You can declare indexers in an interface. To do this, specify the get and/or set keyword, but replace the body of the get or set accessor with a semicolon. Any class or struct that implements the interface must implement the indexer accessors declared in the interface. For example:

interface IRawInt
{
    bool this [ int index ] { get; set; }
}

struct RawInt : IRawInt
{
    ...
    public bool this [ int index ]
    {
        get { ... }
        set { ... }
    }
    ...
}

If you implement the interface indexer in a class, you can declare the indexer implementations as virtual. This allows further derived classes to override the get and set accessors. For example:

class RawInt : IRawInt
{
    ...
    public virtual bool this [ int index ]
    {
        get { ... }
        set { ... }
    }
    ...
}

You can also choose to implement an indexer by using the explicit interface implementation syntax covered in Chapter 12, “Working with Inheritance.” An explicit implementation of an indexer is non-public and non-virtual (and so cannot be overridden). For example:

struct RawInt : IRawInt
{
    ...
    bool IRawInt.this [ int index ]
    {
        get { ... }
        set { ... }
    }
    ...
}