Edit

Share via

Indexers in Interfaces (C# Programming Guide)

  • Article

Indexers can be declared on an interface. Accessors of interface indexers differ from the accessors of class indexers in the following ways:

  • Interface accessors don't use modifiers.
  • An interface accessor typically doesn't have a body.

The purpose of the accessor is to indicate whether the indexer is read-write, read-only, or write-only. You can provide an implementation for an indexer defined in an interface, but this is rare. Indexers typically define an API to access data fields, and data fields can't be defined in an interface.

The following is an example of an interface indexer accessor:

public interface ISomeInterface
{
    //...

    // Indexer declaration:
    string this[int index]
    {
        get;
        set;
    }
}

The signature of an indexer must differ from the signatures of all other indexers declared in the same interface.

Example

The following example shows how to implement interface indexers.

// Indexer on an interface:
public interface IIndexInterface
{
    // Indexer declaration:
    int this[int index]
    {
        get;
        set;
    }
}

// Implementing the interface.
class IndexerClass : IIndexInterface
{
    private int[] arr = new int[100];
    public int this[int index]   // indexer declaration
    {
        // The arr object will throw IndexOutOfRange exception.
        get => arr[index];
        set => arr[index] = value;
    }
}

IndexerClass test = new IndexerClass();
System.Random rand = System.Random.Shared;
// Call the indexer to initialize its elements.
for (int i = 0; i < 10; i++)
{
    test[i] = rand.Next();
}
for (int i = 0; i < 10; i++)
{
    System.Console.WriteLine($"Element #{i} = {test[i]}");
}

/* Sample output:
    Element #0 = 360877544
    Element #1 = 327058047
    Element #2 = 1913480832
    Element #3 = 1519039937
    Element #4 = 601472233
    Element #5 = 323352310
    Element #6 = 1422639981
    Element #7 = 1797892494
    Element #8 = 875761049
    Element #9 = 393083859
*/

In the preceding example, you could use the explicit interface member implementation by using the fully qualified name of the interface member. For example

string IIndexInterface.this[int index]
{
}

However, the fully qualified name is only needed to avoid ambiguity when the class is implementing more than one interface with the same indexer signature. For example, if an Employee class is implementing two interfaces, ICitizen and IEmployee, and both interfaces have the same indexer signature, the explicit interface member implementation is necessary. That is, the following indexer declaration:

string IEmployee.this[int index]
{
}

Implements the indexer on the IEmployee interface, while the following declaration:

string ICitizen.this[int index]
{
}

Implements the indexer on the ICitizen interface.

See also