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• Which statically typed languages support intersection types for function return values?

  - by stakx

  Initial note: This question got closed after several edits because I lacked the proper terminology to state accurately what I was looking for. Sam Tobin-Hochstadt then posted a comment which made me recognise exactly what that was: programming languages that support intersection types for function return values. Now that the question has been re-opened, I've decided to improve it by rewriting it in a (hopefully) more precise manner. Therefore, some answers and comments below might no longer make sense because they refer to previous edits. (Please see the question's edit history in such cases.) Are there any popular statically & strongly typed programming languages (such as Haskell, generic Java, C#, F#, etc.) that support intersection types for function return values? If so, which, and how? (If I'm honest, I would really love to see someone demonstrate a way how to express intersection types in a mainstream language such as C# or Java.) I'll give a quick example of what intersection types might look like, using some pseudocode similar to C#: interface IX { … } interface IY { … } interface IB { … } class A : IX, IY { … } class B : IX, IY, IB { … } T fn() where T : IX, IY { return … ? new A() : new B(); } That is, the function fn returns an instance of some type T, of which the caller knows only that it implements interfaces IX and IY. (That is, unlike with generics, the caller doesn't get to choose the concrete type of T — the function does. From this I would suppose that T is in fact not a universal type, but an existential type.) P.S.: I'm aware that one could simply define a interface IXY : IX, IY and change the return type of fn to IXY. However, that is not really the same thing, because often you cannot bolt on an additional interface IXY to a previously defined type A which only implements IX and IY separately. Footnote: Some resources about intersection types: Wikipedia article for "Type system" has a subsection about intersection types. Report by Benjamin C. Pierce (1991), "Programming With Intersection Types, Union Types, and Polymorphism" David P. Cunningham (2005), "Intersection types in practice", which contains a case study about the Forsythe language, which is mentioned in the Wikipedia article. A Stack Overflow question, "Union types and intersection types" which got several good answers, among them this one which gives a pseudocode example of intersection types similar to mine above.

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• Are there legitimate reasons for returning exception objects instead of throwing them?

  - by stakx

  This question is intended to apply to any OO programming language that supports exception handling; I am using C# for illustrative purposes only. Exceptions are usually intended to be raised when an problem arises that the code cannot immediately handle, and then to be caught in a catch clause in a different location (usually an outer stack frame). Q: Are there any legitimate situations where exceptions are not thrown and caught, but simply returned from a method and then passed around as error objects? This question came up for me because .NET 4's System.IObserver<T>.OnError method suggests just that: exceptions being passed around as error objects. Let's look at another scenario, validation. Let's say I am following conventional wisdom, and that I am therefore distinguishing between an error object type IValidationError and a separate exception type ValidationException that is used to report unexpected errors: partial interface IValidationError { } abstract partial class ValidationException : System.Exception { public abstract IValidationError[] ValidationErrors { get; } } (The System.Component.DataAnnotations namespace does something quite similar.) These types could be employed as follows: partial interface IFoo { } // an immutable type partial interface IFooBuilder // mutable counterpart to prepare instances of above type { bool IsValid(out IValidationError[] validationErrors); // true if no validation error occurs IFoo Build(); // throws ValidationException if !IsValid(…) } Now I am wondering, could I not simplify the above to this: partial class ValidationError : System.Exception { } // = IValidationError + ValidationException partial interface IFoo { } // (unchanged) partial interface IFooBuilder { bool IsValid(out ValidationError[] validationErrors); IFoo Build(); // may throw ValidationError or sth. like AggregateException<ValidationError> } Q: What are the advantages and disadvantages of these two differing approaches?

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• Multi-monitor setup: one widescreen monitor, one regular (4:3) monitor; is this usually possible?

  - by stakx

  I'm trying to find out whether it's generally possible to have a multi-monitor setup with both a widescreen monitor and a regular monitor (ie. one with a 4:3 picture ratio). I'd like to run them in Extended Desktop mode on a Windows Vista machine. The computer is a Dell notebook with an onboard Intel GMA 4500MHD graphics chipset. While I know that this notebook is capable of driving multiple monitors, I don't know if it is also able to drive monitors with varying resolutions. Does anyone know if differing screen resolutions is generally a problem?

