Q:

Review of Connection handling and Data access layer using C#, sql server compact 3.5

I am developing a stand alone application, using sql server compact 3.5 sp2 which runs in process. No Database writes involved. Its purely a reporting application. Read many articles about reusing open db connections in case of sql compact(connection pooling) due to its different behavior from sql server.
Quoting the comments from a quiz opened by Erik Ejlskov Jensen Link, where its discussed an open early close late strategy for sql server compact databases. Based on this, with my limited experience I have implemented a not so complex Connection handling+Data access layer. Basically I am unsure if i am writing it in a recommended way. Please could any one point me in the right direction with rooms for improvement in this connection handling approach i have written?
The DbConnection class

public class FkDbConnection
{
  private static SqlCeConnection conn; 
  private static DataTable table;
  private static SqlCeCommand cmd;
      
  ~FkDbConnection() { conn = null; }

  //This will be called when the main winform loads and connection will be open as long as the main form is open
  public static string ConnectToDatabase()
  {
     try {
      conn = new SqlCeConnection(ConfigurationManager.ConnectionStrings["Connstr"].ConnectionString);
      if (conn.State == ConnectionState.Closed || conn.State == ConnectionState.Broken)
      {
          conn.Open();
      }
      return "Connected";
     }
     catch(SqlCeException e) { return e.Message; }
  }

  public static void Disconnect()
  {
    if (conn.State == ConnectionState.Open || conn.State == ConnectionState.Connecting || conn.State == ConnectionState.Fetching)
    {
      conn.Close();
      conn.Dispose();
      //conn = null; //does conn have to be set to null?
    }
    //else the connection might be already closed due to failure in opening it
    else if (conn.State == ConnectionState.Closed) {
      conn.Dispose();
      //conn = null; //does conn have to be set to null?
    }
  }

/// <summary>
///  Generic Select DataAccess
/// </summary>
/// <param name="sql"> the sql query which needs to be executed by command object </param>
public static DataTable ExecuteSelectCommand(SqlCeCommand comm)
{
if (conn != null && conn.State == ConnectionState.Open)
            {
                #region block using datareader
                using (table = new DataTable())
                {
                    //using statement needed for reader? Its closed below
                    using (SqlCeDataReader reader = comm.ExecuteReader())
                    {
                        table.Load(reader);
                        reader.Close(); //is it needed?
                    }
                }
                #endregion
                # region block using dataadpater
                //I read DataReader is faster?
                //using (SqlCeDataAdapter sda = new SqlCeDataAdapter(cmd))
                //{
                //    using (table = new DataTable())
                //    {
                //        sda.Fill(table);
                //    }
                //}
                #endregion
            //}
            }
            return table;
        }

        /// <summary>
        ///  Get Data
        /// </summary>
        /// <param name="selectedMPs"> string csv, generated from a list of selected posts(checkboxes) from the UI, which forms the field names used in SELECT </param>
        public static DataTable GetDataPostsCars(string selectedMPs)
        {
            DataTable dt;
            //i know this it not secure sql, but will be a separate question to pass column names to select as parameters
            string sql = string.Format(
                "SELECT " + selectedMPs + " "+
                "FROM GdRateFixedPosts");
            using (cmd = new SqlCeCommand(sql,conn))
            {
                cmd.CommandType = CommandType.Text;
                //cmd.Parameters.Add("@fromDateTime",DbType.DateTime);
                //cmd.Parameters.Add("@toDateTime",DbType.DateTime);
                dt = ExecuteSelectCommand(cmd);
            }
            return dt;
        }

    }

The Main UI (Form) in which connection opened, for connection to be open through out. 2 other reporting forms are opened from here. Closing main form closes all, at which point connection is closed and disposed.

private void FrmMain_Load(object sender, EventArgs e)
{
  string str = FkDbConnection.ConnectToDatabase();
  statStDbConnection.Items[0].Text = str;
}

private void FrmMain_FormClosing(object sender, FormClosingEventArgs e)
{
    FkDbConnection.Disconnect();
}

Comments, improvements on this connection class much appreciated. See my questions also inline code
Thank you.
Updated classes as per Erik's suggestion. with a correction on ExecuteSelectCommand() and an additional class which will instantiate command objs in "using" and pass data to the UI. I intent to add separate GetDataForFormX() methods since the dynamic sql for each form may differ. Hope this is ok?
Correction to Erik's code:
public static DataTable ExecuteSelectCommand(SqlCeCommand comm)
{
  var table = new DataTable();
  if (conn != null && conn.State == ConnectionState.Open)
  {
     comm.Connection = conn;
     using (SqlCeDataReader reader = comm.ExecuteReader())
     {
       table.Load(reader);
     }
  }
  return table;
}

New FkDataAccess class for passing Data to UI
public class FkDataAccess
{

  public static DataTable GetDataPostsCars(string selectedMPs)
  {
    var table = new DataTable();
    string sql = string.Format(
                "SELECT " + selectedMPs + " " +
                "FROM GdRateFixedPosts");
    if (FkDbConnection.conn != null && FkDbConnection.conn.State == ConnectionState.Open)
    {
      using (SqlCeCommand cmd = new SqlCeCommand(sql, FkDbConnection.conn))
      {
        cmd.CommandType = CommandType.Text;
       //cmd.Parameters.Add("@fromDateTime",DbType.DateTime);
        table = FkDbConnection.ExecuteSelectCommand(cmd);
      }
    }
  return table;    
  }

  //public static DataTable GetDataXY(string selectedvals)
  // and so on

}

A:

Too much code in your data access class, makes it unreadable and hard to maintain
The SqlCeonnection object will be disposed when you close it (and when the app closes)
You cannot dispose the DataTable if you want to use it elsewhere, and it is an  completely managed object anyway.
It is a good pattern to limit your classes to a single responsibility
public class FkDbConnection
{
  private static SqlCeConnection conn; 

  ~FkDbConnection() { conn = null; }

  //This will be called when the main winform loads and connection will be open as long as the main form is open
  public static void ConnectToDatabase()
  {
      // Handle failure to open in the caller
      conn = new SqlCeConnection(ConfigurationManager.ConnectionStrings["Connstr"].ConnectionString);
      conn.Open();
  }

  public static void Disconnect()
  {
    if (conn != null)
    {
      conn.Close();
    }
  }

public static DataTable ExecuteSelectCommand(SqlCeCommand comm)
{
    var table = new DataTable();
    if (conn != null && conn.State == ConnectionState.Open)                
    {

       comm.Connection = conn;
       using (SqlCeDataReader reader = comm.ExecuteReader())
       {
          table.Load(reader);
       }           
    }
    return table;
}

private void FrmMain_Load(object sender, EventArgs e)
{
  try
  {
     FkDbConnection.ConnectToDatabase();
     statStDbConnection.Items[0].Text = "Connected";
  }
  catch (Exception ex)
  {
     //Inform use that we canot proceed, what she can do to remedy, and exit
  }
}

private void FrmMain_FormClosing(object sender, FormClosingEventArgs e)
{
    FkDbConnection.Disconnect();
}

