methods - 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] {
    get {
      return SomeInternalMethod(strParameter);
    }
  }
}

It looks like it is called as follows:

SomeClass _someClass = new SomeClass();
SomeInterface returnedValue = _someClass["someString"];

I am interested in where this function would be appropriate or what the intent of writing in this style. For example why would this be preferred over simply calling the function?

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.