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• Triple-monitor set-up (2 unique, 1 cloned): Can a VGA splitter be used on one output of a dual-head

  - by stakx

  Background: I'm currently researching hardware components for some kind of information terminal we're building. This application of ours makes use of three output screens: (1) A touch screen where all user input is made; (2) A regular LCD monitor where the requested information is being displayed; and (3) A projector which displays exactly the same signal as screen (2) does. (All screens will run at the same resolution of 1024x768 btw.) Now I figured that using a dual-head video card would be sufficient, let's say a Matrox P690 low-profile PCI card. This would involve having a Y cable connected to the graphics card itself, then two DVI-to-VGA adapters at each end of the Y cable, and then having a VGA splitter on one of the VGA outputs. The following shows the setup in question: 0--1---------2-> VGA (DSUB-15) \ \ ----2-3---------> VGA (DSUB-15) \ \ -----------------> VGA (DSUB-15) 0: graphics card (LFH60 jack) 1: LFH60 to DVI-I dual monitor Y cable 2: DVI-to-VGA adapters 3: VGA splitter cable Question(s): Will this work? I'm particularly concerned about the following points: Can a low-profile PCI video card output a signal which is strong enough for three monitors (even if it's a dual-head card)? Does the combination of so many adapters and splitter cables work? (The LFH-to-DVI cable comes with the video card) Will the VGA splitter cable degrade the signal on the output screen & projector significantly? (If so, would a USB-powered splitter cable remedy this problem?) I can't possibly expect anyone to answer all those questions, but any input is appreciated.

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• DVD playback with Windows Media Player 11 works fine, but when copied to HDD and then played back, t

  - by stakx

  I have several DVDs with short documentaries on it. Since the notebook I'm using (a Dell Latitude E6400) has only one DVD drive, and I might play back those short movies very often, I thought of copying them to the HDD and playing them back from there. However, I've run into a problem, namely stuttering audio. Problem description: When I play back these movies directly from DVD (with Windows Media Player 11 under Windows Vista), everything works fine. Smooth video, no significant audio problems (only the occasional click). But as soon as I copy any of these DVDs to the HDD and try to play them back from there (e.g. using the wmpdvd://drive/title/chapter?contentdir=path protocol, I get stuttering audio — audio playback sounds like a machine gun for a third of a second or so, approx. every 8 seconds. I have tried converting the VOB files from the DVD to another format (ie. ripping), but that resulted in a noticeable downgrade of picture quality. Therefore I thought it best to keep the files in their original format, if possible. Still, I suspect that the stuttering audio is due to some (de-)muxing problem, and that changing the file format might help. (After all, video playback is fine; therefore I don't think that the hardware is too slow for playback.) Only thing is, I don't know how to convert the VOB files to another Windows Media Player-compatible format without quality loss. I hope someone can help me, or give me further pointers on things I could try out to get HDD playback to work without the problem described. Some things I've tried so far, without any success: VOB2MPG, in order to convert the .vob file to a .mpg file. But that changes only the A/V container, not the content. No re-encoding takes place at all. Re-encoding with MPlayer/MEncoder. Lots of quality loss there, and I frankly haven't got the time to test all possible settings combinations available. Disabling all plug-ins, equalizers, etc. in Windows Media Player. Disabling all hardware acceleration on the audio playback device. Further info on the VOB files I'm trying to playback: The video format is MPEG ES, PAL 720x576 pixels @ 24/25 frames per second. The sound stream is uncompressed PCM, 16-bit stereo @ 48kHz. (Might it help if I somehow re-encoded the sound stream at a lower resolution, or as an MP3? If so, how would I do this without changing the video stream?) P.S.: I am limited to using Windows Media Player (11). (I previously tried MPlayer btw., but the video playback quality was surprisingly bad.)

