how to minimize dependency between a form and a class in C#

[STAThread]
static void Main()
{
	Application.EnableVisualStyles();
	Application.SetCompatibleTextRenderingDefault(false);

	var view = new FormMain();
	var controller = new Controller(view, new EmployeeManager(new DataService()));

	Application.Run(view);
}


public partial class FormMain : Form
{
	private Controller _controller;

	public FormMain()
	{
		InitializeComponent();
	}

	public void SetController(Controller controller)
	{
		_controller = controller;
	}

	private void button1_Click(object sender, EventArgs e)
	{
		var newEmp = _controller.SearchEmployee(txtEmployeeID.Text);
                if(null == newEmp) 
                {
                   //Id is not found.
                }
		txtEmployeeName.Text = newEmp.EmployeeName;
		txtEmployeeAddress.Text = newEmp.EmployeeAddress;
	}
}

public class Controller
{
	private FormMain _view;
	private EmployeeManager _manager;


	public Controller(FormMain view, EmployeeManager manager)
	{
		_view = view;
		_manager = manager;
		view.SetController(this);
	}

	public Employee SearchEmployee(string employeeID)
	{
	   return _manager.SearchEmployee(employeeID);
	}

	private void UpdateService(DataService newService)
	{
		_manager = new EmployeeManager(newService);
	}

}

public class EmployeeManager
{

	private DataService _ds;

	public EmployeeManager(DataService DS)
	{
		_ds = DS;
	}

	public Employee SearchEmployee(string employeeID)
	{

		var dt = _ds.GetByEmployeeId(employeeID);

		if (dt.Rows.Count > 0)
		{

			return new Employee(employeeID,
								dt.Rows[0].ItemArray[1].ToString(),
								dt.Rows[0].ItemArray[2].ToString());
		}
		return null;
	}

	//ADD DELETE UPDATE METHODS etc.

}

public class Employee
{
	public string EmployeeID { get; set; }
	public string EmployeeName { get; set; }
	public string EmployeeAddress { get; set; }


	public Employee(string employeeID, string employeeName, string employeeAddress)
	{
		EmployeeID = employeeID;
		EmployeeName = employeeName;
		EmployeeAddress = employeeAddress;
	}
}

public class DataService
{
	public DataTable GetByEmployeeId(string employeeID)
	{
		using (SqlConnection newCon = new SqlConnection(""))
		{
			SqlCommand Cmd = new SqlCommand("SELECT..WHERE emp_id=@employeeID", newCon);
			Cmd.Parameters.Add("@employeeID", SqlDbType.VarChar).Value = employeeID;
			newCon.Open();
			SqlDataReader rdr = Cmd.ExecuteReader();
			DataTable results = new DataTable();
			results.Load(rdr);
			return results;
		}
	}
}