为什么在这个例子中 PropertyInfo.SetValue 不起作用，如何使其起作用？

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英文:

Why is PropertyInfo.SetValue not working in this example and how to make it work?

问题

以下是翻译好的内容：

以下代码似乎无法正常工作，尽管根据文档，使用装箱的部分应该可以工作。我似乎找不到比Value Type Property setting这篇文章更多的信息。

这个问题与Setting a property of a property in a struct?不同，因为这与我正在尝试使用的反射无关。

这段代码只是一个更大项目的一小部分（使用反射完成某些任务），但可以演示问题。

这个示例的特殊之处在于，我们要设置的属性是值类型（结构体），而SetValue似乎无法很好地处理这个问题...

  1. Imports System.Drawing
  3. Module Module1
  4.     Sub Main()
  5.         Console.WriteLine("Starting test")
  6.         Dim s As Size
  7.         Console.WriteLine("Created size: {0}", s)
  8.         s.Width = 5
  9.         Console.WriteLine("Set Width: {0}", s)
  10.         Dim pi = s.GetType().GetProperty("Height")
  11.         Console.WriteLine("get property info: {0}", pi)
  12.         pi.SetValue(s, 10, Nothing)
  13.         Console.WriteLine("setting height with pi: {0}", s)
  14.         Dim box As Object = s 'same result with  RuntimeHelpers.GetObjectValue(s)
  15.         Console.WriteLine("boxing: {0}", box)
  16.         pi.SetValue(box, 10, Nothing)
  17.         Console.WriteLine("setting value: {0}", box)
  18.         Console.WriteLine("")
  19.         Console.WriteLine("End test")
  20.         Console.ReadLine()
  21.     End Sub
  22. End Module

给出的结果是：

  1. Starting test
  2. Created size: {Width=0, Height=0}
  3. Set Width: {Width=5, Height=0}
  4. get property info: Int32 Height
  5. setting height with pi: {Width=5, Height=0} 'expected 5,10
  6. boxing: {Width=5, Height=0}
  7. setting value: {Width=5, Height=0} 'expected 5,10
  9. End test

相同的代码在C#中可以正常工作（使用装箱）：

  1. internal class Program
  2. {
  3.     static void Main(string[] args)
  4.     {
  5.         Console.WriteLine("Starting test");
  6.         Size s= new Size();
  7.         Console.WriteLine("Created size: {0}", s);
  8.         s.Width = 5;
  9.         Console.WriteLine("Set Width: {0}", s);
  10.         PropertyInfo pi = s.GetType().GetProperty("Height");
  11.         Console.WriteLine("get property info: {0}", pi);
  12.         Console.WriteLine("can write?: {0}", pi.CanWrite);
  13.         pi.SetValue(s, 10, null);
  14.         Console.WriteLine("setting height with pi: {0}", s);
  15.         Object box = s; //same result with  RuntimeHelpers.GetObjectValue(s)
  16.         Console.WriteLine("boxing: {0}", box);
  17.         pi.SetValue(box, 10, null);
  18.         Console.WriteLine("setting value: {0}", box);
  19.         Console.WriteLine("");
  20.         Console.WriteLine("End test");
  21.         Console.ReadLine();
  22.     }
  23. }

给出的结果是：

  1. Starting test
  2. Created size: {Width=0, Height=0}
  3. Set Width: {Width=5, Height=0}
  4. get property info: Int32 Height
  5. can write?: True
  6. setting height with pi: {Width=5, Height=0}
  7. boxing: {Width=5, Height=0}
  8. setting value: {Width=5, Height=10}
  10. End test

如何在VB中使其工作？

英文:

The following code does not seem to work, although according to documentation the part with boxing should work. I seem not be able to find more information on this than the article at Value Type Property setting

This question is also not similar as in Setting a property of a property in a struct? as this has nothing to do with reflection, which I am trying to use.

The code is just a small piece of a much larger project (using reflection to get something done) but demonstrates the issue.

The specialty in this example is that the property we want to set is in a value type (struct) and SetValue seems not to be handling this very well...

