What’s the least invasive way to read a locked file in C# (perhaps in unsafe mode)?

You can do it without copying the file, see this article:

The trick is to use FileShare.ReadWrite (from the article):

private void LoadFile()
{
    try
    {
        using(FileStream fileStream = new FileStream(
            "logs/myapp.log",
            FileMode.Open,
            FileAccess.Read,
            FileShare.ReadWrite))
        {
            using(StreamReader streamReader = new StreamReader(fileStream))
            {
                this.textBoxLogs.Text = streamReader.ReadToEnd();
            }
        }
    }
    catch(Exception ex)
    {
        MessageBox.Show("Error loading log file: " + ex.Message);
    }
}