类型为“ System.IO.IOException”的未处理异常。该进程无法访问该文件，因为该文件正在被另一个进程使用

Question

I am trying to write a program to read records from a csv file, process them and write the results to a new file. To do so I constructed a class called CsvWriter that handles writing of headers and appending rows as I read through the main csv file. Initially I was working on it on my mac and I did not get any error whatsoever when using this class but when I switched to windows (to make a windows form application), I kept getting

An unhandled exception of type 'System.IO.IOException' occurred in mscorlib.dll

Additional information: The process cannot access the file 'C:\\Users\\user\\Documents\\correct-list.csv' because it is being used by another process.

The error I'm having is in the CsvWriter class in the following line:

File.WriteAllText(filename, builder.ToString());

I thought using System.IO.File.WriteAllText would flush and close the file automatically. I cannot find any other source of the error. Any help to resolve this error is appreciated.

This is my code:

public class CsvWriter : StreamWriter
{
    private string filename;

    /// <summary>
    /// Initializes a new instance of the <see cref="T:USPSAddressVerification.CsvWriter"/> class.
    /// </summary>
    /// <param name="filename">Filename of the csv file to write on.</param>
    public CsvWriter(string filename) : base(filename)
    {
        this.filename = filename;
    }

    /// <summary>
    /// Writes the header in the csv file.
    /// </summary>
    /// <param name="headers">Array of Headers.</param>
    /// <param name="addHeaders">Add additional headers to the csv file.</param>
    public void WriteHeader(List<string> headers)
    {
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < headers.Count; i++)
        {
            if (i < headers.Count - 1) builder.Append("\"" + headers[i] + "\",");
            else builder.Append("\"" + headers[i] + "\"\n");
        }
        File.WriteAllText(filename, builder.ToString());
    }

    /// <summary>
    /// Appends the row to the csv file.
    /// </summary>
    /// <param name="row">Row to be appended.</param>
    public void AppendRow(List<string> row)
    {
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < row.Count; i++)
        {
            if (i < row.Count - 1) builder.Append("\"" + row[i] + "\",");
            else builder.Append("\"" + row[i] + "\"\n");
        }
        File.AppendAllText(filename, builder.ToString());
    }

}

Answer 1

You are deriving StreamWriter, this opens the file. Instead of using File.WriteAllText you should use the StreamWriter (base) methods to write your contents. Or don't derive StreamWriter.

Answer 2

Your class inherits from the System.IO.StreamWriter. When you are calling your constructor with the filename parameter you are calling the StreamWriter constructor wih this(filename) as well. With that command you create an instance of the StreamWriter class that access the file as long you do not dispose or close it. That is the reason why your File.WriteAllText command will fail every time. Is there any reason why your class inherits from the StreamWriter? File.WriteAllText works without it.

Update 1

Ups my answer was a little bit to late.