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• Workflow Foundation (WF) -- Why does Visual Studio's designer not use my custom ActivityDesignerThem

  - by stakx

  Problem: I am trying to customize a custom Workflow Foundation activity (called CustomActivity) so that it will display with a specific background color. What I've got so far: First, I'm defining a custom ActivityDesignerTheme as follows: public class CustomActivityTheme : ActivityDesignerTheme { public CustomActivityTheme(WorkflowTheme theme) : base(theme) { this.BackColorStart = Color.FromArgb(0xff, 0xf4, 0xf4, 0xf4); this.BackColorEnd = Color.FromArgb(0xff, 0xc0, 0xc0, 0xc0); this.BackgroundStyle = LinearGradientMode.Horizontal; } } Then, I am applying this theme to a custom ActivityDesigner (apparently the theme must be applied to a designer, and not to an activity): [ActivityDesignerTheme(typeof(CustomActivityTheme))] public class CustomActivityDesigner : SequentialActivityDesigner { ... } Ultimately, I am applying the custom designer to my custom Activity: [Designer(typeof(CustomActivityDesigner))] public partial class CustomActivity : SequenceActivity { ... } Now, according to some code examples that I've seen, this should do the trick. However, when I include an instance of my CustomActivity in a workflow, my custom theme is not applied and it is displayed in the Visual Studio Designer as any standard activity would (white background etc.). I tried re-compiling and even re-starting Visual Studio a couple of times, just to make sure the used assembly is up-to-date, but to no avail. My question: What am I missing? Why does Visual Studio's Workflow Designer not respect the CustomActivityTheme when it displays a CustomActivity?

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• Workflow Foundation (WF) -- Why does setting a DependencyProperty to a COM object using SetValue() t

  - by stakx

  Assume that I have a .NET Workflow Foundation (WF) SequenceActivity class with the following property: public IWorkspace Workspace { get; set; } // ^^^^^^^^^^ // important: this is a COM interface type! public static DependencyProperty WorkspaceProperty = DependencyProperty.Register( "Workspace", typeof(IWorkspace), typeof(FoobarActivity)); // <-- this activity class This activity executes some code that sets both of the above like this: this.Workspace = ...; // exact code not relevant; property set to a COM object SetValue(WorkspaceProperty, this.Workspace); The last line (which makes the call to SetValue) results in an ArgumentException for the second parameter (having the value of this.Workspace): Type […].IWorkspace of dependency property Workspace does not match the value's type System.__ComObject.                                           (translated from German, the English exception text might differ slightly) As soon as I register the dependency property with typeof(object) instead of typeof(IWorkspace) as the second parameter, the code executes just fine. However, that would result in the possibility to assign just about any value to the dependency property, and I do not want that. It seems to me that WF dependency properties don't work for COM interop objects.Does anyone have a solution to this?

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• Lifetime issue of IDisposable unmanaged resources in a complex object graph?

  - by stakx

  This question is about dealing with unmanaged resources (COM interop) and making sure there won't be any resource leaks. I'd appreciate feedback on whether I seem to do things the right way. Background: Let's say I've got two classes: A class LimitedComResource which is a wrapper around a COM object (received via some API). There can only be a limited number of those COM objects, therefore my class implements the IDisposable interface which will be responsible for releasing a COM object when it's no longer needed. Objects of another type ManagedObject are temporarily created to perform some work on a LimitedComResource. They are not IDisposable. To summarize the above in a diagram, my classes might look like this: +---------------+ +--------------------+ | ManagedObject | <>------> | LimitedComResource | +---------------+ +--------------------+ | o IDisposable (I'll provide example code for these two classes in just a moment.) Question: Since my temporary ManagedObject objects are not disposable, I obviously have no control over how long they'll be around. However, in the meantime I might have Disposed the LimitedComObject that a ManagedObject is referring to. How can I make sure that a ManagedObject won't access a LimitedComResource that's no longer there? +---------------+ +--------------------+ | managedObject | <>------> | (dead object) | +---------------+ +--------------------+ I've currently implemented this with a mix of weak references and a flag in LimitedResource which signals whether an object has already been disposed. Is there any better way? Example code (what I've currently got): LimitedComResource: class LimitedComResource : IDisposable { private readonly IUnknown comObject; // <-- set in constructor ... void Dispose(bool notFromFinalizer) { if (!this.isDisposed) { Marshal.FinalReleaseComObject(comObject); } this.isDisposed = true; } internal bool isDisposed = false; } ManagedObject: class ManagedObject { private readonly WeakReference limitedComResource; // <-- set in constructor ... public void DoSomeWork() { if (!limitedComResource.IsAlive()) { throw new ObjectDisposedException(); // ^^^^^^^^^^^^^^^^^^^^^^^ // is there a more suitable exception class? } var ur = (LimitedComResource)limitedComResource.Target; if (ur.isDisposed) { throw new ObjectDisposedException(); } ... // <-- do something sensible here! } }

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• Do fluent interfaces significantly impact runtime performance of a .NET application?

