C# what is the point or benefit of an indexer?

Question

Doing some code reading and stumbled upon this snippet that I haven\'t seen before:

public SomeClass {
  public someInterface this[String strParameter] {
            


        

Answer 1

See the language specification, section 10.9, which states:

An Indexer is a member that enables an object to be indexed in the same way as an array.

Indexers and properties are very similar in concept, but differ in the following ways:

• A property is identified by its name, whereas an indexer is identified by its signature.
• A property is accessed through a simple-name (§7.5.2) or a member-access (§7.5.4), whereas an indexer element is accessed through an element-access (§7.5.6.2).
• A property can be a static member, whereas an indexer is always an instance member.
• A get accessor of a property corresponds to a method with no parameters, whereas a get accessor of an indexer corresponds to a method with the same formal parameter list as the indexer.
• A set accessor of a property corresponds to a method with a single parameter named value, whereas a set accessor of an indexer corresponds to a method with the same formal parameter list as the indexer, plus an additional parameter named value.
• It is a compile-time error for an indexer accessor to declare a local variable with the same name as an indexer parameter.
• In an overriding property declaration, the inherited property is accessed using the syntax base.P, where P is the property name. In an overriding indexer declaration, the inherited indexer is accessed using the syntax base[E], where E is a comma separated list of expressions.

Answer 2

That's called an Indexer, they allow you to use List<>, ArrayList, Dictionary<> and all the other collections using an array syntax.

It's just syntactic sugar, but it gives some readability when used right.

Answer 3

You may have already stumbled across something similar before:

var myList = new List<string>();
myList.Add("One");
myList.Add("Two");
myList.Add("Three");

for(int i = 0; i < myList.Count; i++) {
    string s = myList[i];
}

The indexer is the "primary key" of an object that implements a collection. It's just a shorthand way for writing a function like .GetValue(index) - syntactic sugar, if you want. But it also makes the intent clear.

Answer 4

In many cases, the 'index' syntax makes a lot of sense. It is particularly useful if the SomeClass represents some sort of collection.

Answer 5

It an implementation of the index operator [ ].

Answer 6

In my opinion, it's just a syntax convenience. Where would you NOT use it:

public SomeClass {
  private int[] nums;
  public GetVal this[int ind] {
    get {
      return nums[ind]; // this is pointless since array is already indexed
    }
  }
}

Where would you benefit from it:

public Matrix {
  private Dictionary<string, double> matrixData; // Matrix indecies, value
  public double this[int row, int col] {
    get {
      return matrixData[string.Format("{0},{1}", row, col)];
    }
  }
}

As you can see, for some reason, your data is a Dictionary indexed with a string key and you wish to call this with two integer indecies and still do not want to change your data type:

Matrix m = new Matrix();
...
Console.WriteLine( m[1,2] );