using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp2
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
string pyth = System.IO.Directory.GetCurrentDirectory();
pictureBox1.Image = Image.FromFile(pyth + "\\img.gif");
}

private void Form1_Load(object sender, EventArgs e)
{
this.TransparencyKey = this.BackColor;
}
[DllImport("user32.dll")]//拖动无窗体的控件
public static extern bool ReleaseCapture();
[DllImport("user32.dll")]
public static extern bool SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);
public const int WM_SYSCOMMAND = 0x0112;
public const int SC_MOVE = 0xF010;
public const int HTCAPTION = 0x0002;
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
ReleaseCapture();
SendMessage(this.Handle, WM_SYSCOMMAND, SC_MOVE + HTCAPTION, 0);
}
private void 结束ToolStripMenuItem_Click(object sender, EventArgs e)
{
this.Close();
}

private void pictureBox1_MouseClick(object sender, MouseEventArgs e)
{
if (e.Button == MouseButtons.Right)
{
contextMenuStrip1.Show(MousePosition.X, MousePosition.Y);
}
}
}
}

最主要的是这行代码：this.TransparencyKey = this.BackColor;消除了窗口自带的底色。