在任务完成之前等待返回

Question

I have this async code

private async Task<InvalidInvoiceViewModel> ValidateInsertedDataInRawTables(string uploadPath, long batchId)
    {
        var direcotryPath = Path.Combine(uploadPath, batchId.ToString());
        var allFiles = Directory.GetFiles(direcotryPath);
        var invoiceRawList = new List<InvoiceRaw>();
        var invoiceDetailRawList = new List<InvoiceDetailRaw>();
        await Task.Run(() => allFiles.ToList().ForEach(async file => 
        {
            var fileName = Path.GetFileNameWithoutExtension(file);
            switch (fileName)
            {
                case "invoice":
                    invoiceRawList = await ValidateInsertedDataInRawTablesForInvoiceAsync(batchId);
                    break;
                case "invoicedetails":
                    invoiceDetailRawList = await ValidateInsertedDataInRawTablesForInvoiceDetailAsync(batchId);
                    break;
            }
        }));
        var invalidInvoiceViewModel = new InvalidInvoiceViewModel
        {
            InvoiceRawList = invoiceRawList ,
            InvoiceDetailRawList = invoiceDetailRawList 
        };
        return invalidInvoiceViewModel;
    }

But what happens is, the I always get 0 count in both the InvoiceRawList and InvoiceDetailRawList . What I found is that the method returns before the async lambda completes its processing. (In chrome console, the result appears in the network tab, but the debug point in ValidateInsertedDataInRawTablesForInvoiceAsync is hit after that). I am awaiting the Task.Run that I created for this lambda. What am I doing wrong here?

Update after comments

If I do that, it won't compile, I changed it to...

private async Task<InvalidInvoiceViewModel> ValidateInsertedDataInRawTables(string uploadPath, long batchId)
    {
        var direcotryPath = Path.Combine(uploadPath, batchId.ToString());
        var allFiles = Directory.GetFiles(direcotryPath);
        var invoiceRawList = new List<InvoiceRaw>();
        var invoiceDetailRawList = new List<InvoiceDetailRaw>();
        await Task.Run(async () => await allFiles.ToList().ForEach(async file => 
        {
            var fileName = Path.GetFileNameWithoutExtension(file);
            switch (fileName)
            {
                case "invoice":
                    invoiceRawList = await ValidateInsertedDataInRawTablesForInvoiceAsync(batchId);
                    break;
                case "invoicedetails":
                    invoiceDetailRawList = await ValidateInsertedDataInRawTablesForInvoiceDetailAsync(batchId);
                    break;
            }
        }));
        var invalidInvoiceViewModel = new InvalidInvoiceViewModel
        {
            InvoiceRawList = invoiceRawList ,
            InvoiceDetailRawList = invoiceDetailRawList 
        };
        return invalidInvoiceViewModel;
    }

And says cannot await void. Which I know what that means, but then how do I resolve that? List.ForEach is a void method.

Answer 1

You actually want to wait for WHEN ALL of the inner tasks are finished.

Task.WhenAll is your friend in this case.

private async Task<InvalidInvoiceViewModel> ValidateInsertedDataInRawTables(string uploadPath, long batchId)
{
    var direcotryPath = Path.Combine(uploadPath, batchId.ToString());
    var allFiles = Directory.GetFiles(direcotryPath);
    var invoiceRawList = new List<InvoiceRaw>();
    var invoiceDetailRawList = new List<InvoiceDetailRaw>();

    List<Task> tasks = allFiles.Select(file => Task.Run(async () =>     
    {
        var fileName = Path.GetFileNameWithoutExtension(file);
        switch (fileName)
        {
            case "invoice":
                invoiceRawList = await ValidateInsertedDataInRawTablesForInvoiceAsync(batchId);
                break;
            case "invoicedetails":
                invoiceDetailRawList = await ValidateInsertedDataInRawTablesForInvoiceDetailAsync(batchId);
                break;
        }
    })).ToList();

    await Task.WhenAll(tasks);

    var invalidInvoiceViewModel = new InvalidInvoiceViewModel
    {
        InvoiceRawList = invoiceRawList,
        InvoiceDetailRawList = invoiceDetailRawList
    };
    return invalidInvoiceViewModel;
}

Answer 2

You will have to use regular foreach loop if you want to use await inside it.

    private async Task<InvalidInvoiceViewModel> ValidateInsertedDataInRawTables(string uploadPath, long batchId)
    {
        var direcotryPath = Path.Combine(uploadPath, batchId.ToString());
        var allFiles = Directory.GetFiles(direcotryPath);
        var invoiceRawList = new List<InvoiceRaw>();
        var invoiceDetailRawList = new List<InvoiceDetailRaw>();
        await Task.Run(async () =>
        {
            foreach(var file in allFiles)
            {
                var fileName = Path.GetFileNameWithoutExtension(file);
                switch (fileName)
                {
                    case "invoice":
                        invoiceRawList = await ValidateInsertedDataInRawTablesForInvoiceAsync(batchId);
                        break;
                    case "invoicedetails":
                        invoiceDetailRawList = await ValidateInsertedDataInRawTablesForInvoiceDetailAsync(batchId);
                        break;
                }
            }
        });
        var invalidInvoiceViewModel = new InvalidInvoiceViewModel
        {
            InvoiceRawList = invoiceRawList,
            InvoiceDetailRawList = invoiceDetailRawList
        };
        return invalidInvoiceViewModel;
    }

By the way, this code does not make much sense, you are creating invoiceRawList and invoiceDetailRawList just to replace them in every iteration of the loop.