  - by stakx

  I'm currently occupying myself with implementing a fluent interface for an existing technology, which would allow code similar to the following snippet: using (var directory = Open.Directory(@"path\to\some\directory")) { using (var file = Open.File("foobar.html").In(directory)) { // ... } } In order to implement such constructs, classes are needed that accumulate arguments and pass them on to other objects. For example, to implement the Open.File(...).In(...) construct, you would need two classes: // handles 'Open.XXX': public static class OpenPhrase { // handles 'Open.File(XXX)': public static OpenFilePhrase File(string filename) { return new OpenFilePhrase(filename); } // handles 'Open.Directory(XXX)': public static DirectoryObject Directory(string path) { // ... } } // handles 'Open.File(XXX).XXX': public class OpenFilePhrase { internal OpenFilePhrase(string filename) { _filename = filename } // handles 'Open.File(XXX).In(XXX): public FileObject In(DirectoryObject directory) { // ... } private readonly string _filename; } That is, the more constituent parts statements such as the initial examples have, the more objects need to be created for passing on arguments to subsequent objects in the chain until the actual statement can finally execute. Question: I am interested in some opinions: Does a fluent interface which is implemented using the above technique significantly impact the runtime performance of an application that uses it? With runtime performance, I refer to both speed and memory usage aspects. Bear in mind that a potentially large number of temporary, argument-saving objects would have to be created for only very brief timespans, which I assume may put a certain pressure on the garbage collector. If you think there is significant performance impact, do you know of a better way to implement fluent interfaces?

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• Custom activity designers in Workflow Foundation 3.5: How do they work?

  - by stakx

  Intent of this post: I realise that Workflow Foundation is not extremely popular on StackOverflow and that there will probably be not many answers, or none at all. This post is intended as a resource to people trying to customise workflow activities' appearance through custom designer classes. Goals: I am attempting to create a custom designer class for Workflow activities to achieve the following: Make activities look less technical. For example, I don't necessarily want to see the internal object name as the activity's "title" -- instead, I'd like to see something more descriptive. Display the values of certain properties beneath the title text. I would like to see some properties' values directly underneath the title so that I don't need to look somewhere else (namely, at the Properties window). Provide custom drop areas and draw custom internal arrows. As an example, I would like to be able to have custom drop areas in very specific places. What I found out so far: I created a custom designer class deriving from SequentialActivityDesigner as follows: [Designer(typeof(SomeDesigner))] public partial class SomeActivity: CompositeActivity { ... } class PlainDesigner : SequentialActivityDesigner { ... } Through overriding some properties and the OnPaint method, I found out about the following correspondences between the properties and how the activity will be displayed: Figure 1. Relationship between some properties of an SequentialActivityDesigner and the displayed activity. Possible solutions for goal #1 (make activities look less technical) and goal #2 (display values of properties beneath title text): The displayed title can be changed through the Title property. If more room is required to display additional information beneath the title, the TitleHeight property can be increased (ie., override the property and make it return base.TitleHeight + n, where n is some positive integer). Override the OnPaint method and draw additional text in the area reserved through TitleHeight. Open questions: What are the connectors, connections, and connection points used for? They seem to be necessary, but for what purpose? While the drop targets can be got through the GetDropTargets method, it seems that this is not necessarily where the designer will actually place dropped activities. When an activity is dragged across a workflow, the designer displays little green plus signs where activities can be dropped; how does it figure out the locations of these plus signs? How does the designer figure out where to draw connector lines and arrows?

