How to use offscreen render in GLWpfControl to export a image

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I wrote a piece of code in a GLWpfControl ,Like below, but it doesn't work：

     private int FBO=0,CBO=0,DBO = 0,TBO=0;
       private static int rendercount = 0;
       public unsafe  void OffScreenRender()
       {
           Loger.LogInfo("start");
           int defautFrame = 0;
           
           defautFrame= GL.GetInteger(GetPName.FramebufferBinding);
           Loger.LogInfo($"default framebuffer {defautFrame}");
           if(FBO==0)
           {
               FBO = GL.GenFramebuffer();
               Loger.LogInfo($"Create FBO {FBO}");
           }
           GL.BindFramebuffer(FramebufferTarget.Framebuffer, FBO);

           if(CBO==0)
           {
               CBO = GL.GenRenderbuffer();

               Loger.LogInfo($"Create CBO {CBO}");
           }
               GL.BindRenderbuffer(RenderbufferTarget.Renderbuffer, CBO);
               GL.RenderbufferStorage(RenderbufferTarget.Renderbuffer, RenderbufferStorage.Rgba8, 800, 600);
               GL.FramebufferRenderbuffer(FramebufferTarget.Framebuffer, FramebufferAttachment.ColorAttachment0, RenderbufferTarget.Renderbuffer, CBO);

           if(DBO==0)
           {
               DBO = GL.GenRenderbuffer();
               Loger.LogInfo($"Create DBO {DBO}");
           }
               GL.BindRenderbuffer(RenderbufferTarget.Renderbuffer, DBO);
               GL.RenderbufferStorage(RenderbufferTarget.Renderbuffer, RenderbufferStorage.DepthComponent, 800, 600);
               GL.FramebufferRenderbuffer(FramebufferTarget.Framebuffer, FramebufferAttachment.DepthAttachment, RenderbufferTarget.Renderbuffer, DBO);

           if (TBO == 0)
           {
               TBO = GL.GenTexture();
               Loger.LogInfo($"Create TBO {TBO}");
           }
               GL.BindTexture(TextureTarget.Texture2D, TBO);
               GL.TexImage2D(TextureTarget.Texture2D, 0, PixelInternalFormat.Rgba, 800, 600, 0, PixelFormat.Rgba, PixelType.UnsignedInt, IntPtr.Zero);
               GL.FramebufferTexture2D(FramebufferTarget.Framebuffer, FramebufferAttachment.ColorAttachment0, TextureTarget.Texture2D, TBO, 0);
           if (GL.CheckFramebufferStatus(FramebufferTarget.Framebuffer) != FramebufferErrorCode.FramebufferComplete)
           {
               Loger.LogError($"OffScreenRender Framebuffer status is not complete");
           }


           **if (!initialed) InitGL();
           ShaderProgram.ActiveProgram(_program);
           UpdateGL();

           GL.BindFramebuffer(FramebufferTarget.Framebuffer, FBO);
           GL.Viewport(0, 0, 800, 600);
           //GL.Enable(EnableCap.DepthTest); 
           //GL.ClearColor(0.2f, 0.5f, 0.6f, 0.5f);
           GL.BindVertexArray(VAO);

           var g =MyData.Graphics;

           if (MDataType == MyType.Points)
           {
               GL.DrawElements(BeginMode.Points, g.PointsSize, DrawElementsType.UnsignedInt, 0);
           }
           else if (MDataType == MType.Face)
           {

               GL.DrawElements(BeginMode.Triangles, g.TranglesSize, DrawElementsType.UnsignedInt, g.PointsSize * sizeof(uint));
           }
           else
           {
               GL.DrawElements(BeginMode.Lines, g.GridsSize, DrawElementsType.UnsignedInt, (g.PointsSize + g.TranglesSize) * sizeof(uint));
           }
           GL.BindVertexArray(0);**

           byte[] pixels=new byte[4*800*600];
           GL.ReadBuffer(ReadBufferMode.ColorAttachment0);
           fixed(byte* buff=pixels){
               GL.ReadPixels(0, 0, 800, 600, PixelFormat.Rgba, PixelType.UnsignedInt,new IntPtr(buff));
           }
           //Bitmap bmp = new Bitmap(800, 600);

           //{mapType.ToString()}_{DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss-ffff")}
           GL.BindFramebuffer(FramebufferTarget.Framebuffer,defautFrame);
           ShaderProgram.DeactiveProgram();

           using (var fs = new FileStream($"D:\\GLCapture\\{DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss-ffff")}.pixels", FileMode.OpenOrCreate))
           {
               fs.Write(pixels, 0, pixels.Length);
               fs.Flush();
               fs.Close();
           }
           Loger.LogInfo("end");

       }

In this code, the InitGL and UpdateGL used to initialize program and VAO data, as well as the DrawElements section, are the same as what I wrote in GLWpfControl's OnRender(). Here, I want to achieve off screen rendering by binding my own framebuffer to OffScreenRender(), but this framebuffer does not work, when I read by ReadPixels from this buffer, all data in pixels is 0. I want to save the content rendered in GLWpfControl to an image through off screen rendering. What should I do?