GcLogBase.cs

public abstract class GcLogBase : IGcLog
{
    protected string Filename;
    private StreamWriter _fileWriter;

    public void Start(string filename)
    {
        if (string.IsNullOrEmpty(filename))
            throw new ArgumentNullException(nameof(filename));

        if (_fileWriter != null)
            throw new InvalidOperationException("Start can't be called twice: Stop must be called first.");

        _fileWriter = new StreamWriter(filename);
        Filename = filename;

        OnStart();
    }

    public void Stop()
    {
        if (string.IsNullOrEmpty(Filename))
            return;

        OnStop();
        Filename = null;
        _fileWriter.Flush();
        _fileWriter.Dispose();
        _fileWriter = null;
    }

    protected bool WriteLine(string line)
    {
        if (_fileWriter == null)
            return false;   // just in case the method is called AFTER Stop

        _fileWriter.WriteLine(line);

        return true;
    }

    protected abstract void OnStart();

    protected abstract void OnStop();
}