在C#中，可以使用SetWindowPos函數來實現窗口的始終置頂。以下是一個示例代碼：

using System;
using System.Runtime.InteropServices;

public class WindowHelper
{
    [DllImport("user32.dll")]
    public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

    public const uint SWP_NOSIZE = 0x0001;
    public const uint SWP_NOMOVE = 0x0002;
    public const uint SWP_SHOWWINDOW = 0x0040;
    public static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
    public static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);

    public static void SetTopMost(IntPtr hWnd)
    {
        SetWindowPos(hWnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_SHOWWINDOW);
    }

    public static void SetNotTopMost(IntPtr hWnd)
    {
        SetWindowPos(hWnd, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_SHOWWINDOW);
    }
}

使用示例：

using System;
using System.Windows.Forms;

public class MainForm : Form
{
    public MainForm()
    {
        this.Text = "Top Most Window Example";
        this.Size = new System.Drawing.Size(300, 200);

        // 設置窗口為始終置頂
        WindowHelper.SetTopMost(this.Handle);

        // 取消窗口的始終置頂
        // WindowHelper.SetNotTopMost(this.Handle);
    }
}

在上面的示例中，SetTopMost方法用來將窗口設置為始終置頂，SetNotTopMost方法用來取消始終置頂。通過調用這兩個方法，可以實現窗口的始終置頂功能。