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• Who architected / designed C++'s IOStreams, and would it still be considered well-designed by today'

  - by stakx

  First off, it may seem that I'm asking for subjective opinions, but that's not what I'm after. I'd love to hear some well-grounded arguments on this topic. In the hope of getting some insight into how a modern streams / serialization framework ought to be designed, I recently got myself a copy of the book Standard C++ IOStreams and Locales by Angelika Langer and Klaus Kreft. I figured that if IOStreams wasn't well-designed, it wouldn't have made it into the C++ standard library in the first place. After having read various parts of this book, I am starting to have doubts if IOStreams can compare to e.g. the STL from an overall architectural point-of-view. Read e.g. this interview with Alexander Stepanov (the STL's "inventor") to learn about some design decisions that went into the STL. What surprises me in particular: It seems to be unknown who was responsible for IOStreams' overall design (I'd love to read some background information about this — does anyone know good resources?); Once you delve beneath the immediate surface of IOStreams, e.g. if you want to extend IOStreams with your own classes, you get to an interface with fairly cryptic and confusing member function names, e.g. getloc/imbue, uflow/underflow, snextc/sbumpc/sgetc/sgetn, pbase/pptr/epptr (and there's probably even worse examples). This makes it so much harder to understand the overall design and how the single parts co-operate. Even the book I mentioned above doesn't help that much (IMHO). Thus my question: If you had to judge by today's software engineering standards (if there actually is any general agreement on these), would C++'s IOStreams still be considered well-designed? (I wouldn't want to improve my software design skills from something that's generally considered outdated.)

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• Visual Studio IntelliSense - IHideObjectMembers trick doesn't work. What am I missing?

  - by stakx

  The IHideObjectMembers trick can be used e.g. in fluent interface implementations to hide System.Object members from IntelliSense. (If you don't know this trick, you can read up on it via the above link; I'm just repeating the interface's usual declaration here:) using System; using System.ComponentModel; [EditorBrowsable(EditorBrowsableState.Never)] public interface IHideObjectMembers { [EditorBrowsable(EditorBrowsableState.Never)] Type GetType(); [EditorBrowsable(EditorBrowsableState.Never)] int GetHashCode(); [EditorBrowsable(EditorBrowsableState.Never)] string ToString(); [EditorBrowsable(EditorBrowsableState.Never)] bool Equals(object obj); } I'm now supposed to be able to hide System.Object members on another type as follows: public class SomeClass : IHideObjectMembers { ... } or: public class ISomeInterface : IHideObjectMembers { ... } I tried this in both VS 2008 Express and VS 2008 Standard. However, no members are hidden from IntelliSense at all. I have used the EditorBrowsableAttribute in different projects and it always worked well; however, it doesn't work in this particular scenario. If things had worked as expected, I would only have seen the SomeMethodTwo method. Am I missing something?

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• Concise C# code for gathering several properties with a non-null value into a collection?

  - by stakx

  A fairly basic problem for a change. Given a class such as this: public class X { public T A; public T B; public T C; ... // (other fields, properties, and methods are not of interest here) } I am looking for a concise way to code a method that will return all A, B, C, ... that are not null in an enumerable collection. (Assume that declaring these fields as an array is not an option.) public IEnumerable<T> GetAllNonNullAs(this X x) { // ? } The obvious implementation of this method would be: public IEnumerable<T> GetAllNonNullAs(this X x) { var resultSet = new List<T>(); if (x.A != null) resultSet.Add(x.A); if (x.B != null) resultSet.Add(x.B); if (x.C != null) resultSet.Add(x.C); ... return resultSet; } What's bothering me here in particular is that the code looks verbose and repetitive, and that I don't know the initial List capacity in advance. It's my hope that there is a more clever way, probably something involving the ?? operator? Any ideas?

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• Is the F# language reference documentation available in an offline format (PDF, CHM)?

  - by stakx

  I've found several posts on hubFS of people asking if there is, or will be, offline documentation for F#. These posts haven't been answered. So I want to give it a shot and ask the same question here on SO. Where I've looked for offline documentation so far: The April 2010 CTP release of Visual F# (version 2.0) is available for VS 2008, but it doesn't come without an offline help. There's a question on SO about offline documentation for various programming languages, but F# isn't mentioned there at the time of this writing. There is of course Microsoft's F# language reference documentation (available on MSDN), which could be downloaded for offline browsing using e.g. wget. Question: Does anyone know whether any "official" offline documentation is on the way, anytime soon? (And related to this, albeit this probably can't be answered objectively: Would it be reasonable to expect that F# likely won't undergo ECMA or ISO standardization, ie. there likely won't be a standards document describing the language?)