  1. Imports System.Drawing
  3. Module Module1
  4.     Sub Main()
  5.         Console.WriteLine("Starting test")
  6.         Dim s As Size
  7.         Console.WriteLine("Created size: {0}", s)
  8.         s.Width = 5
  9.         Console.WriteLine("Set Width: {0}", s)
  10.         Dim pi = s.GetType().GetProperty("Height")
  11.         Console.WriteLine("get property info: {0}", pi)
  12.         pi.SetValue(s, 10, Nothing)
  13.         Console.WriteLine("setting height with pi: {0}", s)
  14.         Dim box As Object = s 'same result with  RuntimeHelpers.GetObjectValue(s)
  15.         Console.WriteLine("boxing: {0}", box)
  16.         pi.SetValue(box, 10, Nothing)
  17.         Console.WriteLine("setting value: {0}", box)
  18.         Console.WriteLine("")
  19.         Console.WriteLine("End test")
  20.         Console.ReadLine()
  21.     End Sub
  22. End Module

The result given is:

  1. Starting test
  2. Created size: {Width=0, Height=0}
  3. Set Width: {Width=5, Height=0}
  4. get property info: Int32 Height
  5. setting height with pi: {Width=5, Height=0} 'expected 5,10
  6. boxing: {Width=5, Height=0}
  7. setting value: {Width=5, Height=0} 'expected 5,10
  9. End test

The same code is working (with boxing) in C#

  1.     internal class Program
  2.     {
  3.         static void Main(string[] args)
  4.         {
  5.             Console.WriteLine("Starting test");
  6.             Size s= new Size();
  7.             Console.WriteLine("Created size: {0}", s);
  8.             s.Width = 5;
  9.             Console.WriteLine("Set Width: {0}", s);
  10.             PropertyInfo pi = s.GetType().GetProperty("Height");
  11.             Console.WriteLine("get property info: {0}", pi);
  12.             Console.WriteLine("can write?: {0}", pi.CanWrite);
  13.             pi.SetValue(s, 10, null);
  14.             Console.WriteLine("setting height with pi: {0}", s);
  15.             Object box = s; //same result with  RuntimeHelpers.GetObjectValue(s)
  16.             Console.WriteLine("boxing: {0}", box);
  17.             pi.SetValue(box, 10, null);
  18.             Console.WriteLine("setting value: {0}", box);
  19.             Console.WriteLine("");
  20.             Console.WriteLine("End test");
  21.             Console.ReadLine();
  22.         }
  23.     }
  1. Starting test
  2. Created size: {Width=0, Height=0}
  3. Set Width: {Width=5, Height=0}
  4. get property info: Int32 Height
  5. can write?: True
  6. setting height with pi: {Width=5, Height=0}
  7. boxing: {Width=5, Height=0}
  8. setting value: {Width=5, Height=10}
  10. End test

How do I make this work in VB?

答案1

得分: 3

在VB.NET中，当使用装箱的可变结构时，需要使用System.ValueType而不是System.Object（在VB.NET中称为Object，在C#中称为object）。

