Does Constructing a FileStream in C# Trigger Significant I/O Overhead?

When you call the FileStream constructor, the following happens:

• OS Handle Acquisition:
  Creating a new FileStream does immediately invoke an OS call to open the file (i.e., it calls the underlying platform API) and returns a file handle. This means that while you aren’t reading any file data yet, you are performing an I/O operation at the system level.

• Buffer Setup:
  The FileStream constructor sets up internal buffers and state information to facilitate later reads (or writes). This is mostly in-memory work, but it’s part of preparing the stream.

• No File Content Read:
  The constructor does not read file data into memory. It simply “opens” the file. The actual file reads (buffer fills) occur when you invoke methods like Read or BeginRead.

Cost Considerations:

• The operation is “cheap” in terms of in-memory processing (i.e., it’s mostly object allocation and initialization).
• However, in I/O terms, opening the file does involve a system call and interaction with the file system, which may be considered expensive compared to purely in-memory operations. The cost may be more noticeable on remote or slow storage mediums.

Summary:
While the constructor call for FileStream is not expensive in terms of reading data into memory, it is not just a plain old C# object—it does trigger an I/O operation to open the file handle via the operating system.