This is just another SQL C# articles which let you learn how to check for a database existence on SQL Server.

All you have to do query names in sys.databases table and make sure it is available to use. You can create a function for the purpose.

public static bool IsDBExist(string server, string db)
        {
            List<string> list = new List<string>();

            // Open connection to the database
            string conString = null; ;
            
                conString = "server=" + server + ";uid=sa;";
           
            bool has = false;
            try
            {
                using (SqlConnection con = new SqlConnection(conString))
                {
                    if (con.State == ConnectionState.Open) con.Close();
                    con.Open();


                    // Set up a command with the given query and associate
                    // this with the current connection.
                    using (SqlCommand cmd = new SqlCommand("SELECT name from sys.databases where name='" + db + "'", con))
                    {
                        using (IDataReader dr = cmd.ExecuteReader())
                        {

                            while (dr.Read())
                            {
                                list.Add(dr[0].ToString());
                            }
                            if (list.Count > 0) has = true;
                        }
                    }
                    con.Close();
                }
            }
            catch (Exception er)
            {
                MessageBox.Show(er.Message.ToString());
            }
            return has;

        }