An indexer is a member that enables an object to be indexed in the same way as an array. Indexers are declared using indexer-declarations:

indexer-declaration
  • attributesopt indexer-modifiersopt indexer-declarator
  • accessor-declarations
indexer-modifiers
  • indexer-modifier
  • indexer-modifiers indexer-modifier
indexer-modifier
  • new
  • public
  • protected
  • internal
  • private
  • virtual
  • override
  • abstract
indexer-declarator
  • type this [ formal-parameter-list ]
  • type interface-type . this [ formal-parameter-list ]
An indexer-declaration may include a set of attributes , a new modifier, a valid combination of the four access modifiers, and one of the virtual, override, or abstract modifiers. The type of an indexer declaration specifies the element type of the indexer introduced by the declaration. Unless the indexer is an explicit interface member implementation, the type is followed by the keyword this. For an explicit interface member implementation, the type is followed by an interface-type, a ".", and the keyword this. Unlike other members, indexers do not have user-defined names.

The formal-parameter-list specifies the parameters of the indexer. The formal parameter list of an indexer corresponds to that of a method , except that at least one parameter must be specified, and that the ref and out parameter modifiers are not permitted. The type of an indexer and each of the types referenced in the formal-parameter-list must be at least as accessible as the indexer itself The accessor-declarations, which must be enclosed in "{" and "}" tokens, declare the accessors of the indexer. The accessors specify the executable statements associated with reading and writing indexer elements.

Even though the syntax for accessing an indexer element is the same as that for an array element, an indexer element is not classified as a variable. Thus, it is not possible to pass an indexer element as a ref or out parameter.

The formal parameter list of an indexer defines the signature of the indexer. Specifically, the signature of an indexer consists of the number and types of its formal parameters. The element type is not part of an indexer's signature, nor are the names of the formal parameters. The signature of an indexer must differ from the signatures of all other indexers declared in the same class.

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 or a member-access , whereas an indexer element is accessed through an element-access . 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 an error for an indexer accessor to declare a local variable with the same name as an indexer parameter. With these differences in mind, all rules defined apply to indexer accessors as well as property accessors. To understand these concepts please see the Sample code in C#.