What is the correct way of using the string buffer returned by the WindowsPreallocateStringBuffer function?

The most common way of creating an HSTRING is to call Windows­Create­String, but there is also a two-phase creation pattern: First you call Windows­Preallocate­String­Buffer to create a buffer for a future string. You then fill the buffer with stringy goodness and then call Windows­Promote­String­Buffer to convert it to a real HSTRING. (Or you can call Windows­Delete­String­Buffer to change your mind and pretend it never happened.)

The rule for managing the buffer returned by Windows­Preallocate­String­Buffer is that you are expected to write exactly length code units into the buffer. No more. No less. The system already put a terminating null after the end of the buffer; your job is to fill the buffer with the string contents.

For example, if you want to use two-phase creation to create the string hello, you would call Windows­Preallocate­String­Buffer and pass length = 5. Into the resulting buffer, you write the characters h, e, l, l, and o, and that's all. The system already stored the terminating null.

This particular formulation of the rules is important in the case that length = 0.¹ Since the representation of an HSTRING of length zero is the null pointer, there is no actual buffer. What happens is that the system uses a single preallocated buffer (consisting of just a null terminator) to represent all zero-length strings. If you call Windows­Preallocate­String­Buffer, you get a pointer to that preallocated buffer.² Since you passed a length of zero, you are expected to write zero characters to the buffer; in other words, you are expected to do nothing at all with the buffer.

And of course since HSTRINGs are immutable, your permission to modify the buffer ends once you promote the buffer to a string. Once it's been promoted to a string, the entire buffer becomes read-only.

¹ Another way of interpreting this corner case is to say "Don't bother calling Windows­Preallocate­String­Buffer with a string of length zero. Otherwise, go ahead and call it, and you can write that null terminator if you like."

² Arguably, to accommodate the possibiltiy of somebody writing that null terminator, it should return a preallocated writable buffer just large enough to hold that null terminator. It could be the high 16 bits of the length field itself!