Hello Ngoc Trinh,

If you want to select all text in a UltraTextEditor when a tab is displayed, you may want to give focus to the ultraTextEditor and call its SelectAll() method when the form is displayed or the selected tab is changed.

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[Sample code]
    private void Form1_Shown(object sender, EventArgs e)
    {
        ultraTextEditor1.Focus();
        ultraTextEditor1.SelectAll();
    }

    private void ultraTabControl1_SelectedTabChanged(object sender, SelectedTabChangedEventArgs e)
    {
        UltraTab selectedTab = e.Tab;

        if(selectedTab.TabPage.Controls.Count > 0)
        {
            // Assuming that the first control on the selected tab is of UltraTextEditor.
            UltraTextEditor textEditor = selectedTab.TabPage.Controls[0] as UltraTextEditor;
            textEditor.Focus();
            textEditor.SelectAll();
        }
    }
--------------

Could you give it a try? I hope this will help.