Add Property to an existing class in a library

Question

I want to add a Property to the Microsoft.Exchange.WebServices.Data.Folder class. The reason for this is, I need a bool Property "Selected" to bind it in WPF to a checkbox.

First I thought I can use C#-Extensions but currently it's not possible to write a property extension.

Then I created my own class "MyFolder" so I can cast from the Folder class to my class.

It doesn't work either.

public class MyFolder : Folder, INotifyPropertyChanged
{
    private bool selected;

    public bool Selected
    {
        get { return selected; }
        set { selected = value; OnPropertyChanged("Selected"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public MyFolder(ExchangeService service):base(service)
    {

    }

    protected void OnPropertyChanged(string name)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}

Edit:

I know it would be possible to add a property Folder to the class MyFolder. But I thought it must be possible in a elegant way.

public class MyFolder : INotifyPropertyChanged
{
    private bool selected;

    public bool Selected
    {
        get { return selected; }
        set { selected = value; OnPropertyChanged("Selected"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public Folder FolderObject { get; set; }

    public MyFolder(Folder FolderObject)
    {
        this.FolderObject = FolderObject;
    }

    protected void OnPropertyChanged(string name)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}

MyFolder is the class I want to work with, because MyFolder has the selected property. — Joba, Jun 14 '17 at 07:49
I understand that but you said you have to cast `Folder` to `MyFolder`. That's why I'm asking if you are using `Folder` or `MyFolder` in your code. — Mighty Badaboom, Jun 14 '17 at 07:50
Have you look at this question [2246777](https://stackoverflow.com/questions/2246777/raise-an-event-whenever-a-propertys-value-changed) — Francesco Milani, Jun 14 '17 at 07:57
you should wrap the class your trying to bind to in a View model and then you can add the selected property to the view model class see https://msdn.microsoft.com/en-gb/library/hh848246.aspx — MikeT, Jun 14 '17 at 09:00
@MikeT That's a good way, but currently I'm not using MVVM. I will remember it in future. Thanks! — Joba, Jun 14 '17 at 11:35

score 2 · Accepted Answer · answered Jun 14 '17 at 09:29

2

You don't need to add a property to an existing class. As you are in MVVM, why not simply implement a View Model that represents that functionality.

So for example:

class MyFolderViewModel : INotifyPropertyChanged
{
    public bool IsSelected { get; set; }

    private Folder folderObject;
}

answered Jun 14 '17 at 09:29

S. Spindler

546
3
12

Thanks! Thats a good way, but I am not using MVVM. I will remember it in future! – Joba Jun 14 '17 at 11:36
Generally you use MVVM. Your class `MyFolder` acts as a View Model since it implements `INotifyPropertyChanged`. What do you want to achieve with `MyFolder`? – S. Spindler Jun 14 '17 at 12:05

score 1 · Answer 2 · answered Jun 14 '17 at 10:16

I want to add a Property to the Microsoft.Exchange.WebServices.Data.Folder class.

This is indeed not possible since you haven't defined this class yourself and thus you cannot change its definition. Adding a property to a class would require you to recompile it.

Then I created my own class "MyFolder" so I can cast from the Folder class to my class.

You cannot cast a Microsoft.Exchange.WebServices.Data.Folder object to a custom type of yours...this will never work.

What you should do is to create your own class, e.g. MyFolder, that wraps any functionality of the Microsoft.Exchange.WebServices.Data.Folder class that you need. This is basically what you have done already I guess. There is no better way.

You then bind directly to an instance of your client-aware class (MyFolder) that implements the INotifyPropertyChanged interface. This is a good and recommended approach really. Forget about adding properties to a build-in class and casting.

And how can I convert an existing Folder object to a MyFolder object? — Joba, Jun 14 '17 at 11:32
You can't convert it. You should wrap it in your custom class. Then you can simply bind to Folder.SomeProperty of your MyFolder object. — mm8, Jun 14 '17 at 11:38
@Waterfront you can't cast Folder into myFolder however you can set up an implicit or Explixt converter that will allow you to convert see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/implicit — MikeT, Jun 14 '17 at 12:15

Add Property to an existing class in a library

2 Answers2