将你的代码更改为以下内容，它可以正常工作（至少对我来说，在针对.NET 7的Linqpad 7中的VB.NET中可以正常工作）：

（你需要将.Dump()调用替换为Console.WriteLine或类似的方法）

  1. Dim heightProperty = GetType(Size).GetProperty("Height", BindingFlags.Public Or BindingFlags.Instance)
  3. Dim localStruct As Size
  4. localStruct.Width = 5
  5. localStruct.Dump("Should have Width 5")
  7. ' 由于`vbc`将插入一个隐藏的`PropertyInfo.SetValue(Object, Object)`调用，该调用将对`localStruct`进行装箱，因此在修改后无法检索它，实际上将其丢弃，所以这个`SetValue`调用不起作用。
  8. heightProperty.SetValue(localStruct, 10)
  9. localStruct.Dump(("Should have Height 10, but has " + localStruct.ToString()))
  11. Dim boxedAsObject As System.Object = localStruct
  12. LINQPad.Extensions.Dump(boxedAsObject, "boxed") ' 有趣的是，你不能在Object类型的装箱结构上使用扩展方法语法，很奇怪。
  14. ' 由于`vbc`将插入一个隐藏的`PropertyInfo.SetValue(Object, Object)`调用，该调用将重置`boxedAsObject`，所以这个调用不起作用。
  15. heightProperty.SetValue(boxedAsObject, 10)
  16. LINQPad.Extensions.Dump(boxedAsObject, ("Should have Height 10, but has " + boxedAsObject.ToString()))
  18. ' https://www.pcreview.co.uk/threads/cannot-get-propertyinfo-setvalue-to-work-for-a-structure.1418755/
  19. Dim boxedAsValueType As System.ValueType = localStruct
  20. heightProperty.SetValue(boxedAsValueType, 10)
  21. boxedAsValueType.Dump("Will have Height = 10: " + boxedAsValueType.ToString())

这将给出以下输出：

为什么在这个例子中 PropertyInfo.SetValue 不起作用，如何使其起作用？

与C#相比，VB.NET在处理装箱结构时有其特殊规则：当你在VB.NET中使用System.Object作为装箱结构的准顶级类型时，IL编译器将插入对RuntimeHelpers.GetObjectValue的隐藏调用，这将导致堆栈上的装箱结构引用被替换为一个新的装箱结构实例。

（以下IL是使用启用了优化的编译）

  1. IL_0000	ldtoken	Size
  2. IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
  3. IL_000A	ldstr	"Height"
  4. IL_000F	ldc.i4.s	14  // 20
  5. IL_0011	call	Type.GetProperty (String, BindingFlags)
  6. IL_0016	ldloca.s	00    // localStruct
  7. IL_0018	ldc.i4.5	
  8. IL_0019	call	Size.set_Width (Int32)
  9. IL_001E	ldloc.0	   // localStruct
  10. IL_001F	ldstr	"Should have Width 5"
  11. IL_0024	call	Extensions.Dump<Size> (Size, String)
  12. IL_0029	pop	
  13. IL_002A	dup	
  14. IL_002B	ldloc.0	   // localStruct
  15. IL_002C	box	Size
  16. IL_0031	ldc.i4.s	0A  // 10
  17. IL_0033	box	Int32
  18. IL_0038	callvirt	PropertyInfo.SetValue (Object, Object)
  19. IL_003D	ldloc.0	   // localStruct
  20. IL_003E	ldstr	"Should have Height 10, but has "
  21. IL_0043	ldloca.s	00    // localStruct
  22. IL_0045	constrained.	Size
  23. IL_004B	callvirt	Object.ToString ()
  24. IL_0050	call	String.Concat (String, String)
  25. IL_0055	call	Extensions.Dump<Size> (Size, String)
  26. IL_005A	pop	
  27. IL_005B	ldloc.0	   // localStruct
  28. IL_005C	box	Size
  29. IL_0061	stloc.1	   // boxedAsObject
  30. IL_0062	ldloc.1	   // boxedAsObject
  31. IL_0063	call	RuntimeHelpers.GetObjectValue (Object)
  32. IL_0068	ldstr	"boxed"
  33. IL_006D	call	Extensions.Dump<Object> (Object, String)
  34. IL_0072	pop	
  35. IL_0073	dup	
  36. IL_0074	ldloc.1	   // boxedAsObject
  37. IL_0075	call	RuntimeHelpers.GetObjectValue (Object)
  38. IL_007A	ldc.i4.s	0A  // 10
  39. IL_007C	box	Int32
  40. IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
  41. IL_0086	ldloc.1	   // boxedAsObject
  42. IL_0087	call	RuntimeHelpers.GetObjectValue (Object)
  43. IL_008C	ldstr	"Should have Height 10, but has "
  44. IL_0091	ldloc.1	   // boxedAsObject
  45. IL_0092	callvirt	Object.ToString ()
  46. IL_0097	call	String.Concat (String, String)
  47. IL_009C	call	Extensions.Dump<Object> (Object, String)
  48. IL_00A1	pop	
  49. IL_00A2	ldloc.0	   // localStruct
  50. IL_00A3	box	Size
  51. IL_00A8	stloc.2	   // boxedAsValueType
  52. IL_00A9	ldloc.2	   // boxedAsValueType
  53. IL_00AA	ldc.i4.s	0A  // 10
  54. IL_00AC	box	Int32
  55. IL_00B1	callvirt	PropertyInfo.SetValue (Object, Object)
  56. IL_00B6	ldloc.2	   // boxedAsValueType
  57. IL_00B7	ldstr	"Will have Height = 10: "
  58. IL_00BC	ldloc.2	   // boxedAsValueType
  59. IL_00BD	callvirt	ValueType.ToString ()
  60. IL_00C2	call	String.Concat (String, String)
  61. IL_00C7	call	Extensions.Dump<ValueType> (ValueType, String)
  62. IL_00CC	pop	
  63. IL_00CD	ret

