Is there a BeforeUpdate for a C# ComboBox on a Winform

Question

I come from the VBA world, and remember there was a BeforeUpdate call I could make on a combobox. Now I am in C# (and loving it) and I was wondering is there a BeforeUpdate call for a ComboBox on a Winform?

I can make an invisible textbox and store the info I need there and after the update, look at that box for what I need, but I was hoping there was a simplier solution.

Answer 1

You could try ValueMemberChanged, Validating, SelectedIndexChanged, or TextChanged. They don't fire like the BeforeUpdate, but you can look at what will be updated and handle the updated, or refuse it.

Answer 2

You may consider SelectionChangeCommited.

From MSDN:

SelectionChangeCommitted is raised only when the user changes the combo box selection. Do not use SelectedIndexChanged or SelectedValueChanged to capture user changes, because those events are also raised when the selection changes programmatically.

This won't work when you have set your combobox to allow the user to type in the textbox though. Also, it won't tell you what the 'last' selected item was. You will have to cache this information. However, you don't need to store your information in a textbox. You can use a string.

Answer 3

One of the goodies of WF is that you can easily make your own. Add a new class to your project and paste the code below. Compile. Drop the new control from the top of the toolbox onto your form. Implement the BeforeUpdate event.

using System;
using System.ComponentModel;
using System.Windows.Forms;

public class MyComboBox : ComboBox {
  public event CancelEventHandler BeforeUpdate;

  public MyComboBox() {
    this.DropDownStyle = ComboBoxStyle.DropDownList;
  }

  private bool mBusy;
  private int mPrevIndex = -1;

  protected virtual void OnBeforeUpdate(CancelEventArgs cea) {
    if (BeforeUpdate != null) BeforeUpdate(this, cea);
  }

  protected override void OnSelectedIndexChanged(EventArgs e) {
    if (mBusy) return;
    mBusy = true;
    try {
      CancelEventArgs cea = new CancelEventArgs();
      OnBeforeUpdate(cea);
      if (cea.Cancel) {
        // Restore previous index
        this.SelectedIndex = mPrevIndex;
        return;
      }
      mPrevIndex = this.SelectedIndex;
      base.OnSelectedIndexChanged(e);
    }
    finally {
      mBusy = false;
    }
  }
}

Answer 4

Out of the box, there's nothing like that. All of the events that deal with change in the combo box happen after the new value is already selected. At that point there's no way to tell what the value USED to be. You're best bet is what you eluded to. As soon as your ComboBox is populated save the SelectedItem to a temporary variable. Then, hook into the SelectedValueChanged event. At that point, your temporary variable will be your old value, and the SelectedItem will be your current value.

private object oldItem = new object();

        private void button3_Click(object sender, EventArgs e)
        {
            DateTime date = DateTime.Now;
            for (int i = 1; i <= 10; i++)
            {
                this.comboBox1.Items.Add(date.AddDays(i));
            }

            oldItem = this.comboBox1.SelectedItem;
        }

        private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            //do what you need with the oldItem variable
            if (oldItem != null)
            {
                MessageBox.Show(oldItem.ToString());
            }

            this.oldItem = this.comboBox1.SelectedItem;
        }