如何在C#中还原SQL Server 2012数据库.bak文件？

这是我的代码：

private void buttonRestore_Click(object sender,EventArgs e)
{
    try
    {
        openFileDialog1.Filter = "Backup File |*.bak";

        if (openFileDialog1.ShowDialog() == DialogResult.OK)
        {
            string sql = "Alter Database BOQ SET SINGLE_USER WITH ROLLBACK IMMEDIATE;";
            sql += "Restore Database BOQ FROM DISK ='" + openFileDialog1.FileName + "' WITH REPLACE;";

            SqlConnection con = new SqlConnection("Data Source=.; Initial Catalog=BOQ;Integrated Security=True");
            SqlCommand command = new SqlCommand(sql,con);

            con.Open();
            command.ExecuteNonQuery();

            MessageBox.Show("Database Recovered Successfully!");
            con.Close();
            con.Dispose();
        }
    }
    catch (Exception ex) 
    { 
         MessageBox.Show(ex.Message); 
    }
}

但我得到这个错误：