Resource Management: The use Keyword
This topic describes the keyword use and the using function, which can control the initialization and release of resources.
Resources
The term resource is used in more than one way. Yes, resources can be data that an application uses, such as strings, graphics, and the like, but in this context, resources refers to software or operating system resources, such as graphics device contexts, file handles, network and database connections, concurrency objects such as wait handles, and so on. The use of these resources by applications involves the acquisition of the resource from the operating system or other resource provider, followed by the later release of the resource to the pool so that it can be provided to another application. Problems occur when applications do not release resources back to the common pool.
Managing Resources
To efficiently and responsibly manage resources in an application, you must release resources promptly and in a predictable manner. The .NET Framework helps you do this by providing the System.IDisposable interface. A type that implements System.IDisposable has the System.IDisposable.Dispose method, which correctly frees resources. Well-written applications guarantee that System.IDisposable.Dispose is called promptly when any object that holds a limited resource is no longer needed. Fortunately, most .NET languages provide support to make this easier, and F# is no exception. There are two useful language constructs that support the dispose pattern: the use binding and the using function.
use Binding
The use keyword has a form that resembles that of the let binding:
use value = expression
It provides the same functionality as a let binding but adds a call to Dispose on the value when the value goes out of scope. Note that the compiler inserts a null check on the value, so that if the value is null, the call to Dispose is not attempted.
The following example shows how to close a file automatically by using the use keyword.
open System.IO
let writetofile filename obj =
use file1 = File.CreateText(filename)
file1.WriteLine("{0}", obj.ToString() )
// file1.Dispose() is called implicitly here.
writetofile "abc.txt" "Humpty Dumpty sat on a wall."
Multiple instances of use are disposed in the reverse order in which they are declared. That is, the first use will be the last one released.
Note
You can use use in computation expressions, in which case a customized version of the use expression is used. For more information, see Sequences, Async expressions, Task expressions, and Computation Expressions.
using Function
The using function has the following form:
using (expression1) function-or-lambda
In a using expression, expression1 creates the object that must be disposed. The result of expression1 (the object that must be disposed) becomes an argument, value, to function-or-lambda, which is either a function that expects a single remaining argument of a type that matches the value produced by expression1, or a lambda expression that expects an argument of that type. At the end of the execution of the function, the runtime calls Dispose and frees the resources (unless the value is null, in which case the call to Dispose is not attempted).
The following example demonstrates the using expression with a lambda expression.
open System.IO
let writetofile2 filename obj =
using (System.IO.File.CreateText(filename)) ( fun file1 ->
file1.WriteLine("{0}", obj.ToString() )
)
writetofile2 "abc2.txt" "The quick sly fox jumps over the lazy brown dog."
The next example shows the using expression with a function.
let printToFile (file1 : System.IO.StreamWriter) =
file1.WriteLine("Test output");
using (System.IO.File.CreateText("test.txt")) printToFile
Note that the function could be a function that has some arguments applied already. The following code example demonstrates this. It creates a file that contains the string XYZ.
let printToFile2 obj (file1 : System.IO.StreamWriter) =
file1.WriteLine(obj.ToString())
using (System.IO.File.CreateText("test.txt")) (printToFile2 "XYZ")
The using function and the use binding are nearly equivalent ways to accomplish the same thing. The using keyword provides more control over when Dispose is called. When you use using, Dispose is called at the end of the function or lambda expression; when you use the use keyword, Dispose is called at the end of the containing code block. In general, you should prefer to use use instead of the using function.
