Data Access Class C#.NET with Maximum DATABASE

DATA ACCESS CLASS C#.NET with Maximum Database.

it can use with maximum database, sliding change.

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;

namespace Banglar_Dalan
{
class DataAccess
{
  static string _ConnectionString = @"Data Source=.\SQLEXPRESS;
  AttachDbFilename=|DataDirectory|\DatabaseName.mdf;
  Integrated Security=True;
  Connect Timeout=30;
  User Instance=True";

  static SqlConnection _Connection = null;
  public static SqlConnection Connection
  {
      get
      {
          if (_Connection == null)
          {
              _Connection = new SqlConnection(_ConnectionString);
              _Connection.Open();

              return _Connection;
          }
          else if (_Connection.State != System.Data.ConnectionState.Open)
          {
              _Connection.Open();

              return _Connection;
          }
          else
          {
              return _Connection;
          }
      }
  }

  public static DataSet GetDataSet(string sql)
  {
      SqlCommand cmd = new SqlCommand(sql, Connection);
      SqlDataAdapter adp = new SqlDataAdapter(cmd);

      DataSet ds = new DataSet();
      adp.Fill(ds);
      Connection.Close();

      return ds;
  }

  public static DataTable GetDataTable(string sql)
  {
      Console.WriteLine(sql);
      DataSet ds = GetDataSet(sql);

      if (ds.Tables.Count > 0)
          return ds.Tables[0];
      return null;
  }

  public static int ExecuteSQL(string sql)
  {
      SqlCommand cmd = new SqlCommand(sql, Connection);
      return cmd.ExecuteNonQuery();
  }
}
}

It just for Archiving.