private void Window_Loaded(object sender, RoutedEventArgs e)
{
    // 獲取窗體句柄
    IntPtr hwnd = new System.Windows.Interop.WindowInteropHelper(this).Handle;

    // 獲得窗體的 樣式
    int oldstyle = NativeMethods.GetWindowLong(hwnd, NativeMethods.GWL_STYLE);

    // 更改窗體的樣式為無邊框窗體
    NativeMethods.SetWindowLong(hwnd, NativeMethods.GWL_STYLE, oldstyle & ~NativeMethods.WS_CAPTION);

    // SetWindowLong(hwnd, GWL_EXSTYLE, oldstyle & ~WS_EX_LAYERED);
    // 1 | 2 << 8 | 3 << 16  r=1,g=2,b=3 詳見winuse.h文件
    // 設置窗體為透明窗體
    NativeMethods.SetLayeredWindowAttributes(hwnd, 1 | 2 << 8 | 3 << 16, 0, NativeMethods.LWA_ALPHA);

    // 創建圓角窗體  12 這個值可以根據自身項目進行設置
    NativeMethods.SetWindowRgn(hwnd, NativeMethods.CreateRoundRectRgn(0, 0, Convert.ToInt32(this.ActualWidth), Convert.ToInt32(this.ActualHeight), 12, 12), true);
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace SEsoft
{
    public class NativeMethods
    {
        /// <summary>
        /// 帶有外邊框和標題的windows的樣式
        /// </summary>
        public const int WS_CAPTION = 0X00C0000;
 
        /// <summary>
        /// window 擴展樣式 分層顯示
        /// </summary>
        public const int WS_EX_LAYERED = 0x00080000;
 
        /// <summary>
        /// 帶有alpha的樣式
        /// </summary>
        public const int LWA_ALPHA = 0x00000002;
 
        /// <summary>
        /// 顏色設置
        /// </summary>
        public const int LWA_COLORKEY = 0x00000001;
 
        /// <summary>
        /// window的基本樣式
        /// </summary>
        public const int GWL_STYLE = -16;
 
        /// <summary>
        /// window的擴展樣式
        /// </summary>
        public const int GWL_EXSTYLE = -20;
 
        /// <summary>
        /// 設置窗體的樣式
        /// </summary>
        /// <param name="handle">操作窗體的句柄</param>
        /// <param name="oldStyle">進行設置窗體的樣式類型.</param>
        /// <param name="newStyle">新樣式</param>
        [System.Runtime.InteropServices.DllImport("User32.dll")]
        public static extern void SetWindowLong(IntPtr handle, int oldStyle, int newStyle);
 
        /// <summary>
        /// 獲取窗體指定的樣式.
        /// </summary>
        /// <param name="handle">操作窗體的句柄</param>
        /// <param name="style">要進行返回的樣式</param>
        /// <returns>當前window的樣式</returns>
        [System.Runtime.InteropServices.DllImport("User32.dll")]
        public static extern int GetWindowLong(IntPtr handle, int style);
 
        /// <summary>
        /// 設置窗體的工作區域.
        /// </summary>
        /// <param name="handle">操作窗體的句柄.</param>
        /// <param name="handleRegion">操作窗體區域的句柄.</param>
        /// <param name="regraw">if set to <c>true</c> [regraw].</param>
        /// <returns>返回值</returns>
        [System.Runtime.InteropServices.DllImport("User32.dll")]
        public static extern int SetWindowRgn(IntPtr handle, IntPtr handleRegion, bool regraw);
 
        /// <summary>
        /// 創建帶有圓角的區域.
        /// </summary>
        /// <param name="x1">左上角坐標的X值.</param>
        /// <param name="y1">左上角坐標的Y值.</param>
        /// <param name="x2">右下角坐標的X值.</param>
        /// <param name="y2">右下角坐標的Y值.</param>
        /// <param name="width">圓角橢圓的width.</param>
        /// <param name="height">圓角橢圓的height.</param>
        /// <returns>hRgn的句柄</returns>
        [System.Runtime.InteropServices.DllImport("gdi32.dll")]
        public static extern IntPtr CreateRoundRectRgn(int x1, int y1, int x2, int y2, int width, int height);
 
        /// <summary>
        /// Sets the layered window attributes.
        /// </summary>
        /// <param name="handle">要進行操作的窗口句柄</param>
        /// <param name="colorKey">RGB的值</param>
        /// <param name="alpha">Alpha的值，透明度</param>
        /// <param name="flags">附帶參數</param>
        /// <returns>true or false</returns>
        [System.Runtime.InteropServices.DllImport("User32.dll")]
        public static extern bool SetLayeredWindowAttributes(IntPtr handle, uint colorKey, byte alpha, int flags);
    }
}