sentence.Split(' ');\r\n }\r\n\r\n public string this[int wordNumber]\r\n {\r\n get\r\n {\r\n if (wordNumber < 0 || wordNumber > _words.Length - 1)\r\n throw new ArgumentOutOfRangeException(nameof(wordNumber));\r\n\r\n return _words[wordNumber];\r\n }\r\n set\r\n {\r\n if (wordNumber < 0 || wordNumber > _words.Length - 1)\r\n throw new ArgumentOutOfRangeException(nameof(wordNumber));\r\n\r\n if (value == null || value.Trim().Length == 0 || value.Split(' ').Length > 1)\r\n return;\r\n\r\n _words[wordNumber] = value;\r\n }\r\n }\r\n}\r\n<\/pre>\nsentence<\/code> is null.<\/p>\nget<\/code> accessor we\u00a0make sure that the wordNumber<\/code> is within the allowed range and in the set<\/code> accessor\u00a0we do this too and in addition, we make sure the value<\/code> is valid.<\/p>\n<\/a>Create Multi-Dimensional Arrays With Indexers<\/h2>\n
CellStatus<\/code>:<\/p>\npublic enum CellStatus\r\n{\r\n Empty,\r\n X,\r\n O\r\n}<\/pre>\nTicTacToe<\/code> class:<\/p>\npublic class TicTacToe\r\n{\r\n private const int _rowCount = 3;\r\n private const int _colCount = 3;\r\n private CellStatus[,] _patch = new CellStatus[_rowCount, _colCount];\r\n public CellStatus this[int row, int col]\r\n {\r\n get\r\n {\r\n if (row >= _rowCount || row < 0)\r\n throw new ArgumentOutOfRangeException(nameof(row));\r\n if (col >= _colCount || col < 0)\r\n throw new ArgumentOutOfRangeException(nameof(col));\r\n\r\n return _patch[row, col];\r\n }\r\n set\r\n {\r\n if (row >= _rowCount || row < 0)\r\n throw new ArgumentOutOfRangeException(nameof(row));\r\n if (col >= _colCount || col < 0)\r\n throw new ArgumentOutOfRangeException(nameof(col));\r\n if (!Enum.IsDefined(value))\r\n return;\r\n if (value == CellStatus.Empty)\r\n return;\r\n if (_patch[row, col] != CellStatus.Empty)\r\n return;\r\n _patch[row, col] = value;\r\n }\r\n }\r\n\r\n public override string ToString()\r\n {\r\n string s = \"\";\r\n for (int i = 0; i < _rowCount; i++)\r\n {\r\n for (int j = 0; j < _colCount; j++)\r\n {\r\n s += _patch[i, j].ToString();\r\n s += \"\\t\";\r\n }\r\n s += Environment.NewLine;\r\n }\r\n\r\n return s;\r\n }\r\n}<\/pre>\nTicTacToe<\/code> class as a two-dimensional array:<\/p>\nvar ticTacToe = new TicTacToe();\r\nticTacToe[0, 0] = CellStatus.X;\r\nConsole.WriteLine(ticTacToe.ToString());\r\n\r\n\/\/ The output is:\r\n\/\/ X Empty Empty\r\n\/\/ Empty Empty Empty\r\n\/\/ Empty Empty Empty<\/pre>\n
<\/a>Overloaded Indexers<\/h2>\n
private static (int, int) Convert(int cellNumber)\r\n{\r\n if(cellNumber <= 0)\r\n throw new ArgumentOutOfRangeException(nameof(cellNumber));\r\n\r\n return ((cellNumber - 1) \/ _rowCount, (cellNumber - 1) % _colCount);\r\n}<\/pre>\npublic CellStatus this[int cellNumber]\r\n{\r\n get\r\n {\r\n var result = Convert(cellNumber);\r\n return this[result.Item1, result.Item2];\r\n }\r\n set\r\n {\r\n var result = Convert(cellNumber);\r\n this[result.Item1, result.Item2] = value;\r\n }\r\n}\r\n<\/pre>\nTicTacToe<\/code> class with one index:<\/p>\nticTacToe[9] = CellStatus.O;\r\nConsole.WriteLine(ticTacToe.ToString());\r\n\r\n\/\/ The output is:\r\n\/\/ X Empty Empty\r\n\/\/ Empty Empty Empty\r\n\/\/ Empty Empty O<\/pre>\n
<\/a>Indexers in Interfaces<\/h2>\n
public interface IIndexerInterface\r\n{ \r\n string this[int index] { get; }\r\n}\r\n\r\npublic class IndexerClass : IIndexerInterface\r\n{\r\n public string this[int index] \r\n { \r\n get => \"Hello from class.\"; \r\n }\r\n}\r\n<\/pre>\nget<\/code> interface accessor does not have a body but after a default interface methods <\/strong>feature\u00a0in C# 8.0, we can write a default body.<\/p>\nstring this [string name] \r\n{\r\n get => \"Hello from interface.\";\r\n}<\/pre>\nIndexerClass<\/code> class because the get<\/code> accessor has a default body.<\/p>\n<\/a>Comparison Between Properties and Indexers in C#<\/h2>\n
\n\n
\n\t \nIndexer<\/th> Property<\/th>\n<\/tr>\n<\/thead>\n \n\t Defined by this keyword.<\/td> Defined by a name.<\/td>\n<\/tr>\n \n\t Accessed by an index.<\/td> Accessed by a name.<\/td>\n<\/tr>\n \n\t Can be only an instance member.<\/td> Can be a static or an instance member.<\/td>\n<\/tr>\n \n\t A get accessor has the same formal parameter list
\nas the indexer<\/td>A get accessor has no parameters.<\/td>\n<\/tr>\n \n\t A set accessor has the same formal parameter list
\nas the indexer, and also to the value parameter.<\/td>A set accessor contains the implicit value parameter.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n When We Should Use Indexers?<\/h2>\n