Non-integer indexed Indexer properties in C#

Question

I want to have an indexed property in C#:

public Boolean IsSelected[Guid personGuid]
{
   get {
      Person person = GetPersonByGuid(personGuid);
      return person.IsSelected;
   }
   set {
      Person person = GetPersonByGuid(personGuid);
      person.IsSelected = value;
   }
}
public Boolean IsApproved[Guid personGuid]
{
   get {
      Person person = GetPersonByGuid(personGuid);
      return person.IsApproved;
   }
   set {
      Person person = GetPersonByGuid(personGuid);
      person.IsApproved= value;
   }
}

Visual Studio complains on the non-integer indexer syntax:

i know .NET supports non-Integer indexors.

---

In another language i would write:

private
   function GetIsSelected(ApproverGUID: TGUID): Boolean;
   procedure SetIsSelected(ApproverGUID: TGUID; Value: Boolean);
   function GetIsApproved(ApproverGUID: TGUID): Boolean;
   procedure SetIsApproved(ApproverGUID: TGUID; Value: Boolean);
public
   property IsSelected[ApproverGuid: TGUID]:Boolean read GetIsSelected write SetIsSelected;
   property IsApproved[ApproverGuid: TGUID]:Boolean read GetIsApproved write SetIsApproved;
end;

Answer 1

Your syntax is incorrect:

public Boolean this[Guid personGuid]
{
   get {
      Person person = GetPersonByGuid(personGuid);
      return person.IsSelected;
   }
   set {
      Person person = GetPersonByGuid(personGuid);
      person.IsSelected = value;
   }
}

Indexers are declared using the this keyword - you can't use your own name.

From Using Indexers (C# Programming Guide):

To declare an indexer on a class or struct, use the this keyword

---

Additionally, it is only possible to have one indexer that accepts a type - this is a limitation of the indexer syntax of C# (might be an IL limitation, not sure).

Answer 2

Indexers only work with the this keyword. See here.

The this keyword is used to define the indexers.

Answer 3

Just like Matt Burland and Oded said, indexers only work with this keyword, so you need to have a proxy class with the interface you need:

public class PersonSelector
{
    private MyClass owner;

    public PersonSelector(MyClass owner)
    {
        this.owner = owner;
    }

    public bool this[Guid personGuid]
    {
       get {
          Person person = owner.GetPersonByGuid(personGuid);
          return person.IsSelected;
       }
       set {
          Person person = owner.GetPersonByGuid(personGuid);
          person.IsSelected = value;
       }
    }

}

public class MyClass
{
    public MyClass()
    {
        this.IsSelected = new PersonSelector(this);
    }   

    public PersonSelector IsSelected { get; private set; }

    ...
}

Answer 4

@Jojan answered here:

C# 3.0 spec

"Overloading of indexers permits a class, struct, or interface to declare multiple indexers, provided their signatures are unique within that class, struct, or interface."

or if your data set is small you can IList

public IList<Boolean> IsSelected
{
   get { ... }
}

public IList<Boolean> IsApproved
{
   get { .... }
}

or use Multiple Indexers technique using Interfaces

Answer 5

Actually you can have multiple indexers accepting types.

However you can not have two indexers with the same signature. Same signature means the parameter number and types - so the above code has two indexers with same signature.

If the code changed to :

    Boolean this[string x, Guid personguid]

and :

    Boolean this[Guid personguid]

it should work.