Open DateTimePicker C# control programmatically

Try the following

//part of the usings
using System.Runtime.InteropServices;

//declares
[DllImport("user32.dll")]
private static extern bool PostMessage(
IntPtr hWnd, // handle to destination window
Int32 msg, // message
Int32 wParam, // first message parameter
Int32 lParam // second message parameter
);

const Int32 WM_LBUTTONDOWN = 0x0201;

//method to call dropdown
private void button1_Click(object sender, EventArgs e)
{
    Int32 x = dateTimePicker1.Width - 10;
    Int32 y = dateTimePicker1.Height / 2;
    Int32 lParam = x + y * 0x00010000;

    PostMessage(dateTimePicker1.Handle, WM_LBUTTONDOWN, 1,lParam);

}

On my system (Windows 7, .NET 35) the other solutions did not work. I found another solution on a MS discussion site that did work.

using System.Runtime.InteropServices;

public static class Extensions
{
    [DllImport("user32.dll", SetLastError = true)]
    private static extern int SendMessage(IntPtr hWnd, uint Msg, int wParam, int lParam);

    private const uint WM_SYSKEYDOWN = 0x104;

    public static void Open(this DateTimePicker obj)
    {
        SendMessage(obj.Handle, WM_SYSKEYDOWN, (int)Keys.Down, 0);
    }
}

Source : http://social.msdn.microsoft.com/Forums/windows/en-US/f2f0b213-d57a-46de-b924-e21b7ac0882e/programmatically-open-the-calendar-of-the-datetimepicker-control?forum=winforms

Usage:

dateTimePicker1.Open();

Warnings. This will not work if the dateTimePicker1 is a Control on DataGridView (ie if you try to make a pop-up DatePicker on the DGV). It does work if the Control is added to the Form instead. What will happen is that the synthesized cursor "down" event will be swallowed by the DGV, and will move the current cell pointer down one instead of drop-drop the Calendar of the DTP.

Credit goes to astander for providing the solution, which makes a very nice extension:

using System.Linq;
using System.Runtime.InteropServices;

public static class Extensions {

  [DllImport("user32.dll", CharSet = CharSet.Auto)]
  private static extern int PostMessage(IntPtr hwnd, Int32 wMsg, Int32 wParam, Int32 lParam);

  public static void Open(this DateTimePicker obj) {
    const int WM_LBUTTONDOWN = 0x0201;
    int width = obj.Width - 10;
    int height = obj.Height / 2;
    int lParam = width + height * 0x00010000; // VooDoo to shift height
    PostMessage(obj.Handle, WM_LBUTTONDOWN, 1, lParam);
  }

}

Usage:

dateTimePicker1.Open();

This way, you can reuse your Extension anytime you'd like, over and over, in any form using any DateTimePicker control.