Iam reading a data from file and writing data to same fie IO Exception is occured file is already used by another process

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C#

lock(_syncobj)
{
    using (FileStream fileStream = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read)
    {
    // coding to read data
    }
    
    FileStream fileStream = new FileStream(filename, FileMode.Create, FileAccess.Write, FileShare.None);
    using (TextWriter tw = new StreamWriter(fileStream))
    {
    //coding to write data
    }
}

I have used FileShare.Read for reading and FileShare.None for writing and used lock also but also some IO Exception is occured stating that the file is used by another process.