C# 如何在文本框为空时防止按 Tab 键

Question

Below code is work fine but how can I stop tab key when my textbox.text is empty how I put my if logic in ProcessCmdKey event?

bool a = true;
protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == Keys.Tab && !a)
    {
        return true;
    }
    return base.ProcessCmdKey(ref msg, keyData);
}

Answer 1

Override ProcessTabKey() and return true if you want to suppress Tab movement. You could check this.ActiveControl if you want it to work only for TextBoxes.

Here's a version that only suppresses Tab for those TextBoxes listed in the "SelectTextBoxes" List:

public partial class Form1 : Form
{

    public Form1()
    {
        InitializeComponent();
        SelectTextBoxes = new List<TextBox>() { textBox1, textBox2, textBox3 }; // list the textboxes here
    }

    private List<TextBox> SelectTextBoxes = new List<TextBox>(); 

    protected override bool ProcessTabKey(bool forward)
    {
        Control ctl = this.ActiveControl;
        if (ctl != null && ctl is TextBox)
        {
            TextBox tb = (TextBox)ctl;
            if (SelectTextBoxes.Contains(tb) && tb.Text.Length == 0)
            {
                return true;
            }
        }
        return base.ProcessTabKey(forward); // process TAB key as normal
    }

}

Answer 2

Modify the code to the following:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == Keys.Tab && string.IsNullOrEmpty(textbox.Text))
        return true;

    return base.ProcessCmdKey(ref msg, keyData);
}