Windows Forms data binding

Question

So, my question is about the exact methodology behind windows form data binding.

I wrote a simple code, where i created a View, an IViewModel interface and a ViewModel.

interface IVM
{
}

and

public class Vm : IVM
{
    int number;
    public int Number
    {
        get
        {
            return this.number;
        }

        set
        {
            this.number = value;
        }
    }
}

the form looks like:

public partial class Form1 : Form
{
    private IVM vm;

    public Form1()
    {
        InitializeComponent();
        this.vm = new Vm();

        this.iVMBindingSource.DataSource = this.vm;
    }
}

and the related designer part is:

this.textBox1.DataBindings.Add(new System.Windows.Forms.Binding("Text", this.iVMBindingSource, "Number", true));
...
this.iVMBindingSource.DataSource = typeof(WindowsFormsApplication1.IVM);

You can clearly see that IViewModel interface does not publish a Number property, but the concrete ViewModel class has a Number property.

Although in design time i can't use the designer to bind the property (since IVM has no Number prop), i can manually write "iVMBindingSource - Number" into the textbox's Test property, to bind it.

My question is, how does binding work EXACTLY? Why don't I receive a runtime error, while trying to access IVM's not existing Number property? (I tested and it actually changes the VM's Number prop properly)

Does it use some kind of reflection? How does this "magic" binding string works?

Thanks for your answers!

DerApe · Accepted Answer

Jup it's done by reflection. I just checked the code and the binding is done by the Binding class. There is a method called CheckBindings which ensures the property you want to bind on is available. It basically works like this:

if (this.control != null && this.propertyName.Length > 0)
{
  // ...certain checks...
  // get PropertyDescriptorCollection (all properties)
  for (int index = 0; index < descriptorCollection.Count; ++index)
  {
    // select the descriptor for the requested property
  }
  // validation
  // setup binding
}

As Ike mentioned, you can find the source code here: http://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/Binding.cs,3fb776d540d0e8ac

MSDN Reference: https://msdn.microsoft.com/en-us/library/system.windows.forms.binding(v=vs.110).aspx

Windows Forms data binding

Answers (2)

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