Use Control.ControlCollection.Find.
TextBox tbx = this.Controls.Find("textBox1", true).FirstOrDefault() as TextBox;
tbx.Text = "found!";
EDIT for asker:
Control[] tbxs = this.Controls.Find(txtbox_and_message[0,0], true);
if (tbxs != null && tbxs.Length > 0)
{
tbxs[0].Text = "Found!";
}
You can use:
f.Controls[name];
Where f is your form variable. That gives you the control with name name.
TextBox tbx = this.Controls.Find("textBox1", true).FirstOrDefault() as TextBox;
tbx.Text = "found!";
If Controls.Find is not found "textBox1" => error. You must add code.
If(tbx != null)
Edit:
TextBox tbx = this.Controls.Find("textBox1", true).FirstOrDefault() as TextBox;
If(tbx != null)
tbx.Text = "found!";
Source: Stackoverflow.com