C# - How to create a text file and add a line of text to it?

I have a C# script that creates a file and I would like to update it to add some text to the file.

This is the code that works perfectly to create the file:

String filepath = "";
filepath = "This/is/a/dynamic/file/path.txt";

System.IO.File.Create(filepath);

I would like to add some code to add a line of text to the file. Below is what I have tried, but when I run it I get this error:

error: the process cannot access the file 'This/is/a/dynamic/file/path.txt' because it is being used by another process.

String filepath = "";
filepath = "This/is/a/dynamic/file/path.txt";

System.IO.File.Create(filepath);

System.IO.File.AppendAllText(filepath, "Text to add to file" + Environment.NewLine);

Any help would be much appreciated, and forgive me if this is a dumb question as I'm very new to C#.

When you use File.Create it returns a FileStream which is IDisposable, you should always use a using statement to ensure it is disposed. This will ensure it releases the file.

using FileStream file = File.Create(filepath);

However, File.AppendAllText will create the file if it doesn't exist and does not return a Stream and it closes the file when done, so no need to worry, just remove the create line all together.

https://learn.microsoft.com/en-us/dotnet/api/system.io.file.appendalltext?view=net-7.0

As mentioned yearlier you missed .Close() immediately after file creation.

Instead I suggest using StreamWriter that will create if doesn't exists, or write into existing file. It also support append or override modes

using (StreamWriter outputFile = new StreamWriter("This/is/a/dynamic/file/path.txt", true)))
{
   outputFile.WriteLine("Text to add to file");
}

Your file is still being used (or kept open) by the File.Create() method. So you need to close it after creating the file.

File.Create(filepath).Close();
File.AppendAllText(filepath, "Your text\n");