onload()和onpaint()事件的执行顺序

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Drawing;
using System.Windows.Forms;

namespace onload_demo_1
{
    class Program:Form
    {
        static void Main(string[] args)
        {
            Application.Run(new Program());
        }
        public Program()
        {
            Text = "Onload Demo";
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            Graphics grfx = e.Graphics;
            grfx.DrawString("this is a demo", Font, new SolidBrush(Color.Red), 0, 0);
        }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            MessageBox.Show("click okay and the Form will appear");
        }

        protected override void OnShown(EventArgs e)
        {
            base.OnShown(e);
            MessageBox.Show("Are you ready ?");
        }
    }
}

从以上示例可以看出，先onload 事件，再执行onpaint事件；

可以参考如下网址：

https://www.codeproject.com/questions/1114117/override-the-onload-function-of-a-form

Yes, you can override the Form OnLoad event:

C#

 

protected override void OnLoad(EventArgs e)
{
   base.OnLoad(e);
   MessageBox.Show("click okay and the Form will appear");
}

But, do you really want the user to see nothing but a MessageBox when they start the app ? If you want to first show the Form, and then put up a MessageBox, try this:

C#

 

protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    MessageBox.Show("Are you ready ?");
}

But, I wonder why it is ... what special circumstances require ... you ... need to do this, because:

Just double-clicking on the Form design-surface in design-mode will cause the automatic generation of a Form Load EventHandler:

C#

 

private void Form1_Load(object sender, EventArgs e)
{
    // do whatever ...
}

which is wired-up to the Form ... you can look in the Designer.cs file and see how the connection is made:

C#

 

this.Load += new System.EventHandler(this.Form1_Load);

Or, you can define instantiate (create) your own EventHandler and wire it up to the Form:

C#

 

public Form1()
{
    InitializeComponent();
    this.Load += MyOnLoad;
}

private void MYOnLoad(object sender, EventArgs e)
{
    // do whatever
}

Or, you can define an EventHandler for the 'Shown event, as shown in the second code example here in order to the Form appear before the MessageBox appears. 

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2022年8月3日更新

Form在show的时候没有激发onshown事件处理

1. Form.OnShown是在Form.OnLoad方法中通过Form.BeginInvoke调用到的。在10000次Focus之后，OnShown就调不到了。

2. 通过跟踪当前Application的Message，发现9999次Focus会导致9999个自定义事件（0xc03b），然后会收到一个0xc337。但是10000次Focus之后，最后的0xc337就消失了。如果这时再发一个0xc03b，会收到两个0xc337。证明超过10000之后，0xc337的消息处理就出了问题。

3. 怀疑是BeginInvoke的问题，所以在10000Focus之后，没有调用Form2.Show，而是通过BeginInvoke调用自己的一个方法，果然会失败。但是，如果在10000Focus之后，调用Application.DoEvents，则不会出错。

4. 通过反编译代码，得知BeginInvoke是通过PostMessage发送一个自定义事件（这里即0xc337，Control.threadCallbackMessage），然后再WndProc中处理这个Message来实现的。