Inheritance / Polymorphism

Grant McConville

Hi, I would like to know how to add a variable to collection or list in a inheriated class.
Any help would be much appreciated.

This is what I would to use it in

class MyGrid : DataGridView

{

// Code to add the enum MyStyle to the Columns Collection, calling it DisplayStyle

private void PaintCell(object sender, DataGridViewCellPaintingEventArgs e)

{

if (this.Columns[e.ColumnIndex].DisplayStyle == MyStyle.Currency)

{

// code here

}

enum MyStyle

{

Currency, Accounting, Other

}

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes15y ago
Accepted answer
You can't do it like that because it's not the DataGridViewColumnCollection that you have to derive from, it's the DataGridViewColumn class itself because that's the class to which the new property is being added.

Suppose this derived class is called MyDataGridViewColumn.

As the link I posted indicated, you need to create a derived class from DataGridViewCell (say MyDataGridViewCell), create an instance of it and set the CellTemplate property of MyDataGridViewColumn to this instance.

MyDataGridViewCell would need to be given a public property called MyDisplayStyle (or whatever you're going to call it) with a backing field to store the enum value.

MyDataGridViewColumn would then need to wrap this property (via CellTemplate), so that it could be set for individual columns in the DataGridViewCollectionCollection. I'm not sure whether you'd need to rewrap all the other properties of MyDataGridViewCell or whether you could just inherit the wrappings from DataGridViewColumn (hopefully the latter).

For both of your custom classes, you also need to override the Clone() method - as mentioned in the link - to encompass the new property.

As it's really an expert job to do all this, it might be a good idea to look at one of the 6 existing derived classes of DataGridViewColumn using a disassembler such as Redgate Reflector (or the new JetBrains dotPeek) to see how the Microsoft programmers have added new functionality. The chances of getting it right otherwise are probably slim.
Grant McConville15y ago
Thank you for your advice, I will look at a different way to do this
Grant McConville15y ago
I was thinking about this on the way home from work and this is what I thought might work

public class MyDataGrid : DataGridView
{
public class MyColumns : DataGridViewColumnCollection
{
public Char ColumnType;
}

public MyColumns Cols;

public MyDataGrid()
{
CellPainting += new DataGridViewCellPaintingEventHandler(DrawCell);
Cols = new MyColumns();
}

private void DrawCell(object sender, DataGridViewCellPaintingEventArgs e)
{
CellPainting -= new DataGridViewCellPaintingEventHandler(DrawCell);
if (this.Cols[e.ColumnIndex].ColumnType == "D") //Decimal
{
// Code for drawing cell in decimal view
}
CellPainting += new DataGridViewCellPaintingEventHandler(DrawCell);
}
}

However, this.Cols[e.ColumnIndex].ColumnType does not work (DataGridViewColumn does not contain a definition for 'ColumnType' and no extension method etc etc)
It has a few other errors as well, such as the line 'public class MyColumns : DataGridViewColumnCollection' saying DataGridViewColumnCollection does not contain a constructor that takes 0 arguments

Am I even close to looking at this correctly, or am I completely wrong

Thanks
Grant McConville
Grant McConville15y ago
Thank you for this information.

Is there any chance you could post some code so I can understand this more clearly.
At the moment I do not know where to start

Thank you
Grant McConville
Vulpes15y ago
As its name suggests, the DataGridViewColumnCollection is a collection of DataGridViewColumn objects.

So, you need to derive from DataGridViewColumn to add a new property called DisplayStyle of type MyStyle. This in turn means that (amongst other things) you have to initialize the CellTemplate property to a new instance of a related type derived from DataGridViewCell (see http://msdn.microsoft.com/en-us/library/system.windows.forms.datagridviewcolumn.aspx for details).

Another problem is that some of the classes which derive from DataGridViewColumn, such as DataGridViewComboBoxColumn, already have a DisplayStyle property and so introducing a public property of the same name in its parent class will introduce a naming conflict and these classes may no longer work properly. So, if you are going to do this, I'd choose some other name for the property which none of the framework derived classes already have,
Grant McConville15y ago
The collection is Columns
The variable I would like to add is a enum called MyStyle

Thankyou
15y ago
here which is your variable and your collection ?