Thread-safe extension method?

Question

I have a very simple extension method that looks like this:

public static string ToUserPageTimeFormat(this DateTime TheTime)
{
    return TheTime.Month + "." + TheTime.Day + "." + TheTime.Year + "."  + TheTime.Hour + "." + TheTime.Minute;
}

I made it in one line. Is this guaranteed to be thread safe?

Answer 1

Yes, it's thread-safe. Essentially, your method will have its own private copy of the DateTime argument since it is passed by value - a copy is first created and then handed to the method. This copy is private to the method and isn't visible to other threads - and therefore can't possibly be mutated by them.

This would not be the case if you had used a ref parameter:

// Not thread-safe.
public static string ToUserPageTimeFormat(ref DateTime TheTime){ ... }

In such a hypothetical scenario, the argument could be mutated on another thread in the midst of execution of this method. The fact that DateTime is an immutable type is irrelevant in this case since it is a struct, and a struct does not own its own storage.

For example, it would then be possible for this method to return an "impossible" formatted date such as "2.31.2012.14.33", resulting from "torn" reads in the midst of multiple write operations.

Answer 2

Yes, DateTime is a struct, and therefore is copied to the functional call instead of just passing a reference. This is due to structs being value types and not reference types.