而等效的C#的IL则没有这些隐藏调用：

  1. IL_0000	ldtoken	Size
  2. IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
  3. IL_000A	ldstr	"Height"
  4. IL_000F	ldc.i4.s	14  // 20
  5. IL_0011	call	Type.GetProperty (String, BindingFlags)
  6. IL_0016	ldloca.s	00    // localStruct
  7. IL_0018	initobj	Size
  8. IL_001E	ldloca.s	00    // localStruct
  9. IL_0020	ldc.i4.5	
  10. IL_0021	call	Size.set_Width (Int32)
  11. IL_0026	ldloc.0	   // localStruct
  12. IL_0027	ldstr	"Should have Width 5"
  13. IL_002C	call	Extensions.Dump<Size> (Size, String)
  14. IL_0031	pop	
  15. IL_0032	dup	
  16. IL_0033	ldloc.0	   // localStruct
  17. IL_0034	box	Size
  18. IL_0039	ldc.i4.s	0A  // 10
  19. IL_003B	box	Int32
  20. IL_0040	callvirt	PropertyInfo.SetValue (Object, Object)
  21. IL_0045	ldloc.0	   // localStruct
  22. IL_0046	ldstr	"Should have Height 10"
  23. IL_004B	call	Extensions.Dump<Size> (Size, String)
  24. IL_0050	pop	
  25. IL_0051	ldloc.0	   // localStruct
  26. IL_0052	box	Size
  27. IL_0057	stloc.1	   // boxedAsObject
  28. IL_0058	dup	
  29. IL_0059	ldloc.1	   // boxedAsObject
  30. IL_005A	ldc.i4.s	0A  // 10
  31. IL_005C	box	Int32
  32. IL_0061	callvirt	PropertyInfo.SetValue (Object, Object)
  33. IL_0066	ldloc.1	   // boxedAsObject
  34. IL_0067	ldstr	"boxedAsObject"
  35. IL_006C	call	Extensions.Dump<Object> (Object, String)
  36. IL_0071	pop	
  37. IL_0072	ldloc.0	   // localStruct
  38. IL_0073	box	Size
  39. IL_0078	stloc.2	   // boxedAsValueType
  40. IL_0079	ldloc.2	   // boxedAsValueType
  41. IL_007A	ldc.i4.s	0A  // 10
  42. IL_007C	box	Int32
  43. IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
  44. IL_0086	ldloc.2	   // boxedAsValueType
  45. IL_0087	ldstr	"boxedAsValueType"
  46. IL_008C	call	Extensions.Dump<ValueType> (ValueType, String)
  47. IL_0091	pop	
  48. IL_0092	ret
英文:

TL;DR: In VB.NET you need to use System.ValueType when working with boxed mutable structs instead of System.Object (aka Object in VB.NET and object in C#).

