C#

    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            filter ft = new filter();
            Application.AddMessageFilter(ft);

            Form1 f = new Form1();
            f.Show();
            Application.Run();
        }
    }

    public class filter : IMessageFilter
    {

        #region IMessageFilter 成员

        const int WM_CREATE = 0x00001;
        public bool PreFilterMessage(ref Message m)
        {
<code>            if (m.Msg == WM_CREATE)
            {
                MessageBox.Show("A");
                return true;
            }</code>
            return false;
        }

        #endregion
    }

    public partial class Form1 : Form
    {
        #region Designer
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        /// <param name="disposing">如果应释放托管资源，为 true；否则为 false。</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows 窗体设计器生成的代码

        /// <summary>
        /// 设计器支持所需的方法 - 不要
        /// 使用代码编辑器修改此方法的内容。
        /// </summary>
        private void InitializeComponent()
        {
            this.button1 = new System.Windows.Forms.Button();
            this.SuspendLayout();
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(0, 0);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75, 23);
            this.button1.TabIndex = 0;
            this.button1.Text = "button1";
            this.button1.UseVisualStyleBackColor = true;
            this.button1.Click += new System.EventHandler(this.button1_Click);
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(609, 265);
            this.Controls.Add(this.button1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);

        }

        #endregion

        private System.Windows.Forms.Button button1;
        #endregion

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
        }


        private void button1_Click(object sender, EventArgs e)
        {
            Form1 f = new Form1();
            f.Show();
        }

        const int WM_CREATE = 0x00001;
        protected override void WndProc(ref Message m)
        {
<code>            if (m.Msg == WM_CREATE)
            { MessageBox.Show("B"); }
            base.WndProc(ref m);</code>
        }
    }