The desktop does not work in the text field of Compact Framework 3.5.

I use the following code to set focus in a text box:

public void setFieldInFocus(TabPage tabPage)
    {
        foreach (Control t in tabPage.Controls)
        {
            if (t is TextBox)
            {
                if (t.Name == fieldInFocus)
                {
                    t.Focus();              

                }
            }

        }

    }

The cursor appears in the text box, however, when I try to enter text using the mobile keyboard, nothing happens until I click on the text box, even if the cursor is already in the text box. Then I click on the text box and try to enter text using the mobile keyboard, and it works. Why should I still click on the text box to enter text? What can I do to solve this problem?

note that I use compact 3.5 frames and textField.Select (); not available.

thank