假设我理解正确，这里是我对问题解释的回答

Assuming I understand the question correctly, here's my answer to the interpretation of the question

private void Form1_Load(object sender, EventArgs e)
{
    foreach (TextBox tb in this.Controls.OfType<TextBox>())
    {
        tb.Enter += new EventHandler(textBoxAll_Enter);
        tb.Leave += new EventHandler(textBoxAll_Leave);
    }
}

private void textBoxAll_Enter(object sender, EventArgs e)
{
    ((TextBox)sender).Text = "textbox gained focus";
}

private void textBoxAll_Leave(object sender, EventArgs e)
{
    ((TextBox)sender).Text = "textbox lost focus";
}