Nullable ComboBox

[Andrus]

I want to create generic bound NullableComboBo x class which allows to enter null values. I havent found a way to store null value to DataSource.

To reproduce:

1. Run the code
2. Clear text
3. Press button.

Observed:

Value is not null

Expected:

Value must be null

How to fix ?

Andrus.


using System.Windows. Forms;
using System.Collecti ons.Generic;
using System.Componen tModel;

class testForm : Form {

Storage s = new Storage();

testForm() {
Customer c = new Customer();
c.Id = "J";
c.Name = "John";

List<Customer> l = new List<Customer>( );
l.Add(c);

NullableComBobo x cm = new NullableComBobo x();
cm.DisplayMembe r = "Name";
cm.ValueMember = "Id";
cm.DataSource = l;

s.CustId = "J";

cm.DataBindings .Add("SelectedV alue", s, "CustId", true,
System.Windows. Forms.DataSourc eUpdateMode.OnV alidation, null);

Controls.Add(cm );
Button b = new Button();
b.Top = 80;
b.Click += new System.EventHan dler(b_Click);
Controls.Add(b) ;

}

void b_Click(object sender, System.EventArg s e) {
if (s.CustId == null)
MessageBox.Show ("Value is null");
else
MessageBox.Show ("Value is NOT null " + s.CustId.ToStri ng());

}

class Customer {
public string Id { get; set; }
public string Name { get; set; }

}

class Storage {
public string CustId { get; set; }
}

static void Main() {
Application.Run (new testForm());
}
}

/// <summary>
/// Combobox which allows to enter null value
/// </summary>
public class NullableComBobo x : ComboBox {

protected override void OnValidating(Ca ncelEventArgs e) {

if (Text.Trim().Le ngth == 0) {
//SelectedItem = null;
SelectedIndex = -1;
base.OnValidati ng(e);
return;
}

int pos = FindString(Text .Trim());
if (pos >= 0) {
SelectedIndex = pos;
base.OnValidati ng(e);
return;
}

e.Cancel = true;
MessageBox.Show ("Invalid entry");
base.OnValidati ng(e);
}
}

[Plater]

Observed:

Value is not null

Expected:

Value must be null

How to fix ?

Andrus.

if (Value =="")
   Value=null;