Change your code to this and it works (for me, at least, in the VB.NET in Linqpad 7 targeting .NET 7):

(You'll want to replace the .Dump() calls with Console.WriteLine or similar)

  1. Dim heightProperty = GetType( Size ).GetProperty( "Height", BindingFlags.Public Or BindingFlags.Instance )
  3. Dim localStruct As Size
  4. localStruct.Width = 5
  5. localStruct.Dump( "Should have Width 5" )
  7. ' This `SetValue` call below won't work because `vbc` will insert a hidden `PropertyInfo.SetValue(Object, Object)` call which will box `localStruct` such that you can't retrieve it after it's mutated, so effectively discarding it.
  8. heightProperty.SetValue( localStruct, 10 )
  9. localStruct.Dump( ( "Should have Height 10, but has " + localStruct.ToString() ) )
  11. Dim boxedAsObject As System.Object = localStruct
  12. LINQPad.Extensions.Dump( boxedAsObject, "boxed" ) ' interestingly, you can't use extension-method syntax on Object-typed boxed structs, weird.
  14. heightProperty.SetValue( boxedAsObject, 10) ' This won't work because `vbc` will insert a hidden `PropertyInfo.SetValue(Object, Object)` call which will reset `boxedAsObject`.
  15. LINQPad.Extensions.Dump( boxedAsObject, ( "Should have Height 10, but has " + boxedAsObject.ToString() )  )
  17. ' https://www.pcreview.co.uk/threads/cannot-get-propertyinfo-setvalue-to-work-for-a-structure.1418755/
  18. Dim boxedAsValueType As System.ValueType = localStruct
  19. heightProperty.SetValue( boxedAsValueType, 10 )
  20. boxedAsValueType.Dump( "Will have Height = 10: " + boxedAsValueType.ToString() )

Gives me this output:

为什么在这个例子中 PropertyInfo.SetValue 不起作用，如何使其起作用？

VB.NET has its own special rules for handling boxed structs compared to C#: when you use System.Object as a _quasi-_top-type for boxed structs in VB.NET then the IL compiler will insert hidden calls to RuntimeHelpers.GetObjectValue which will cause the boxed struct reference on the stack to be replaced with a new boxed struct instance.

(The IL below was compiled with optimizations enabled)

  1. IL_0000	ldtoken	Size
  2. IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
  3. IL_000A	ldstr	"Height"
  4. IL_000F	ldc.i4.s	14  // 20
  5. IL_0011	call	Type.GetProperty (String, BindingFlags)
  6. IL_0016	ldloca.s	00    // localStruct
  7. IL_0018	ldc.i4.5	
  8. IL_0019	call	Size.set_Width (Int32)
  9. IL_001E	ldloc.0	   // localStruct
  10. IL_001F	ldstr	"Should have Width 5"
  11. IL_0024	call	Extensions.Dump<Size> (Size, String)
  12. IL_0029	pop	
  13. IL_002A	dup	
  14. IL_002B	ldloc.0	   // localStruct
  15. IL_002C	box	Size
  16. IL_0031	ldc.i4.s	0A  // 10
  17. IL_0033	box	Int32
  18. IL_0038	callvirt	PropertyInfo.SetValue (Object, Object)
  19. IL_003D	ldloc.0	   // localStruct
  20. IL_003E	ldstr	"Should have Height 10, but has "
  21. IL_0043	ldloca.s	00    // localStruct
  22. IL_0045	constrained.	Size
  23. IL_004B	callvirt	Object.ToString ()
  24. IL_0050	call	String.Concat (String, String)
  25. IL_0055	call	Extensions.Dump<Size> (Size, String)
  26. IL_005A	pop	
  27. IL_005B	ldloc.0	   // localStruct
  28. IL_005C	box	Size
  29. IL_0061	stloc.1	   // boxedAsObject
  30. IL_0062	ldloc.1	   // boxedAsObject
  31. IL_0063	call	RuntimeHelpers.GetObjectValue (Object)
  32. IL_0068	ldstr	"boxed"
  33. IL_006D	call	Extensions.Dump<Object> (Object, String)
  34. IL_0072	pop	
  35. IL_0073	dup	
  36. IL_0074	ldloc.1	   // boxedAsObject
  37. IL_0075	call	RuntimeHelpers.GetObjectValue (Object)
  38. IL_007A	ldc.i4.s	0A  // 10
  39. IL_007C	box	Int32
  40. IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
  41. IL_0086	ldloc.1	   // boxedAsObject
  42. IL_0087	call	RuntimeHelpers.GetObjectValue (Object)
  43. IL_008C	ldstr	"Should have Height 10, but has "
  44. IL_0091	ldloc.1	   // boxedAsObject
  45. IL_0092	callvirt	Object.ToString ()
  46. IL_0097	call	String.Concat (String, String)
  47. IL_009C	call	Extensions.Dump<Object> (Object, String)
  48. IL_00A1	pop	
  49. IL_00A2	ldloc.0	   // localStruct
  50. IL_00A3	box	Size
  51. IL_00A8	stloc.2	   // boxedAsValueType
  52. IL_00A9	ldloc.2	   // boxedAsValueType
  53. IL_00AA	ldc.i4.s	0A  // 10
  54. IL_00AC	box	Int32
  55. IL_00B1	callvirt	PropertyInfo.SetValue (Object, Object)
  56. IL_00B6	ldloc.2	   // boxedAsValueType
  57. IL_00B7	ldstr	"Will have Height = 10: "
  58. IL_00BC	ldloc.2	   // boxedAsValueType
  59. IL_00BD	callvirt	ValueType.ToString ()
  60. IL_00C2	call	String.Concat (String, String)
  61. IL_00C7	call	Extensions.Dump<ValueType> (ValueType, String)
  62. IL_00CC	pop	
  63. IL_00CD	ret

