原理:重写当前窗体 OnPaint 方法, 如下:

protected override void OnPaint(PaintEventArgs e)
        {
            e.Graphics.DrawImage((Image)bit, new Point(0, 0));
        }

一. 窗体设计:

#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.label1 = new System.Windows.Forms.Label();
            this.SuspendLayout();
            //
            // label1
            //
            this.label1.BackColor = System.Drawing.Color.Transparent;
            this.label1.Location = new System.Drawing.Point(261, 56);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(17, 16);
            this.label1.TabIndex = 0;
            this.label1.Click += new System.EventHandler(this.label1_Click);
            //
            // Form1
            //
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.BackColor = System.Drawing.SystemColors.Control;
            this.ClientSize = new System.Drawing.Size(393, 317);
            this.Controls.Add(this.label1);
            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
            this.Name = "Form1";
            this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
            this.Text = "非矩形窗体";
            this.TransparencyKey = System.Drawing.SystemColors.Control;
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);

}

#endregion

二. 代码控制

public partial class Form1 : Form
    {
        Bitmap bit;
        public Form1()
        {
            InitializeComponent();
        }

private void Form1_Load(object sender, EventArgs e)
        {
            bit = new Bitmap("wordImage.bmp");
            bit.MakeTransparent(Color.Blue);
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            e.Graphics.DrawImage((Image)bit, new Point(0, 0));
        }
        private void label1_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }