Data Binding doesn't work when I assign a new object instance to the bound variable

Question

I have an object that I have bound to a control on a form using C# WinForms (targetting .NET 4.5.2). I have implemented INotifyPropertyChanged, and when I modif

Answer 1

Ivan showed you how to solve the problem. Here in this answer I'll try to show you what the problem is and why it works this way.

Short answer

Your label is bound to an object not the variable. The variable is just like a pointer to the object. So replacing the variable doesn't have any impact on the object which your label is using as data source. Just the variable points to the new object. But your label is using the previous object. It has its own pointer to the previous object.

Consider these facts to know what is happening:

1. User currentUser = new User("Example Name");

   When you assign an object to a variable, in fact the variable points to the object.

2. lblName.DataBindings.Add("Text", currentUser, "Name");
   You performed data-binding to the object. The control will be bound to the object not the variable.

3. currentUser.Name = "Blahblah";

   You changed the value of Name property and since you implemented INotifyPropertyChanged the control will be notified of changes and will refresh its Text.

But the main statement which didn't work as you expected:

4. currentUser = new User("New Name");

   Here you made the currentUser variable point to another object. It doesn't have anything to do with the previous object which it was pointing to. It just points to another object.

   The Binding which you added to the label, still uses the previous object.

   So it's normal to not have any notification, because the object which is in use by the label didn't changed.

How the BindingSource solved the problem?

The BindingSource raises ListChanged event when you assign a new object to its DataSource and it makes the BindingObject fetch new value from DataSource and refresh Text property. You can take a look at this post: How to make a binding source aware of changes in its data source?

Answer 2

To get the desired behavior, you should not bind directly to the concrete instance like you do currently:

lblName.DataBindings.Add("Text", currentUser, "Name");

Instead, you need some intermediary. The easiest is to use the BindingSource component for that purpose.

Add a BindingSource field to the form:

private BindingSource dataSource;

Then initialize it and bind the controls to it one time (usually in form Load event):

dataSource = new BindingSource { DataSource = typeof(User) };
lblName.DataBindings.Add("Text", dataSource, "Name");
// ...

Now anytime you want to bind to a User instance, you simply assign it to the DataSource property of the BindingSource:

initial:

dataSource.DataSource = new User("Example Name");

later:

dataSource.DataSource = new User("New Name");