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;

`C# does not limit the index type to integer.` - From [here](http://msdn.microsoft.com/en-us/library/2549tw02.aspx). Not sure why it complains. — Oded, Oct 10 '12 at 17:56
@MohsenAfshin i didn't miss the `this`, i just called it `IsSelected`. If it were called `this`, then it would mean there's only one indexed property. — Ian Boyd, Oct 10 '12 at 18:07

Oded · Accepted Answer · 2012-10-10T18:07:37.090

7

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).

edited Oct 10 '12 at 18:07

answered Oct 10 '12 at 17:57

Oded

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That would mean that i can't have two (e.g. `IsSelected[Guid personGuid]` and `IsApproved[Guid personGuid]`) indexed properties? – Ian Boyd Oct 10 '12 at 18:03
@IanBoyd - True. One limitation of indexers. Though you could have an `IsApproved[string reallyAGUID]` and parse into a GUID (ugly hack, but you see where I am going). – Oded Oct 10 '12 at 18:05
@IanBoyd: Correct. You can have multiple indexors if they have different types of keys, but you can't have indexes work how you want them to. You'd be better off with a `ToggleSelected(Guid personGuid)` and an `ToggleApproval(Guid personGuid)` methods, or something similar. – Matt Burland Oct 10 '12 at 18:05
Update your answer with *"cannot do - limitation of indexers"* and i'll accept it. – Ian Boyd Oct 10 '12 at 18:06

Matt Burland · Answer 2 · 2012-10-10T18:27:53.207

5

Indexers only work with the this keyword. See here.

The this keyword is used to define the indexers.

edited Oct 10 '12 at 18:27

answered Oct 10 '12 at 17:57

Matt Burland

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@AnthonyPegram: I've been proven wrong enough times to be careful about making absolute statements. – Matt Burland Oct 10 '12 at 18:06
i think that the compiler has a way of shutting that whole thing down. – Ian Boyd Oct 10 '12 at 18:08

score 1 · Answer 3 · answered Oct 10 '12 at 18:03

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; }

    ...
}

score 0 · Answer 4 · edited May 23 '17 at 11:56

@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

score 0 · Answer 5 · edited Oct 18 '13 at 15:14

0

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.

edited Oct 18 '13 at 15:14

Andrii Kalytiiuk

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answered Oct 18 '13 at 14:53

wade

1

Non-integer indexed Indexer properties in C#

5 Answers5