Indexers and Properties

pagates · February 24th, 2005, 11:02 AM

Hi All,

I may be a little confused on indexers and properties. Is there any way to a have a parameterized property?

For instance, say a control I am creating has a number of labels on it. I would like to have properties called "LabelColor" and "Text" for this class, but would like to pass in an index to tell which label to update.

For instance, here is a simplified class:

Code:

public class MyClass
{
       private ArrayList m_MyLabelList = new ArrayList();
               |
       public Color LabelColor
       {
            get
            { 
               return (m_MyLabelList[SomeIndex].ForeColor); 
            }
            set 
            { 
               m_MyLabelList[SomeIndex].ForeColor = value;
            }
       }

       public string Text
       {
            get
            { 
               return (m_MyLabelList[SomeIndex].Text); 
            }
            set 
            { 
               m_MyLabelList[SomeIndex].Text = value;
            }
       }
}

and then from code that uses this class:

Code:

       public MyClass test = new MyClass();
       Color ThisColor = test[0].Color;
       string ThisText = test[0].Text;
       test[1].Color = Color.Green;
       test[1].Text  = "This is label #1";
       test[2].Color = Color.DarkKhaki;
       test[2].Text = "This is label #2";

Any hints or pointers to code examples would be most appreciated.

Thanks,
pagates

mehdi62b · February 24th, 2005, 12:01 PM

use Indexers instead,

Code:

public Color this[int index]
{
     get
     {
          //check for a valid index 
          return (m_MyLabelList[index].ForeColor); 
     }
     set 
     { 
          //check for a valid index
          m_MyLabelList[index].ForeColor = value;
     }
}

_____________
Mehdi.
software student.

pagates · February 24th, 2005, 12:23 PM

Thanks, Mehdi, but that doesn't completely solve my problem.

In the example you've given, the indexer will (correctly) let me set the color. However, I cannot set the text.

If I try to do multiple indexers with different return types, such as:

Code:

public Color this[int index]
{
     get
     {
          //check for a valid index 
          return (m_MyLabelList[index].ForeColor); 
     }
     set 
     { 
          //check for a valid index
          m_MyLabelList[index].ForeColor = value;
     }
}

public string this[int index]
{
     get
     {
          //check for a valid index 
          return (m_MyLabelList[index].Text); 
     }
     set 
     { 
          //check for a valid index
          m_MyLabelList[index].Text = value;
     }
}

I get an "Class 'MyClass' already defines a member called 'this' with the same parameter types" error.

Is this just a shortcoming of C# (VB6 allowed indexed properties)?

Thanks,
PAGates

mehdi62b · February 24th, 2005, 03:37 PM

not shortcoming of C#(However I don't think VB6 could allow this),
if you write,

Code:

int index=0; 
object o=objMyClass[index];

compiler doesn't know what should return(string or Color)so in your case use methods simply or expose Labels in your indexer..

_____________
Mehdi.
software student.

pagates · February 24th, 2005, 03:50 PM

Hi Medhi,

Thanks - that's what I was afraid of. I still say it's a shortcoming (single indexer and/or non-indexed properties).

FYI - it was possible in VB6 (I used it quite a bit):

Code:

Public Property Get Text(ByVal Index As Integer) As String
    ' Do Error Checking and stuff
    Text = SomeClass(Index).Text
End Property

Public Property Let Text(ByVal Index As Integer, ByVal New_Text As String)
  ' Do Some Error Handling, etc.
  SomeClass(Index).Text = New_Text
  PropertyChanged "Text" & Index
End Property

Thanks again,
PAGates

mehdi62b · March 15th, 2005, 08:08 AM

I didn't get your code,

in C# if you declare two things like,

everything TheSameName(TheSameArguments){}

compiler gives that error(IMO it's completely logical).

_____________
Mehdi.
software student.