scope of 'use' keyword using F#

Question

Assume the following code:

let fn () =
  use a = new System.IO.StreamWriter ("d:\\test.txt")
  a

let b = fn ()
b.Write "t"

It gives following error:

ObjectDisposedException: Cannot write to a closed TextWriter.

Why is that?

score 8 · Accepted Answer · answered Jun 10 '11 at 12:14

8

use is scoped to the function fn so when a is returned it also disposed.

The easiest way to cure this is:

let fn () =
  let a = new System.IO.StreamWriter ("d:\\test.txt")
  a

let main() = 
  use b = fn ()
  b.Write "t"
main()

answered Jun 10 '11 at 12:14

Robert

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score 5 · Answer 2 · edited May 23 '17 at 12:20

5

To add a bit more to Robert's (correct) answer: you can achieve even finer control over the scope of use with scoping constructs (parentheses or begin/end). See Brian's answer to a related question.

edited May 23 '17 at 12:20

Community

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answered Jun 10 '11 at 14:14

Daniel

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Thanks, I have to admit I didn't know that was possible. – Robert Jun 10 '11 at 14:43

score 2 · Answer 3 · answered Jun 10 '11 at 12:12

2

The resource will be disposed as soon as the variable (in your case a) goes out of scope.

This means that after the call to fn the StreamWriter will already be disposed.

answered Jun 10 '11 at 12:12

Joachim Sauer

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And why does it not wait until I stop using 'b' when 'b' = 'a'? – Oldrich Svec Jun 10 '11 at 12:15
1

Because it looks at the **variable** `a` and doesn't "follow" the value held by that variable. – Joachim Sauer Jun 10 '11 at 12:15

scope of 'use' keyword using F#

3 Answers3