Problem writing to a text file C#

Here is my program:

protected int CheckExisting(string item_id)
{
            StreamReader sr = new StreamReader(@"D:\ItemID.txt");
            string line = sr.ReadLine();

            while (line != null)
            {
                if (0 == string.Compare(line, item_id))
                    return 1;

                line = sr.ReadLine();
            }

            sr.Close();
            return 0;
}

protected void WriteNewLog(string item_id)
{
            using (StreamWriter sw = F开发者_StackOverflow社区ile.AppendText(@"D:\ItemID.txt"))
            {
                sw.WriteLine(item_id);
            }
}

protected void xHandler(int num)
{
    for(int i= 0; i< num; i++)
        if (0 == CheckExisting(item_id))
        {                       
            WriteNewLog(item_id);
        }
}

When o run the program, an exception unhandled occur: "The process cannot access the file 'D:\ItemID.txt' because it is being used by another process." Can u guys help me fix that? Thanks so much!

If this executes:

if (0 == string.Compare(line, item_id))
    return 1;

then you won't close your StreamReader. Use a using block when reading as well as writing.

Additionally:

• I'd suggest using bool instead of an integer to indicate yes/no results
• I'd use a simple equality check instead of calling Compare and checking the results against 0
• If you're using .NET 4, File.ReadLines is a simpler way of reading lines
• LINQ makes it easier to implement CheckExisting to start with
• The name xHandler doesn't follow .NET naming conventions
• Underscores in parameter names don't follow .NET naming conventions
• I would suggest using braces for clarity everywhere, even where they're not strictly necessary
• I would put the constants in comparisons at the end of the comparison instead of at the start; I believe most people find that more readable
• There's no indication that the CheckExisting and WriteNewLog methods need to be protected rather than private
• You're not actually using your i variable in xHandler

Here's the implementation I'd use:

const string FileName = @"D:\ItemID.txt";

private bool CheckExisting(string itemId)
{
    return File.ReadLines(FileName)
               .Contains(itemId);
}

private void WriteNewLog(string itemId)
{
    using (TextWriter writer = File.AppendText(FileName))
    {
        writer.WriteLine(itemId);
    }
}

// Adjust name appropriately
protected void FooHandler(int num)
{
    for (int i = 0; i < num; i++)
    {
        // Probably use i here somewhere?
        if (!CheckExisting(itemId))
        {
            WriteNewLog(itemId);
        }  
    }    
}

try putting the stream reader in a using so that its dispose is called.

right now your code will return before sr.Close() is called.

You should either use using or try/finally to make sure your recourse is closed, in your code the exception is thrown because of return 1; the method will exit without closing the stream and so you getting file in use by another process. example on try/finally

StreamReader sr = new StreamReader(@"D:\ItemID.txt");
try
{
    string line = sr.ReadLine();

    while (line != null)
    {
        if (0 == string.Compare(line, item_id))
            return 1;

        line = sr.ReadLine();
    }
}
finally
{ 
    sr.Close();
}

return 0;

You are not closing the file when you return 1 from CheckExisting. Also use the using (StringReader rdr = ...).

Because the file is still locked open by your process, it cannot be opened again. That's why you see this Exception: The process cannot access the file 'D:\ItemID.txt' because it is being used by another process.