WinForms two way bindings don't work when change property value c#

Question

I have this code, and one TextBox control txtTest.

public partial class Form1 : Form
{

    public Test TestProperty { get; set; }

    public Form1()
    {
        InitializeComponent();
        TestProperty = new Test();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        TestProperty.Value = 15;
        txtTest.DataBindings.Add("Text", TestProperty, 
            nameof(TestProperty.Value),false, 
            DataSourceUpdateMode.OnPropertyChanged); // textTest has value 15
        TestProperty.Value = 10; // textTest has value 15
    }
}

public class Test
{
    public int Value { get; set; }
}

On form load I assign value 15 to Value property, after that I add binding for txtTest control on Text property, now my control show txtTest.Text = "15". After binding I assign new value to my Value property 10. Now my txtTest control still shows value 15. If I enter for example 20 in my text box control via form I have value 20 in property Value.

For this case why I don't have value 10 in my txtTest.Text property?

Answer 1

To support two-way databinding for your business objects, you need to raise change notification when data in your object changes. To do so, you can use either of the following options:

• You can implement INotifyPropertyChanged for your class. (✔ Preferred)

• Or you can implement [PropertyName]Changed pattern for you property; assuming you have XXXX property, then you need to have XXXXChanged event in your class.

To learn more about data binding and change notification, take a look at the following documentations:

• How to: Implement the INotifyPropertyChanged Interface
• How to: Apply the PropertyNameChanged Pattern
• Change Notification in Windows Forms Data Binding

You can find a more useful documentations in the following doc:

• Windows Forms Data Binding

Example - Implement INotifyPropertyChanged

using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
public class Customer : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    string firstName;
    public string FirstName
    {
        get { return firstName; }
        set {
            if (value != firstName) {
                firstName = value;
                NotifyPropertyChanged();
            }
        }
    }
    string lastName;
    public string LastName
    {
        get { return lastName; }
        set {
            if (value != lastName) {
                lastName = value;
                NotifyPropertyChanged();
            }
        }
    }
}

Example 2 - Apply [PropertyName]Changed pattern

using System;
public class Customer 
{
    public event EventHandler FirstNameChanged;
    public event EventHandler LastNameChanged;
    protected void OnFirstNameChanged(EventArgs e)
    {
        FirstNameChanged?.Invoke(this, e);
    }
    protected void OnLastNameChanged(EventArgs e)
    {
        LastNameChanged?.Invoke(this, e);
    }
    string firstName;
    public string FirstName
    {
        get { return firstName; }
        set {
            if (value != firstName) {
                firstName = value;
                OnFirstNameChanged(EventArgs.Empty);
            }
        }
    }
    string lastName;
    public string LastName
    {
        get { return lastName; }
        set {
            if (value != lastName){
                lastName = value;
                OnLastNameChanged(EventArgs.Empty);
            }
        }
    }
}