Forms controls two-way databinding to class properties has strange behaviour

Question

To learn databinding I have a test-project with copied example code. I have a test class with some properties and on a form I have some textboxes, that should be two-way bound to the class properties:

    public class Test : INotifyPropertyChanged
    {
        private string strProp1;
        private string strProp2;
        public string StrProp1
        {
            get {return strProp1; }
            //set { strProp1 = value; OnPropertyChanged("StrProp1"); }
            set { strProp1 = value; OnPropertyChanged(); }
        }
        public string StrProp2
        {
            get { return strProp2; }
            set { strProp2 = value; OnPropertyChanged(); }
        }
///.... more properties
        //---------------------------------
        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;

            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
     }

//// in Form class and Load() ////

Test tst=new Test();
            txt1.DataBindings.Add("Text", tst, "StrProp1", true, DataSourceUpdateMode.OnPropertyChanged);
            txt2.DataBindings.Add("Text", tst, "StrProp2", true, DataSourceUpdateMode.OnPropertyChanged);

Now, when I edit only textbox txt2 I see with the debugger that following code/calls are made:

• set { strProp2... // OK
• OnPropertyChanged() // OK
• getters of all properties // unnecessary?
• OnPropertyChanged() // unnecessary, second time. Why is that?
• getters of all properties // unnecessary, second time!

I don't understand the bindings fully, yet. What is my coding error? Is there a better way to do two-way binding? OnPropertyChanged(): why creating every time handler = PropertyChanged? What purpose has propertyName in PropertyChangedEventArgs(propertyName))? It is not used anyway? Thanks for help / advice.

Databinding do updates only on necessary properties, without double call to all the class getters.

Answer 1

databinding is usefull with Tables here is an example

DataTable table = new DataTable();
public Form1()
     {
      InitializeComponent();

            table.Columns.Add("strProp1", typeof(string));
            table.Columns.Add("strProp2", typeof(string));

            DataRow dr = table.NewRow();
            dr["strProp1"] = "";
            dr["strProp2"] = "";
            table.Rows.Add(dr);
            textBox1.DataBindings.Add("Text", table, "strProp1", false);
            textBox2.DataBindings.Add("Text", table, "strProp2", false);

}

  private void button1_Click(object sender, EventArgs e)
  {
            DataRow dr = table.Rows[0];
            dr["strProp1"] = "welcome" + DateTime.Now.Millisecond;
            dr["strProp2"] = "hello" + DateTime.Now.Millisecond;
  }

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