Q:

Concurrent Reads and Write to an Excel file

Is it possible to have only one process do WRITE and many doing a READ operation on an excel file? I am using ExcelPackage(EPPlus) for this. 
To demo, I wrote two console app one to write iteratively and another to read. Running them concurrently will cause a failure on either side.
WRITE
// 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++;
    }
}

READ
//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();
}

A:

I slightly changed my code by making the WRITE program lock the file before writing and READ to be resilient when a failure happens:
WRITE: Use a FileStream and lock it before attempting to write. This will prevent the WRITE from failing
READ: added a retry mechanism implementing a try/catch block
Modified code:
WRITE
// 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++;
    }
}

READ
//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();
}

in the actual app's code I set the limit of READ's WHILE to 3 to retry twice if first read attempt failed. That proves to be more than enough in my case (as the WRITEs are short; append a row at a time) and the app is running well over a month.

