Foreach Control in form, how can I do something to all the TextBoxes in my Form?

A lot of the above work. Just to add. If your textboxes are not directly on the form but are on other container objects like a GroupBox, you will have to get the GroupBox object and then iterate through the GroupBox to access the textboxes contained therein.

foreach(Control t in this.Controls.OfType<GroupBox>())
{
   foreach (Control tt in t.Controls.OfType<TextBox>())
   {
        // do stuff
   }
}

private IEnumerable<TextBox> GetTextBoxes(Control control)
{
    if (control is TextBox textBox)
    {
        yield return textBox;
    }

    if (control.HasChildren)
    {
        foreach (Control ctr in control.Controls)
        {
            foreach (var textbox in GetTextBoxes(ctr))
            {
                yield return textbox;
            }
        }
    }
}

I found this to work very well, but initially I have my textboxes on a panel so none of my textboxes cleared as expected so I added

this.panel1.Controls.....  

Which got me to thinking that is you have lots of textboxes for different functions and only some need to be cleared out, perhaps using the multiple panel hierarchies you can target just a few and not all.

foreach ( TextBox tb in this.Controls.OfType<TextBox>()) {
  ..
}