The 'await' operator can only be used with an async lambda expression [duplicate]

Question

I'm trying to copy a list of files to a directory. I'm using async / await. But I've been getting this compilation error

The 'await' operator can only be used within an async lambda expression. Consider marking this lambda expression with the 'async' modifier.

This is what my code looks like

async Task<int> CopyFilesToFolder(List<string> fileList, 
            IProgress<int> progress, CancellationToken ct)
{
    int totalCount = fileList.Count;
    int processCount = await Task.Run<int>(() =>
    {
        int tempCount = 0;
        foreach (var file in fileList)
        {
            string outputFile = Path.Combine(outputPath, file);

            await CopyFileAsync(file, outputFile); //<-- ERROR: Compilation Error 

            ct.ThrowIfCancellationRequested();
            tempCount++;
            if (progress != null)
            {
                progress.Report((tempCount * 100 / totalCount)));
            }

        }

        return tempCount;
    });
    return processCount;
}


private async Task CopyFileAsync(string sourcePath, string destinationPath)
{
    using (Stream source = File.Open(sourcePath, FileMode.Open))
    {
        using (Stream destination = File.Create(destinationPath))
        {
            await source.CopyToAsync(destination);
        }
    }

}

Pls can anyone point out what am I missing here ?

Answer 1

int processCount = await Task.Run<int>(() =>

Should be

int processCount = await Task.Run<int>(async () =>

Remember that a lambda is just shorthand for defining a method. So, your outer method is async, but in this case you're trying to use await within a lambda (which is a different method than your outer method). So your lambda must be marked async as well.