WPF Dictionary Binding by key does not work - binding

Question

I really don't get it.
I created a viewmodel-class, which contains two objects, like this one:
public class MyViewModel{
public MyFirstObject FirstObject {get;set;}
public MySecondObject SecondObject {get;set;}
...
// Constructors and so on...
}
The FirstObject looks like this:
public class MyFirstObject {
public int Id {get;set;}
...
// Constructors and so on...
}
}
And MySecondObject contains a Dictionary:
public class MySecondObject{
public Dictionary<int, string> MyDict {get;set;}
...
// Constructors and so on...
}
}
What I want to do now, is to get the dictionarys value for a key, which is a refence to MyFirstObject.Id - like this:
mySecondObject.myDict[myFirstObject.Id]
Doing this in C# is easy, but now I want to do this in xaml.
I created a UserControl and set the DataContext to a MyViewModel-reference.
After that, I tried this:
<TextBox Text={Binding SecondObject[FirstObject.Id]} ../>
Unfortunately this is not working, because the key 'FirstObject.Id' can not be found.
Has anyone an idea how to fix that?
Thanks a lot!
CodeCannibal

Answer 1

Don't do this in XAML. Just create a new property on your viewmodel which encapsulates this logic.
You can also delegate the PropertyChangedEvent to propagate your value changes:
public class MyViewModel : INotifyPropertyChanged {
private MyFirstObject firstObject;
public MyFirstObject FirstObject
{
get { return firstObject; }
set { firstObject = value; PropertyChanged("MyDictValue"); }
}
private MySecondObject secondObject;
public MySecondObject SecondObject
{
get { return secondObject; }
set
{
secondObject = value; PropertyChanged("MyDictValue");
// you can subscribe here also for your MyDict changes if it is an
// observable and you can call PropertyChanged("MyDictValue");
// in the change event etc
}
}
public string MyDictValue
{
get { return SecondObject.MyDict[FirstObject.Id]; }
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
protected void RaisePropertyChanged(string propertyName)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
And bind to your new property:
<TextBox Text={Binding MyDictValue} ../>
With this approach you will have a place inside the MyDictValue to add additional logic like handling if FirstObject or SecondObject is null etc.