how to disable tabpages in C# and botton next until the all fields are filled?

// C#
private void tabControl1_SelectedIndexChanged(object sender, System.EventArgs e)
{
    // Check Credentials Here

    if ((CredentialCheck.Checked = true) & (tabControl1.SelectedTab == tabPage2)) 
    {
        tabControl1.SelectedTab = tabPage2;
    }
    else if ((CredentialCheck.Checked = false) & (tabControl1.SelectedTab == tabPage2))
    {
        MessageBox.Show("Unable to load tab. You have insufficient access privileges.");
        tabControl1.SelectedTab = tabPage3;
    }
}