从有效对象到位图的转换结果为空C＃

Question

我正在尝试使用线程来处理彼此平行的多个图像。在代码中，您将看到我给了mehtod'imgprocessing'一个位图，但是图像处理仅接收对象参数。因此我必须将其类型转换为位图，并在某些时候抛出异常：“参数无效”。显然，位图在某些时候变为null。 它仅在几个线程已经完成处理后才发生。 问题是：为什么在类型转换期间我的位图为null？我不知道为什么它会停止正常投放。 码- 主要：

 int i = 0;
            float[] percentcount = new float[6];
            string[] sc = Directory.GetFiles(@"C:\Users\User\Desktop\gn clouds-snow", "*.jpg");
            Bitmap[] bt = new Bitmap[sc.Length];
            Thread[] threads = new Thread[sc.Length];
            foreach (string file in sc)
            {
                Console.WriteLine("number of image: " + i + " / " + sc.Length);
                bt[i] = new Bitmap(file);
                threads[i] = new Thread(imgProcessing);
                threads[i].Start(bt[i]);
                i++;
            }

            foreach(Thread thread in threads)
            {
                thread.Join();
            }

            Console.WriteLine("highest snow percent:" + ((double)maxgray / sc.Length) * 100);
            Console.WriteLine("lowest:" + (((double)maxwhite) / sc.Length) * 100);

imgprocessing函数：

private static void imgProcessing(Object obj1)
        {
            Bitmap bt = new Bitmap((Image)obj1); //exception occurs here
            //Bitmap //ft = new Bitmap(bt);
            Color color = new Color();
            Ipn[] Iparams = new Ipn[6];
            int nm = 2211840;
            int i = 0;
            int j = 0;
            int k = 0;
            //int holder = 0;
            float hue = 0;
            float brt = 0;
            float sat = 0;
            GiveId(Iparams);

                for (i = 0; i < bt.Height; i++)
                {
                    for (j = 0; j < bt.Width; j++)
                    {
                        color = bt.GetPixel(j, i);
                        hue = color.GetHue();
                        sat = color.GetSaturation();
                        brt = color.GetBrightness();
                        #region if statements
                        //white
                        if (brt > 0.7)
                        {
                            Iparams[5].Percent++;
                            //ft.SetPixel(j,i,Color.White);
                        }

                        //gray
                        else if (sat <= 0.25)
                        {
                            Iparams[4].Percent++;
                            //ft.SetPixel(j, i, Color.Gray);
                        }
                        //black
                        else if (color.R <= 15 && color.G <= 15 && color.B <= 15 && brt < 0.3)
                        {
                            Iparams[3].Percent++;
                            //ft.SetPixel(j, i, Color.Black);
                        } 

                        //red
                        else if (hue > 0 && hue <= 59)
                        {
                            Iparams[0].Percent++;
                            //ft.SetPixel(j, i, Color.Red);
                        }
                        //green 
                        else if (hue > 60 && hue <= 150)
                        {
                            Iparams[1].Percent++;
                            //ft.SetPixel(j, i, Color.Green);
                        }

                        //blue
                        else if (hue > 150 && hue <= 270)
                        {
                            Iparams[2].Percent++;
                            //ft.SetPixel(j, i, Color.Blue);
                        }
                        #endregion
                    }
                }


                for (k = 0; k < Iparams.Length; k++)
                {
                    Iparams[k].Percent = ((float)Iparams[k].Percent / nm) * 100;
                }

                //bubblesort(Iparams);

                lock (lockobject)
                {
                    #region space code
                    //calculate the color percentages for each color

                    //if (Iparams[0].ID == ID.gray && Iparams[1].ID == ID.blue)
                    //{
                    //    counter2++;
                    //}
                    //else if (Iparams[1].ID == ID.white && Iparams[0].ID == ID.blue)
                    //{
                    //    counter1++;
                    //}
                    #endregion

                    #region ocean code
                    //if (Iparams[0].ID == ID.blue)
                    //{
                    //    Interlocked.Increment(ref counter1);
                    //}
                    //else
                    //    Interlocked.Increment(ref counter2);

                    //holder = (int)(Iparams[0].Percent - Iparams[1].Percent);
                    //Interlocked.Add(ref dis, holder);
                    #endregion

                    ////ft.Save(@"C:\Users\User\Desktop\results\" + name + ".jpg");
                    //Interlocked.Increment(ref name);
                    #region veg code
                    //if (Iparams[0].ID == ID.green)
                    //{
                    //    Interlocked.Increment(ref counter1);
                    //    holder = (int)(Iparams[0].Percent - Iparams[1].Percent);
                    //    Interlocked.Add(ref dis, holder);
                    //}
                    //else
                    //    Interlocked.Increment(ref counter2);
                    #endregion 
                    #region snow&clouds code
                    /*understanding what is the highest percentages of white and gray and the lowest.
                     * 
                     */
                    double graypercent = Convert.ToDouble(Iparams[4].Percent);
                    double whitepercent = Convert.ToDouble(Iparams[5].Percent);
                    if(Iparams[4].Percent > maxgray)
                    {
                        Interlocked.Exchange(ref maxgray, graypercent);
                    }
                    else if(Iparams[4].Percent < mingray)
                    {
                        Interlocked.Exchange(ref mingray, graypercent);
                    }

                    if(Iparams[5].Percent > maxwhite)
                    {
                        Interlocked.Exchange(ref maxwhite, Iparams[5].Percent);
                    }
                    else if(Iparams[5].Percent < minwhite)
                    {
                        Interlocked.Exchange(ref minwhite, Iparams[5].Percent);
                    }
                    #endregion 

                }