File being used by another process after using File.Create()

The File.Create method creates the file and opens a FileStream on the file. So your file is already open. You don't really need the file.Create method at all:

string filePath = @"c:\somefilename.txt";
using (StreamWriter sw = new StreamWriter(filePath, true))
{
    //write to the file
}

The boolean in the StreamWriter constructor will cause the contents to be appended if the file exists.

    File.Create(FilePath).Close();
    File.WriteAllText(FileText);

I want to update this answer to say that this is not really the most efficient way to write all text. You should only use this code if you need something quick and dirty.

I was a young programmer when I answered this question, and back then I thought I was some kind of genius for coming up with this answer.