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Data Source=|DataDirectory|\Data.sdf;Connect Timeout=10					

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public static class Access
{
    public static SqlConnection sqlConnection = new SqlConnection(Properties.Settings.Default.DataConnectionString);
   
    public static void TestConnection()
    {
        if (sqlConnection.State != ConnectionState.Open)
        {
            if (sqlConnection.State == ConnectionState.Closed)
            {
                sqlConnection.Open();
            }
        }
    }
}					

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using System.Data.SqlServerCe;

string connectionString = @"Data Source=c:\Temp\Data.sdf";
using (SqlCeConnection conn = new SqlCeConnection(connectionString))
{
  conn.Open();                
}