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With the exception of stack and memory, all objects that own resources should be subtypes of the .NET type System.IDisposable. This is the primary way you can recognize primitive resources and objects that wrap resources. The System.IDisposable interface has a single method; in F# syntax, it can be defined as follows: namespace System type IDisposable = abstract Dispose: unit -> unit One of the simplest approaches to managing IDisposable objects is to give each resource a lifetime: that is, some well-defined portion of the program execution for which the object is active. This is even easier when the lifetime of a resource is lexically scoped, such as when a resource is allocated on entry to a function and deallocated on exit. In this case, the resource can be tied to the scope of a particular variable, and you can protect and dispose of a value that implements IDisposable by using a use binding instead of a let binding. For example, in the following code, three values implement IDisposable, all of which are bound using use: /// Fetch a web page let http (url: string) = let req = System.Net.WebRequest.Create url use resp = req.GetResponse() use stream = resp.GetResponseStream() use reader = new System.IO.StreamReader(stream) let html = reader.ReadToEnd() html
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CHAPTER 8 MASTERING F#: COMMON TECHNIQUES
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In all three cases, the objects (a WebResponse, a Stream, and a StreamReader) are automatically closed and disposed at the end of an execution of the function. To see what s going on, notice that a use binding is syntactic sugar. For example, the following use var = expr in body is shorthand for this: let var = expr try body finally match var with | null -> () | _ -> var.Dispose() A number of important types implement IDisposable; Table 8-1 shows some of them. You can use tables such as this to chart the portions of the .NET Framework that reveal operating system functionality to .NET applications. Table 8-1. A Selection of the Types That Implement IDisposable
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Namespace
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System.IO System.Drawing System.Drawing.Text System.Drawing.Drawing2D System.Drawing.Imaging System.Net System.Net.Sockets System.Data.SqlClient System.Threading System.Web.UI System.Web.UI.WebControls System.Windows.Forms Microsoft.Win32
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Some Types Implementing IDisposable
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BinaryReader, BinaryWriter, FileSystemWatcher, IsolatedFileStorage, Stream, TextReader, TextWriter, Brush, BufferedGraphics, Font, FontFamily, Graphics, Icon, Image, Pen, Region, TextureBrush, FontCollection, CustomLineCap, GraphicsPath, GraphicsPathIterator, Matrix, EncoderParameter, ImageAttributes, WebResponse, Socket, TcpClient, SqlBulkCopy, SqlCommand, SqlConnection, SqlTransaction, Timer, WaitHandle, AutoResetEvent, ManualResetEvent, Mutex, Semaphore, Control, HttpApplication, Button, CheckBox, DataGrid, Button, CheckBox, Cursor, Control, DataGrid, Form, RegistryKey,
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Tip A tool such as Visual Studio can help you determine when a type has implemented IDisposable. When you
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rest your mouse pointer over a value, you normally see this noted on the information displayed for a value.
When Will the Runtime Clean Up for You
People often ask if the .NET Common Language Runtime automatically cleans up resources such as file handles the same way it cleans up memory. It s true that when an object gets garbage collected, it may be finalized; if the object is well-implemented, this results in it deallocating any unmanaged resources, closing any outstanding file connections, and releasing any operating system resources. However, although it s appropriate to rely on finalization when prototyping, you should never rely on finalization in code where you re hitting resource limits. For example, let s say you have a loop where you open files using System.IO.File.OpenRead. If you forget to close the file handles, you may quickly allocate thousands of file handles. If you re lucky, the garbage collector may finalize these before you run out of OS resources; but if not, one of your File.OpenRead calls will fail with an exception, even if the file exists on disk. You should also be aware of the potential for memory stickiness. Memory stickiness occurs when the .NET Common Language Runtime is unable to garbage-collect memory even though objects have become unreachable. This happens especially when long-running computations and inactive callbacks hold on to object handles related to the earlier phases of execution of a program. Memory stickiness can also lead to objects never being finalized, reinforcing that you shouldn t rely on finalization to release nonmemory resources. Memory profiling tools such as CLRProfiler are indispensable when you re tracking down memory leaks in production code or long-running applications.
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