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• Can a conforming C# compiler optimize away a local (but unused) variable if it is the only strong re

  - by stakx

  The title says it all, but let me explain: void Case_1() { var weakRef = new WeakReference(new object()); GC.Collect(); // <-- doesn't have to be an explicit call; just assume that // garbage collection would occur at this point. if (weakRef.IsAlive) ... } In this code example, I obviously have to plan for the possibility that the new'ed object is reclaimed by the garbage collector; therefore the if statement. (Note that I'm using weakRef for the sole purpose of checking if the new'ed object is still around.) void Case_2() { var unusedLocalVar = new object(); var weakRef = new WeakReference(unusedLocalVar); GC.Collect(); // <-- doesn't have to be an explicit call; just assume that // garbage collection would occur at this point. Debug.Assert(weakReferenceToUseless.IsAlive); } The main change in this code example from the previous one is that the new'ed object is strongly referenced by a local variable (unusedLocalVar). However, this variable is never used again after the weak reference (weakRef) has been created. Question: Is a conforming C# compiler allowed to optimize the first two lines of Case_2 into those of Case_1 if it sees that unusedLocalVar is only used in one place, namely as an argument to the WeakReference constructor? i.e. is there any possibility that the assertion in Case_2 could ever fail?

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• How does the CLR (.NET) internally allocate and pass around custom value types (structs)?

  - by stakx

  Question: Do all CLR value types, including user-defined structs, live on the evaluation stack exclusively, meaning that they will never need to be reclaimed by the garbage-collector, or are there cases where they are garbage-collected? Background: I have previously asked a question on SO about the impact that a fluent interface has on the runtime performance of a .NET application. I was particuarly worried that creating a large number of very short-lived temporary objects would negatively affect runtime performance through more frequent garbage-collection. Now it has occured to me that if I declared those temporary objects' types as struct (ie. as user-defined value types) instead of class, the garbage collector might not be involved at all if it turns out that all value types live exclusively on the evaluation stack. What I've found out so far: I did a brief experiment to see what the differences are in the CIL generated for user-defined value types and reference types. This is my C# code: struct SomeValueType { public int X; } class SomeReferenceType { public int X; } . . static void TryValueType(SomeValueType vt) { ... } static void TryReferenceType(SomeReferenceType rt) { ... } . . var vt = new SomeValueType { X = 1 }; var rt = new SomeReferenceType { X = 2 }; TryValueType(vt); TryReferenceType(rt); And this is the CIL generated for the last four lines of code: .locals init ( [0] valuetype SomeValueType vt, [1] class SomeReferenceType rt, [2] valuetype SomeValueType <>g__initLocal0, // [3] class SomeReferenceType <>g__initLocal1, // why are these generated? [4] valuetype SomeValueType CS$0$0000 // ) L_0000: ldloca.s CS$0$0000 L_0002: initobj SomeValueType // no newobj required, instance already allocated L_0008: ldloc.s CS$0$0000 L_000a: stloc.2 L_000b: ldloca.s <>g__initLocal0 L_000d: ldc.i4.1 L_000e: stfld int32 SomeValueType::X L_0013: ldloc.2 L_0014: stloc.0 L_0015: newobj instance void SomeReferenceType::.ctor() L_001a: stloc.3 L_001b: ldloc.3 L_001c: ldc.i4.2 L_001d: stfld int32 SomeReferenceType::X L_0022: ldloc.3 L_0023: stloc.1 L_0024: ldloc.0 L_0025: call void Program::TryValueType(valuetype SomeValueType) L_002a: ldloc.1 L_002b: call void Program::TryReferenceType(class SomeReferenceType) What I cannot figure out from this code is this: Where are all those local variables mentioned in the .locals block allocated? How are they allocated? How are they freed? Why are so many anonymous local variables needed and copied to-and-fro only to initialize my two local variables rt and vt?

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