Why I'm getting exception System.Reflection.TargetInvocationException: 'Exception has been thrown by the target of an invocation.' when trying to drawstring on images in a loop ?

Question

ExternalException: A generic error occurred in GDI+.

System.Reflection.TargetInvocationException
HResult=0x80131604
Message=Exception has been thrown by the target of an invocation.
Source=mscorlib
StackTrace:
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Delegate.DynamicInvokeImpl(Object[] args)
at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)
at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
at System.Windows.Forms.Control.InvokeMarshaledCallbacks()
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at Extract.Program.Main() in D:\Csharp Projects\Extract\Program.cs:line 19

This exception was originally thrown at this call stack:
System.Drawing.Image.Save(string, System.Drawing.Imaging.ImageCodecInfo, System.Drawing.Imaging.EncoderParameters)
System.Drawing.Image.Save(string, System.Drawing.Imaging.ImageFormat)
Extract.Form1.DownloadAsync() in Form1.cs
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task)
System.Runtime.CompilerServices.TaskAwaiter.GetResult()
Extract.Form1.DownloadImages() in Form1.cs
System.Runtime.CompilerServices.AsyncMethodBuilderCore.ThrowAsync.AnonymousMethod__6_0(object)

Inner Exception 1:
ExternalException: A generic error occurred in GDI+.

Both Lists images and dates contain 23 items. The exception is not out of index but the exception throw on the last item 23.
It's drawing the text on all the images but in the end of the loop on item 23 it's throwing the exception.

 if (images.Length > 0)
                             {
                                 for (int i = 0; i < images.Length; i++)
                                 {
                                     drawOnImage.DrawText(dates[i].ToString("ddd, dd MMM yyy HH':'mm"), images[i]);
                                 }
                             }

Maybe because both Lists contain the same amount of items 23 ? how should I solve it then ?

private async Task DownloadAsync()
        {
            using (var client = new WebClient())
            {
                client.DownloadFileCompleted += (s, e) =>
                {
                    if (e.Error == null)
                    {
                        urlsCounter--;

                        var t = urls;

                        if (urlsCounter == 0)
                        {
                            CheckIfImagesExist();

                            btnRadarPath.Enabled = true;
                            btnSatellitePath.Enabled = true;

                            radCounter = 0;
                            satCounter = 0;

                            lblStatus.Text = "Completed.";

                            dates = rad.dates;
                            var images = System.IO.Directory.GetFiles(radarFolderImagesDownload,
                                  "*.gif", SearchOption.AllDirectories).OrderBy(x => x).ToArray();

                            Array.Sort(images, new MyComparer(false));

                            if (images.Length > 0)
                            {
                                for (int i = 0; i < images.Length; i++)
                                {
                                    drawOnImage.DrawText(dates[i].ToString("ddd, dd MMM yyy HH':'mm"), images[i]);
                                }
                            }

                            GetImagesFiles();
                        }
                    }
                    else
                    {
                        string error = e.Error.ToString();
                    }
                };

                client.DownloadDataCompleted += (s, e) =>
                {
                    if (e.Error == null)
                    {
                        urlsCounter--;

                        var t = urls;

                        if (urlsCounter == 0)
                        {
                            CheckIfImagesExist();

                            btnRadarPath.Enabled = true;
                            btnSatellitePath.Enabled = true;

                            radCounter = 0;
                            satCounter = 0;

                            lblStatus.Text = "Completed.";

                            dates = rad.dates;
                            var images = System.IO.Directory.GetFiles(radarFolderImagesDownload,
                                  "*.gif", SearchOption.AllDirectories).OrderBy(x => x).ToArray();

                            Array.Sort(images, new MyComparer(false));

                            if (images.Length > 0)
                            {
                                for (int i = 0; i < images.Length; i++)
                                {
                                    drawOnImage.DrawText(dates[i].ToString("ddd, dd MMM yyy HH':'mm"), images[i]);
                                }
                            }

                            GetImagesFiles();
                        }
                    }
                };

                client.DownloadProgressChanged += (s, e) => tracker.SetProgress(e.BytesReceived, e.TotalBytesToReceive);
                client.DownloadProgressChanged += (s, e) => lblAmount.Text = tracker.SizeSuffix(e.BytesReceived) + "/" + tracker.SizeSuffix(e.TotalBytesToReceive);
                client.DownloadProgressChanged += (s, e) => lblSpeed.Text = tracker.GetBytesPerSecondString();
                client.DownloadProgressChanged += (s, e) => myLong = Convert.ToInt64(client.ResponseHeaders["Content-Length"]);
                client.DownloadProgressChanged += (s, e) =>
                {
                    progressBar1.Value = e.ProgressPercentage;
                    label1.Text = e.ProgressPercentage + "%";
                };

                for (int i = 0; i < urls.Count; i++)
                {
                    tracker.NewFile();

                    if (urls[i].Contains("Radar"))
                    {
                        await client.DownloadFileTaskAsync(new Uri(urls[i]), radarFolderImagesDownload + "\\image" + radCounter + ".gif");

                        radCounter++;
                    }
                    else
                    {
                        lblStatus.Text = "Downloading satellite";

                        using (MemoryStream ms = new MemoryStream(await client.DownloadDataTaskAsync(new Uri(urls[i]))))
                        {
                            Image img = Image.FromStream(ms, true);
                            img.Save(satelliteFolderImagesDownload + "\\image" + satCounter + ".gif", System.Drawing.Imaging.ImageFormat.Gif);
                        }

                        satCounter++;
                    }
                }
            }
        }

Answer 1

It looks like the exception is being thrown from inside your Form1.DownloadAsync method when you're calling Image.Save.

It's hard to diagnose further without seeing more of your Form1 class but whenever I've encountered the error "A generic error occurred in GDI+" it's often been because I have two open two images attempting to save to the same location & one's locked the file, then the second throws that exception because it can't gain exclusive write access to it.