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RuntimeHelpers.GetObjectValue DiscussionI was hoping to start a dialog about this shared method which is used by the VB.NET compiler. From what I understand, it's very similar to boxing, except that it will always create a new box on the heap for an existing box. I can't really understand the need for this. My thought is that it's to avoid the situation where 2 objects point to the same value type (boxed) and then accidentally modifying fields in one reflects in the other. However, I don't think that this accidental behavior is possible to achieve. Therefore, I'm not really sure what the advantage of this is. See: http://msdn.microsoft.com/chats/transcripts/vstudio/vstudio_072903.aspx "Host: Cameron (Microsoft) Q: Why does the compiler emit a call to System.Runtime.CompilerServices.RuntimeHelpers.GetObjectValue whenever you pass an Object reference to an Object parameter? A: The reason is because we do not want valuetype aliasing to occur. For example, assigning between Objects would copy only the _reference_ to a boxed valuetype. Modifying what the user thinks is a unique instance would in fact modify both copies, which we thought would be surprising and bad. Using reflection and CallByName isn't impossible because you can use System.ValueType. Assigning or passing System.ValueType's will NOT generate calls to GetObjectValue. In other words, you can force aliasing by using System.ValueType instead of Object to manipulate your valuetypes." So "valuetype aliasing" seems to be when another valuetype points to the same box if I understand correctly. Is this what others have gathered? Also, if anyone is able to use system.valuetype to achieve some sort of differing behavior, I'd like to hear about it as I had difficulty. Thanks... -Ben
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"Ben R." <benr@newsgroup.nospam> schrieb: Imagine you have to set a structure's field's value using reflection. If > I was hoping to start a dialog about this shared method which is used by > the > VB.NET compiler. From what I understand, it's very similar to boxing, > except > that it will always create a new box on the heap for an existing box. I > can't > really understand the need for this. My thought is that it's to avoid the > situation where 2 objects point to the same value type (boxed) and then > accidentally modifying fields in one reflects in the other. However, I > don't > think that this accidental behavior is possible to achieve. Therefore, I'm > not really sure what the advantage of this is. > > See: > > http://msdn.microsoft.com/chats/transcripts/vstudio/vstudio_072903.aspx > > "Host: Cameron (Microsoft) > Q: Why does the compiler emit a call to > System.Runtime.CompilerServices.RuntimeHelpers.GetObjectValue whenever you > pass an Object reference to an Object parameter? > > A: The reason is because we do not want valuetype aliasing to occur. For > example, assigning between Objects would copy only the _reference_ to a > boxed > valuetype. Modifying what the user thinks is a unique instance would in > fact > modify both copies, which we thought would be surprising and bad. Using > reflection and CallByName isn't impossible because you can use > System.ValueType. Assigning or passing System.ValueType's will NOT > generate > calls to GetObjectValue. In other words, you can force aliasing by using > System.ValueType instead of Object to manipulate your valuetypes." > > So "valuetype aliasing" seems to be when another valuetype points to the > same box if I understand correctly. Is this what others have gathered? > Also, > if anyone is able to use system.valuetype to achieve some sort of > differing > behavior, I'd like to hear about it as I had difficulty. you are passing the structure directly to the 'SetValue' method, only a copy would be passed. The same occurs when boxing the value into an 'Object' prior to passing it to the method. Instead you can use the trick shown below: \\\ Dim o as <structure type> Dim x As ValueType = o GetType(...).GetField(...).SetValue(x, <value>) o = CType(x, <structure type>) /// -- M S Herfried K. Wagner M V P <URL:http://dotnet.mvps.org/> V B <URL:http://dotnet.mvps.org/dotnet/faqs/> Ben,
What is wrong with boxing, there seems to be a C hype that it is wrong, nobody could tell me what is wrong with it. (beside some two or three bytes plus some time which is really not measurable and which is not even a fraction from the time used by in my eyes more obscure methods as reflection). Cor Show quoteHide quote "Ben R." <benr@newsgroup.nospam> schreef in bericht news:9EA5C12C-865A-4CC1-AE79-BC19BF8A17F4@microsoft.com... > Hi all, > > I was hoping to start a dialog about this shared method which is used by > the > VB.NET compiler. From what I understand, it's very similar to boxing, > except > that it will always create a new box on the heap for an existing box. I > can't > really understand the need for this. My thought is that it's to avoid the > situation where 2 objects point to the same value type (boxed) and then > accidentally modifying fields in one reflects in the other. However, I > don't > think that this accidental behavior is possible to achieve. Therefore, I'm > not really sure what the advantage of this is. > > See: > > http://msdn.microsoft.com/chats/transcripts/vstudio/vstudio_072903.aspx > > "Host: Cameron (Microsoft) > Q: Why does the compiler emit a call to > System.Runtime.CompilerServices.RuntimeHelpers.GetObjectValue whenever you > pass an Object reference to an Object parameter? > > A: The reason is because we do not want valuetype aliasing to occur. For > example, assigning between Objects would copy only the _reference_ to a > boxed > valuetype. Modifying what the user thinks is a unique instance would in > fact > modify both copies, which we thought would be surprising and bad. Using > reflection and CallByName isn't impossible because you can use > System.ValueType. Assigning or passing System.ValueType's will NOT > generate > calls to GetObjectValue. In other words, you can force aliasing by using > System.ValueType instead of Object to manipulate your valuetypes." > > So "valuetype aliasing" seems to be when another valuetype points to the > same box if I understand correctly. Is this what others have gathered? > Also, > if anyone is able to use system.valuetype to achieve some sort of > differing > behavior, I'd like to hear about it as I had difficulty. > > Thanks... > > -Ben Hi Cor,
The only things I've heard are the things you mentioned (mainly the overhead of extra allocation / dereferencing). I think the performance hit comes when boxing / unboxing happens repeatedly in a loop. -Ben Show quoteHide quote "Cor Ligthert [MVP]" wrote: > Ben, > > What is wrong with boxing, there seems to be a C hype that it is wrong, > nobody could tell me what is wrong with it. (beside some two or three bytes > plus some time which is really not measurable and which is not even a > fraction from the time used by in my eyes more obscure methods as > reflection). > > Cor > > "Ben R." <benr@newsgroup.nospam> schreef in bericht > news:9EA5C12C-865A-4CC1-AE79-BC19BF8A17F4@microsoft.com... > > Hi all, > > > > I was hoping to start a dialog about this shared method which is used by > > the > > VB.NET compiler. From what I understand, it's very similar to boxing, > > except > > that it will always create a new box on the heap for an existing box. I > > can't > > really understand the need for this. My thought is that it's to avoid the > > situation where 2 objects point to the same value type (boxed) and then > > accidentally modifying fields in one reflects in the other. However, I > > don't > > think that this accidental behavior is possible to achieve. Therefore, I'm > > not really sure what the advantage of this is. > > > > See: > > > > http://msdn.microsoft.com/chats/transcripts/vstudio/vstudio_072903.aspx > > > > "Host: Cameron (Microsoft) > > Q: Why does the compiler emit a call to > > System.Runtime.CompilerServices.RuntimeHelpers.GetObjectValue whenever you > > pass an Object reference to an Object parameter? > > > > A: The reason is because we do not want valuetype aliasing to occur. For > > example, assigning between Objects would copy only the _reference_ to a > > boxed > > valuetype. Modifying what the user thinks is a unique instance would in > > fact > > modify both copies, which we thought would be surprising and bad. Using > > reflection and CallByName isn't impossible because you can use > > System.ValueType. Assigning or passing System.ValueType's will NOT > > generate > > calls to GetObjectValue. In other words, you can force aliasing by using > > System.ValueType instead of Object to manipulate your valuetypes." > > > > So "valuetype aliasing" seems to be when another valuetype points to the > > same box if I understand correctly. Is this what others have gathered? > > Also, > > if anyone is able to use system.valuetype to achieve some sort of > > differing > > behavior, I'd like to hear about it as I had difficulty. > > > > Thanks... > > > > -Ben > > > Ben,
There were some C# guys who were telling all the time that boxing was bad. Than I made a test and asked them to do that. They are not telling it anyore, you need billions of loops with unboxing to make it visible that it takes time. Cor Show quoteHide quote "Ben R." <benr@newsgroup.nospam> schreef in bericht news:6AB37B8F-9FDD-479D-8F4D-DABF6DF81E0D@microsoft.com... > Hi Cor, > > The only things I've heard are the things you mentioned (mainly the > overhead > of extra allocation / dereferencing). I think the performance hit comes > when > boxing / unboxing happens repeatedly in a loop. > > -Ben > > "Cor Ligthert [MVP]" wrote: > >> Ben, >> >> What is wrong with boxing, there seems to be a C hype that it is wrong, >> nobody could tell me what is wrong with it. (beside some two or three >> bytes >> plus some time which is really not measurable and which is not even a >> fraction from the time used by in my eyes more obscure methods as >> reflection). >> >> Cor >> >> "Ben R." <benr@newsgroup.nospam> schreef in bericht >> news:9EA5C12C-865A-4CC1-AE79-BC19BF8A17F4@microsoft.com... >> > Hi all, >> > >> > I was hoping to start a dialog about this shared method which is used >> > by >> > the >> > VB.NET compiler. From what I understand, it's very similar to boxing, >> > except >> > that it will always create a new box on the heap for an existing box. I >> > can't >> > really understand the need for this. My thought is that it's to avoid >> > the >> > situation where 2 objects point to the same value type (boxed) and then >> > accidentally modifying fields in one reflects in the other. However, I >> > don't >> > think that this accidental behavior is possible to achieve. Therefore, >> > I'm >> > not really sure what the advantage of this is. >> > >> > See: >> > >> > http://msdn.microsoft.com/chats/transcripts/vstudio/vstudio_072903.aspx >> > >> > "Host: Cameron (Microsoft) >> > Q: Why does the compiler emit a call to >> > System.Runtime.CompilerServices.RuntimeHelpers.GetObjectValue whenever >> > you >> > pass an Object reference to an Object parameter? >> > >> > A: The reason is because we do not want valuetype aliasing to occur. >> > For >> > example, assigning between Objects would copy only the _reference_ to a >> > boxed >> > valuetype. Modifying what the user thinks is a unique instance would in >> > fact >> > modify both copies, which we thought would be surprising and bad. Using >> > reflection and CallByName isn't impossible because you can use >> > System.ValueType. Assigning or passing System.ValueType's will NOT >> > generate >> > calls to GetObjectValue. In other words, you can force aliasing by >> > using >> > System.ValueType instead of Object to manipulate your valuetypes." >> > >> > So "valuetype aliasing" seems to be when another valuetype points to >> > the >> > same box if I understand correctly. Is this what others have gathered? >> > Also, >> > if anyone is able to use system.valuetype to achieve some sort of >> > differing >> > behavior, I'd like to hear about it as I had difficulty. >> > >> > Thanks... >> > >> > -Ben >> >> >> Hi Ben,
You can use following code to test: Public Structure MyStr Public a As String End Structure Module Module1 Sub Main() Dim s As New MyStr Dim boxed As Object = s Dim t As Type = boxed.GetType() t.GetField("a").SetValue(boxed, "aa") s = CType(boxed, MyStr) Console.WriteLine(s.a) '<==== in C# you get "aa" in the output, nothing in VB Dim boxedVT As ValueType = s t = boxedVT.GetType() t.GetField("a").SetValue(boxedVT, "aa") s = CType(boxedVT, MyStr) Console.WriteLine(s.a) ' <==== this will print "aa", since using ValueType explictly bypass the GetObjectValue() End Sub End Module Sincerely, Walter Wang (waw***@online.microsoft.com, remove 'online.') Microsoft Online Community Support ================================================== Get notification to my posts through email? Please refer to http://msdn.microsoft.com/subscriptions/managednewsgroups/default.aspx#notif ications. If you are using Outlook Express, please make sure you clear the check box "Tools/Options/Read: Get 300 headers at a time" to see your reply promptly. Note: The MSDN Managed Newsgroup support offering is for non-urgent issues where an initial response from the community or a Microsoft Support Engineer within 1 business day is acceptable. Please note that each follow up response may take approximately 2 business days as the support professional working with you may need further investigation to reach the most efficient resolution. The offering is not appropriate for situations that require urgent, real-time or phone-based interactions or complex project analysis and dump analysis issues. Issues of this nature are best handled working with a dedicated Microsoft Support Engineer by contacting Microsoft Customer Support Services (CSS) at http://msdn.microsoft.com/subscriptions/support/default.aspx. ================================================== This posting is provided "AS IS" with no warranties, and confers no rights. Thanks Walter, this was a great example to show the point.
-Ben Show quoteHide quote "Walter Wang [MSFT]" wrote: > Hi Ben, > > You can use following code to test: > > Public Structure MyStr > Public a As String > End Structure > > Module Module1 > Sub Main() > Dim s As New MyStr > Dim boxed As Object = s > Dim t As Type = boxed.GetType() > t.GetField("a").SetValue(boxed, "aa") > s = CType(boxed, MyStr) > Console.WriteLine(s.a) '<==== in C# you get "aa" in the output, > nothing in VB > > Dim boxedVT As ValueType = s > t = boxedVT.GetType() > t.GetField("a").SetValue(boxedVT, "aa") > s = CType(boxedVT, MyStr) > Console.WriteLine(s.a) ' <==== this will print "aa", since > using ValueType explictly bypass the GetObjectValue() > End Sub > End Module > > Sincerely, > Walter Wang (waw***@online.microsoft.com, remove 'online.') > Microsoft Online Community Support > > ================================================== > Get notification to my posts through email? Please refer to > http://msdn.microsoft.com/subscriptions/managednewsgroups/default.aspx#notif > ications. If you are using Outlook Express, please make sure you clear the > check box "Tools/Options/Read: Get 300 headers at a time" to see your reply > promptly. > > Note: The MSDN Managed Newsgroup support offering is for non-urgent issues > where an initial response from the community or a Microsoft Support > Engineer within 1 business day is acceptable. Please note that each follow > up response may take approximately 2 business days as the support > professional working with you may need further investigation to reach the > most efficient resolution. The offering is not appropriate for situations > that require urgent, real-time or phone-based interactions or complex > project analysis and dump analysis issues. Issues of this nature are best > handled working with a dedicated Microsoft Support Engineer by contacting > Microsoft Customer Support Services (CSS) at > http://msdn.microsoft.com/subscriptions/support/default.aspx. > ================================================== > > This posting is provided "AS IS" with no warranties, and confers no rights. > > 
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