在ObservableCollection中添加元素会导致异常 Index was out of range。

Question

我有一段代码，允许用户选择一个文件，添加一些关于它的信息到一个可观察的集合，并显示上传进度，当完成时，它将图像绑定到一个图像视图。它一次就能正常工作，但如果你重复这个过程，就会出现一个异常。

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.43(intptr,intptr)
  at at (wrapper native-to-managed) Android.Runtime.DynamicMethodNameCounter.43(intptr,intptr)

插入新元素并获取其索引的代码。

switch(x){

 // some code...

    case 6:

        // .... some code 


            _ = Task.Run(async () => {
            try
            {
                int i = default;

            Msg newmsg2 = new Msg() { UploadProgress = 0.0, UploadProgressVisibility = true, IsImageSend = true, ImageSend = "@drawable/icon_default" };

            valuesLock.EnterWriteLock();
            try
            {
                // THE ISSUE IS RELATED WITH THIS WORK
                Device.BeginInvokeOnMainThread(() => ListaMsg.Add(newmsg2));
            }
            finally
            {
                valuesLock.ExitWriteLock();
                valuesLock.EnterReadLock();
                try
                {
                    // THE ISSUE IS RELATED WITH THIS WORK

                    i = ListaMsg.IndexOf(newmsg2); 

                    // Some data can be added while the progress is being updated, I need to get the index of this exactly Item, can't get it with Count-1

                }
                finally
                {
                    valuesLock.ExitReadLock();
                    Resultado a = await Task.Run(()=>UploadFile(filename, i));
                    if (a.status == "ok")
                    {
                        valuesLock.EnterWriteLock();
                        try
                        {
                            Device.BeginInvokeOnMainThread(() =>
                            {
                                MyList[i].msg = "Image was sent";
                                MyList[i].status = true;
                                MyList[i].ImageSend = "mydomain/assets/libs/thumb.php?h=150&w=150&img=" + a.msg;
                            });
                        }
                        finally
                        {
                            valuesLock.ExitWriteLock();
                            SendMsg(6, a.msg, i);
                        }
                    }
                    else
                    {
                        valuesLock.EnterWriteLock();
                        try
                        {
                            Device.BeginInvokeOnMainThread(() =>
                            {
                                MyList[i].ImageSend = "@drawable/icon_default";
                                MyList[i].icon = "\uf057";
                                MyList[i].IconColor = Xamarin.Forms.Color.Red;
                            });
                        }
                        finally { valuesLock.ExitWriteLock(); }
                    }
                }
            }
        }
        catch (Exception e)
        {
            ...
        }
    });
break;
}

上载方式

public async Task<Resultado> UploadFile(string url, string filepath)
{
  //some code

  webclient.UploadProgressChanged += new UploadProgressChangedEventHandler(UploadProgressCallback);

  //some code
}

最后是 ProgressChanged 回调。

private void UploadProgressCallback(object sender, UploadProgressChangedEventArgs e, int idreference)
{
    double temp = ((double)e.BytesSent) / filesize;
    Device.BeginInvokeOnMainThread(() => {
        valuesLock.EnterWriteLock();
        try
        {
            // THE EXCEPTION IS BEING THREW AT LINE BELOW, idreference value is -1 and not the correct index
            MyList[idreference].UploadProgress = temp;
        }
        finally { valuesLock.ExitWriteLock(); }
    });
}

为什么只有一次有效？是我错过了还是做错了什么？

Answer 1

问题的根源是包含IndexOf()方法的部分。它是在主线程上的祖先执行之前执行的，索引总是-1。通过修改来解决。

Device.BeginInvokeOnMainThread(() => ... );

改为

await Device.InvokeOnMainThreadAsync(() => { ... } );