要搭建一個C# WebSocket 服務器，可以使用ASP.NET Core框架來實現。以下是一個簡單的示例代碼：

using System;
using System.Net;
using System.Net.WebSockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;

public class Startup
{
    public void Configure(IApplicationBuilder app)
    {
        app.UseWebSockets();
        app.Use(async (context, next) =>
        {
            if (context.WebSockets.IsWebSocketRequest)
            {
                WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
                await Echo(context, webSocket);
            }
            else
            {
                await next();
            }
        });
    }

    private async Task Echo(HttpContext context, WebSocket webSocket)
    {
        var buffer = new byte[1024];
        WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);

        while (!result.CloseStatus.HasValue)
        {
            await webSocket.SendAsync(new ArraySegment<byte>(buffer, 0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);

            result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
        }

        await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>();
}

在這個示例中，我們創建了一個ASP.NET Core的Startup類，配置了一個WebSocket中間件來處理WebSocket請求。在Echo方法中實現了一個簡單的Echo服務器，收到客戶端消息后直接將消息原樣返回給客戶端。

要運行這個WebSocket服務器，可以在項目文件夾下執行以下命令：

dotnet run

這將啟動一個本地的WebSocket服務器，可以通過WebSocket客戶端連接到它。