...whereas the IL for the equivalent C# lacks those hidden calls:

  1. IL_0000	ldtoken	Size
  2. IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
  3. IL_000A	ldstr	"Height"
  4. IL_000F	ldc.i4.s	14  // 20
  5. IL_0011	call	Type.GetProperty (String, BindingFlags)
  6. IL_0016	ldloca.s	00    // localStruct
  7. IL_0018	initobj	Size
  8. IL_001E	ldloca.s	00    // localStruct
  9. IL_0020	ldc.i4.5	
  10. IL_0021	call	Size.set_Width (Int32)
  11. IL_0026	ldloc.0	   // localStruct
  12. IL_0027	ldstr	"Should have Width 5"
  13. IL_002C	call	Extensions.Dump<Size> (Size, String)
  14. IL_0031	pop	
  15. IL_0032	dup	
  16. IL_0033	ldloc.0	   // localStruct
  17. IL_0034	box	Size
  18. IL_0039	ldc.i4.s	0A  // 10
  19. IL_003B	box	Int32
  20. IL_0040	callvirt	PropertyInfo.SetValue (Object, Object)
  21. IL_0045	ldloc.0	   // localStruct
  22. IL_0046	ldstr	"Should have Height 10"
  23. IL_004B	call	Extensions.Dump<Size> (Size, String)
  24. IL_0050	pop	
  25. IL_0051	ldloc.0	   // localStruct
  26. IL_0052	box	Size
  27. IL_0057	stloc.1	   // boxedAsObject
  28. IL_0058	dup	
  29. IL_0059	ldloc.1	   // boxedAsObject
  30. IL_005A	ldc.i4.s	0A  // 10
  31. IL_005C	box	Int32
  32. IL_0061	callvirt	PropertyInfo.SetValue (Object, Object)
  33. IL_0066	ldloc.1	   // boxedAsObject
  34. IL_0067	ldstr	"boxedAsObject"
  35. IL_006C	call	Extensions.Dump<Object> (Object, String)
  36. IL_0071	pop	
  37. IL_0072	ldloc.0	   // localStruct
  38. IL_0073	box	Size
  39. IL_0078	stloc.2	   // boxedAsValueType
  40. IL_0079	ldloc.2	   // boxedAsValueType
  41. IL_007A	ldc.i4.s	0A  // 10
  42. IL_007C	box	Int32
  43. IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
  44. IL_0086	ldloc.2	   // boxedAsValueType
  45. IL_0087	ldstr	"boxedAsValueType"
  46. IL_008C	call	Extensions.Dump<ValueType> (ValueType, String)
  47. IL_0091	pop	
  48. IL_0092	ret

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