System.IO.File.Close() missing; prevents writing idiomatic F# code

[wyoung]

Although I realize that .NET Core is intentionally stripped down relative to the full .NET platform, this method is necessary for idiomatic (i.e. single-assignment) programming with F# on Windows. Without it, the only way to close a file is to delete the File object which requires coping with nulls, a thing F# tries very hard to avoid.

Explicitly closing a file is often necessary on Windows due to its mandatory file locking semantics. For example, you cannot typically move or a file to another directory while a File object is open for writing on that file.

Lacking a Close() method, single-assignment code such as this:

 open System.IO

 let file = File.Create("myfile", ...)
 ... do stuff with file
 file.Close()
 ... do something with myfile on disk

must be rewritten to use a mutable file object:

 open System.IO

 let mutable file = File.Create("myfile", ...)
 ... do stuff with file
 file <- null
 ... do something with myfile on disk

Although that looks like a simple fix, now all the F# code dealing with file has to check for null before using the value, a very un-F# style of coding.

(Moved from [https://github.com/dotnet/cli/issues/1776])

[stephentoub]

You can't just use Dispose() instead of Close()? (Note that System.IO.File.Create returns a FileStream, not a File.)

[svick]

I thought the idiomatic ways to do this in F# are use and using. Why can't you use those?