Slowness in C#.net windows form resize when form has many dropdownlist controls

Question

I made a windows form with many dropdown list which are placed inside a grid(not a datagrid) and when I use anchor=left right top bottom and dock=fill options in all of them, the form resize gets slow on runtime.

What's the problem with dock and anchor options?

Is it a general problem with forms? Is it a general disadvantage of using .net components and windows forms?

I don't have any custom events handled on control resize, so the problem is about the controls Microsoft developed. Should I avoid using dock=fill?

What type of grid are you using? A TableLayoutPanel or a DataGridView? — Kevin McCormick, Jan 06 '12 at 20:01
Do you have anything in the form's OnPaint override or the form's paint event? — Chris Dunaway, Jan 06 '12 at 20:15
All of these controls will need to redraw when they move because of the anchor. Redrawing a lot of controls takes time. A simple fix is hiding the 'grid' with the form's ResizeBegin event and making it visible again with the ResizeEnd event. — Hans Passant, Jan 06 '12 at 20:17

score 19 · Accepted Answer · edited May 23 '17 at 12:28

19

It's a normal that it consumes your processor as on every resize move form forms resize table layout panel which forces resize and reposition child controls (dropdowns), as there is anchor property setuped.

There is a trick to avoid to create a frustration for user when resizing:

Use SuspendLayout() and ResumeLayout() on BeginResize() and EndResize() event handlers.

Look here for concrete example:

Prevent window redraw when resizing c# windows forms

Should work for you.

edited May 23 '17 at 12:28

Community

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answered Jan 06 '12 at 20:17

Tigran

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Actually it is not problem of winform or some layout panel, it's problem of combobox control. It's very slow control, it takes a lot of time on creation and resizing. – 23W Sep 24 '19 at 05:27
For me this only works when making the form bigger for some reason. When I make it smaller again, the anchor doesn't update. – Icoryx Oct 20 '22 at 09:57

sɐunıɔןɐqɐp · Answer 2 · 2021-06-22T07:21:35.673

WinForms drawing performance relies exclusively on CPU processor, single-threaded. This means you will face several performance issues if your form draws pictures, or any other complex/numerous UI elements. So you may want to disable unnecessary redrawing events to optimize the overall performance.

The dirty trick below will give you a huge performance gain by disabling redrawing when resizing occurs in your form. This code below should be placed in your form's class:

protected override void OnResize(EventArgs e)
{
    this.SuspendDrawing();
    base.OnResize(e);
    this.ResumeDrawing();
}

protected override void OnResizeBegin(EventArgs e)
{
    this.SuspendDrawing();
    base.OnResizeBegin(e);
}

protected override void OnResizeEnd(EventArgs e)
{
    base.OnResizeEnd(e);
    this.ResumeDrawing();
}

protected override void OnClosing(CancelEventArgs e)
{
    this.SuspendDrawing();
    base.OnClosing(e);
    this.ResumeDrawing();
}

And this is the extension method which does all the trick:

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace Example
{
    public static class ControlExtensions
    {
        [DllImport("user32.dll")]
        public static extern int SendMessage(IntPtr hWnd, Int32 wMsg, bool wParam, Int32 lParam);

        private const int WM_SETREDRAW = 11;

        public static void SuspendDrawing(this Control control) => SendMessage(control.Handle, WM_SETREDRAW, false, 0);

        public static void ResumeDrawing(this Control control)
        {
            SendMessage(control.Handle, WM_SETREDRAW, true, 0);
            control.Refresh();
        }
    }
}

Downvoted because functions for these things already exist (see accepted answer), so P/Invoke is unnecessary overkill. Plus, your solution requires inheriting from the control. — Sod Almighty, Jul 28 '21 at 12:22
Well, overkill or not, it works, and it has been commonly used to considerably improve responsiveness. — sɐunıɔןɐqɐp, Jul 31 '21 at 17:14
So have the built-in functions such as `Control.SuspendLayout()`. Your externs are redundant. — Sod Almighty, Aug 02 '21 at 02:47
Sorry but you are completely wrong, it has indeed a completely different performance effect, just try it by yourself in a complex WinForms application of yours. — sɐunıɔןɐqɐp, Aug 16 '21 at 11:08

Slowness in C#.net windows form resize when form has many dropdownlist controls

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