并发读取和写入Excel文件

Question

是否可以只对一个进程执行WRITE，而对一个Excel文件执行许多读操作？我为此使用ExcelPackage（EPPlus）。

为了演示，我编写了两个控制台应用程序，一个用于迭代编写，另一个用于读取。同时运行它们会在任何一方导致故障。

写

// simply write to a column
var fileLocation = "D:\\Book.xlsx";
FileInfo fi = new FileInfo(fileLocation);

int i = 1;
while (1 == 1)  //ALERT: an infinite loop!
{
    using (ExcelPackage excelPackage = new ExcelPackage(fi))
    {
        ExcelWorksheet worksheet = excelPackage.Workbook.Worksheets["Sheet1"];
        var row = worksheet.Row(2);

        worksheet.Cells[$"A{i}"].Value = "Test " + i.ToString();
        excelPackage.Save();
        i++;
    }
}

读

//simply populate a list reading excel
var fileLocation = "D:\\Book.xlsx";
FileInfo fi = new FileInfo(fileLocation);
List<string> list = new List<string>();
ExcelWorksheet worksheet = null;
int i = 1;
while (1 == 1)  //ALERT: an infinite loop!
{
    using (ExcelPackage excelPackage = new ExcelPackage(fi))
    {
        worksheet = excelPackage.Workbook.Worksheets["Sheet1"];
        if (worksheet.Cells[i, 1].Value != null)
        {
            list.Add(worksheet.Cells[i, 1].Value.ToString());
        }
    }

    list.Clear();
}

Answer 1

我通过使WRITE程序在写入之前锁定文件并略微更改了代码，并在发生故障时使READ保持弹性：

写：在尝试写入之前，使用FileStream并将其锁定。这样可以防止WRITE失败

阅读：添加了实现try / catch块的重试机制

修改后的代码：

写

// simply write to a column
var fileLocation = "D:\\Book.xlsx";
FileInfo fi = new FileInfo(fileLocation);

int i = 1;
while (1 == 1)  //ALERT: an infinite loop!
{

    using (ExcelPackage excelPackage = new ExcelPackage(fi))
    {
        ExcelWorksheet worksheet = excelPackage.Workbook.Worksheets["Sheet1"];
        worksheet.Cells[$"A{i}"].Value = "Test " + i.ToString();

        using (var fs = new FileStream(fileLocation, FileMode.Open, FileAccess.ReadWrite, FileShare.Read))
        {
            fs.Lock(0, fs.Length);
            excelPackage.SaveAs(fs);
            try
            {
                fs.Unlock(0, fs.Length); // this raises an exception if fs unlocked already by itself
            }
            catch (IOException ex) when (ex.Message.ToLower().StartsWith("the segment is already unlocked.",
                StringComparison.InvariantCultureIgnoreCase))
            {
                // NOP; just ignore if already unlocked
            }
        }
        i++;
    }
}

读

//simply populate a list reading excel
var fileLocation = "D:\\Book.xlsx";
FileInfo fi = new FileInfo(fileLocation);
List<string> list = new List<string>();
ExcelWorksheet worksheet = null;
int i = 1;
while (1 == 1)  //ALERT: an infinite loop!
{
    try
    {
        using (ExcelPackage excelPackage = new ExcelPackage(fi))
        {
            worksheet = excelPackage.Workbook.Worksheets["Sheet1"];
            if (worksheet.Cells[i, 1].Value != null)
            {
                list.Add(worksheet.Cells[i, 1].Value.ToString());
            }

            Console.WriteLine(worksheet.Dimension.Rows.ToString()); // just prove that it read
        }
    }

    catch (Exception ex) when (
        ex is IOException &&
        ex.Message.StartsWith("The process cannot access the file because another process has locked a portion of the file.", StringComparison.InvariantCultureIgnoreCase))
    {
        Console.WriteLine($"Attempt: {i}");
    }

    list.Clear();
}

在实际应用程序的代码中，如果第一次读取尝试失败，我将READ'S WHILE的限制设置为3，以重试两次。在我看来，这已经足够了（因为WRITEs很短；每次添加一行），并且该应用程序运行了一个多月。