Yes, you can, try the following
long length = writer.BaseStream.Length;//will give unexpected output if autoflush is false and write has been called just before
Note: writer.BaseStream.Length
property can return unexpected results since StreamWriter
doesn't write immediately. It caches so to get expected output you need AutoFlush = true
writer.AutoFlush = true; or writer.Flush();
long length = writer.BaseStream.Length;//will give expected output