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Working with .tmp file for writing data

I'm wondering if there is a best practice when it comes to working with .tmp file for writing data. I like to make an .tmp that will be use in the filestream and then when I close the writer, I like to rename the file. Is there a way to rename file extension?
FileStream stream2 = new FileStream(fileName, FileMode.Create, FileAccess.ReadWrite);
StreamWriter streamWriter2 = new StreamWriter(stream2);
streamWriter2.WriteLine(textToAdd);
streamWriter2.Close();
string changed = Path.ChangeExtension(fileName, .txt);
File.Move(path, changed);

Here's how I would do this:
// Build a FileInfo object for your temp destination, this gives us
// access to a handful of useful file manipulation methods
var yourFile = new FileInfo(#"C:\temp\testfile.tmp");
// open a StreamWriter to write text to the file
using (StreamWriter sw = yourFile.CreateText())
{
// Write your text
sw.WriteLine("Test");
// There's no need to call Close() when you're using usings
}
// "Rename" the file -- this is the fastest way in C#
yourFile.MoveTo(#"C:\temp\testfile.txt");

You can use Path.GetFilenameWithoutExtension to remove the extension and then just add the one you want.

Create and write file txt

How can I create and then modify writing on this file?
string fileName = #"C:\...\MioFile.txt";
In main:
File.CreateText(fileName);
Then when I would edit the file by adding text.
StreamWriter writer = new StreamWriter(fileName);
sw.WriteLine("Hello"+variable);
sw.Close();
But the file is empty and I cannot write anything.
I would like create a file.txt and I would like for this file to always add new information every time I call it in writing mode. A kind of "log file".

Use File.AppendAllText instead of StreamWriter. Its simple:
File.AppendAllText(filename, "Hello"+variable);

You have sw.WriteLine, But your streamwriter is called "writer". That might be the problem.

I like to use the "using" statements:
//full path
var fileName = #"C:\Users\...\Desktop\newFile2.txt";
//Get the stream in FileMode.Append (will create or open)
using (var fileStream = new FileStream(fileName,FileMode.Append))
{
//pass the fileStream into the writer.
using (var writer = new StreamWriter(fileStream))
{
writer.WriteLine("{0} => file appended", DateTime.Now);
}//dispose writer
}//dispose fileStream

The process cannot access the file when using StreamWriter

Basically I want to create a file if not existing then write message to it.
if (!File.Exists(filePath + fileName))
File.Create(filePath + fileName);
StreamWriter sr = new StreamWriter(filePath + fileName,false);
How to deal with this error?
The process cannot access the file 'c:\blahblah' because it is being used by another process.

File.Create opens a FileStream (http://msdn.microsoft.com/en-us/library/d62kzs03.aspx).
As you didn't dispose it, the file remains locked and subsequent accesses to the file will fail because of this situation if these are performed from other handles (i.e. other FileStream or the whole StreamWriter).
This code demonstrates how you should work with IDisposable objects like FileStream:
if (!File.Exists(filePath + fileName))
{
File.Create(filePath + fileName).Dispose();
using(StreamWriter sr = new StreamWriter(filePath + fileName,false))
{
}
}

Why not just use the StreamWriter constructor that takes in the file name?
StreamWriter sr = new StreamWriter(filePath + fileName);
From MSDN:
The path parameter can be a file name, including a file on a Universal Naming Convention (UNC) share. If the file exists, it is overwritten; otherwise, a new file is created.
Very minor point but you could consider using Path.Combine when concatenating file names and folder paths.

Simplify your code by using single method to create and open a file:
using (FileStream fs = File.OpenWrite(path))
{
Byte[] info = new UTF8Encoding(true)
.GetBytes("This is to test the OpenWrite method.");
fs.Write(info, 0, info.Length);
}
MSDN: (File.OpenWrite Method)
Opens an existing file or creates a new file for writing.

Writing in files ASP.NET C# and NOT locking them afterwards

I am getting this error: The process cannot access the file (...) because it is being used by another process.
I have tried to use
File.WriteAllText;
StreamWriter sw = new StreamWriter(myfilepath);
sw.Write(mystring);
sw.Close();
sw.Dispose();
;
using (FileStream fstr = File.Create(myfilepath))
{
StreamWriter sw = new StreamWriter(myfilepath);
sw.Write(mystring);
sw.Close();
sw.Dispose();
fstr.Close();
}
All I am trying to do is to access a file, write on it, then close it. I might be making a silly mistake but I would like to understand what I am doing wrong and why. How to make sure that the file is closed and not to cause this error again.
Helped by the answers so far I did this:
using (FileStream fstr = File.Open(myfilepath,FileMode.OpenOrCreate,FileAccess.ReadWrite))
{
StreamWriter sw = new StreamWriter(fstr);
sw.Write(mystring);
sw.Close();
}
It seems to be better because it seems to close/stop the process of my file if I try to access another file on the second time I access the page. But if I try to access the same file on a second time, it gives me the error again.

Why not just use:
System.IO.File.WriteAllText(myfilepath, mystring");
That should not lock your file.
Internally WriteAllText uses FileShare.Read and releases that lock as soon as it is done writing.

"because it is being used by another process" that's the clue. Do you by chance have the file open in Notepad or something?
You may need to set the sharing mode when you open the file to allow readers, and ask only for the permission you need (write access).
http://msdn.microsoft.com/en-us/library/5h0z48dh.aspx

I would like to thank everyone for the help.
In fact, apart from this code I found out that I had a stremReader still opened somewhere else after the code above. At the end I changed the code I had before for this:
using (FileStream fstr = File.Open(myfile, FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
StreamWriter sw = new StreamWriter(fstr);
sw.Write(mystring);
sw.Flush();
sw.Dispose();
}
and on my StreamReader I did this:
StreamReader sr = new StreamReader(myfile);
string sometext = sr.ReadToEnd();
sr.Dispose();
I could also use this:
File.ReadAllText(myfile);
If there is something that I could have done in a better way please tell me.
Thank you very much.

Try this
FileStream fs = new FileStream(myfilepath, FileMode.Create, FileAccess.Write);
byte[] bt = System.Text.Encoding.ASCII.GetBytes(mystring);
fs.Write(bt, 0, bt.Length);
fs.Close();

StreamWriter is not able to write in file

My code is
System.IO.StreamWriter objStreamWriter = new System.IO.StreamWriter(File);
objStreamWriter.Write(txtEditor.Text);
objStreamWriter.Close();
txtEditor.Text = string.Empty;
I got a message The file has been modified out side of............. but my text file is empty. When in debug mode, I got a value of textEditor and path is not a problem. Am I missing some stupid things.
Thanks.

You have to verify the content of txtEditor before you write it to disk file.
string text=txtEditor.Text;
if(text.Trim.Length!=0)
{
using(System.IO.StreamWriter objStreamWriter = new System.IO.StreamWriter(File))
{
objStreamWriter.Write(text);
}
}

Use the StreamWriter by the "using" keyword for correct writing in to textfile.
using (StreamWriter writer = new StreamWriter("important.txt"))
{
writer.Write("Word ");
writer.WriteLine("word 2");
writer.WriteLine("Line");
}
Refer to the C# Using StreamWriter for more info