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

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...

Imports System.Drawing

Module Module1
    Sub Main()
        Console.WriteLine("Starting test")
        Dim s As Size
        Console.WriteLine("Created size: {0}", s)
        s.Width = 5
        Console.WriteLine("Set Width: {0}", s)
        Dim pi = s.GetType().GetProperty("Height")
        Console.WriteLine("get property info: {0}", pi)
        pi.SetValue(s, 10, Nothing)
        Console.WriteLine("setting height with pi: {0}", s)
        Dim box As Object = s 'same result with  RuntimeHelpers.GetObjectValue(s)
        Console.WriteLine("boxing: {0}", box)
        pi.SetValue(box, 10, Nothing)
        Console.WriteLine("setting value: {0}", box)
        Console.WriteLine("")
        Console.WriteLine("End test")
        Console.ReadLine()
    End Sub
End Module

The result given is:

Starting test
Created size: {Width=0, Height=0}
Set Width: {Width=5, Height=0}
get property info: Int32 Height
setting height with pi: {Width=5, Height=0} 'expected 5,10
boxing: {Width=5, Height=0}
setting value: {Width=5, Height=0} 'expected 5,10

End test

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

    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Starting test");
            Size s= new Size();
            Console.WriteLine("Created size: {0}", s);
            s.Width = 5;
            Console.WriteLine("Set Width: {0}", s);
            PropertyInfo pi = s.GetType().GetProperty("Height");
            Console.WriteLine("get property info: {0}", pi);
            Console.WriteLine("can write?: {0}", pi.CanWrite);
            pi.SetValue(s, 10, null);
            Console.WriteLine("setting height with pi: {0}", s);
            Object box = s; //same result with  RuntimeHelpers.GetObjectValue(s)
            Console.WriteLine("boxing: {0}", box);
            pi.SetValue(box, 10, null);
            Console.WriteLine("setting value: {0}", box);
            Console.WriteLine("");
            Console.WriteLine("End test");
            Console.ReadLine();
        }
    }

Starting test
Created size: {Width=0, Height=0}
Set Width: {Width=5, Height=0}
get property info: Int32 Height
can write?: True
setting height with pi: {Width=5, Height=0}
boxing: {Width=5, Height=0}
setting value: {Width=5, Height=10}

End test

How do I make this work in VB?

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)

Dim heightProperty = GetType( Size ).GetProperty( "Height", BindingFlags.Public Or BindingFlags.Instance )

Dim localStruct As Size
localStruct.Width = 5
localStruct.Dump( "Should have Width 5" )

' 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.
heightProperty.SetValue( localStruct, 10 )
localStruct.Dump( ( "Should have Height 10, but has " + localStruct.ToString() ) )

Dim boxedAsObject As System.Object = localStruct
LINQPad.Extensions.Dump( boxedAsObject, "boxed" ) ' interestingly, you can't use extension-method syntax on Object-typed boxed structs, weird.

heightProperty.SetValue( boxedAsObject, 10) ' This won't work because `vbc` will insert a hidden `PropertyInfo.SetValue(Object, Object)` call which will reset `boxedAsObject`.
LINQPad.Extensions.Dump( boxedAsObject, ( "Should have Height 10, but has " + boxedAsObject.ToString() )  )

' https://www.pcreview.co.uk/threads/cannot-get-propertyinfo-setvalue-to-work-for-a-structure.1418755/
Dim boxedAsValueType As System.ValueType = localStruct
heightProperty.SetValue( boxedAsValueType, 10 )
boxedAsValueType.Dump( "Will have Height = 10: " + boxedAsValueType.ToString() )

Gives me this output:

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)

IL_0000	ldtoken	Size
IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
IL_000A	ldstr	"Height"
IL_000F	ldc.i4.s	14  // 20
IL_0011	call	Type.GetProperty (String, BindingFlags)
IL_0016	ldloca.s	00    // localStruct
IL_0018	ldc.i4.5	
IL_0019	call	Size.set_Width (Int32)
IL_001E	ldloc.0	   // localStruct
IL_001F	ldstr	"Should have Width 5"
IL_0024	call	Extensions.Dump<Size> (Size, String)
IL_0029	pop	
IL_002A	dup	
IL_002B	ldloc.0	   // localStruct
IL_002C	box	Size
IL_0031	ldc.i4.s	0A  // 10
IL_0033	box	Int32
IL_0038	callvirt	PropertyInfo.SetValue (Object, Object)
IL_003D	ldloc.0	   // localStruct
IL_003E	ldstr	"Should have Height 10, but has "
IL_0043	ldloca.s	00    // localStruct
IL_0045	constrained.	Size
IL_004B	callvirt	Object.ToString ()
IL_0050	call	String.Concat (String, String)
IL_0055	call	Extensions.Dump<Size> (Size, String)
IL_005A	pop	
IL_005B	ldloc.0	   // localStruct
IL_005C	box	Size
IL_0061	stloc.1	   // boxedAsObject
IL_0062	ldloc.1	   // boxedAsObject
IL_0063	call	RuntimeHelpers.GetObjectValue (Object)
IL_0068	ldstr	"boxed"
IL_006D	call	Extensions.Dump<Object> (Object, String)
IL_0072	pop	
IL_0073	dup	
IL_0074	ldloc.1	   // boxedAsObject
IL_0075	call	RuntimeHelpers.GetObjectValue (Object)
IL_007A	ldc.i4.s	0A  // 10
IL_007C	box	Int32
IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
IL_0086	ldloc.1	   // boxedAsObject
IL_0087	call	RuntimeHelpers.GetObjectValue (Object)
IL_008C	ldstr	"Should have Height 10, but has "
IL_0091	ldloc.1	   // boxedAsObject
IL_0092	callvirt	Object.ToString ()
IL_0097	call	String.Concat (String, String)
IL_009C	call	Extensions.Dump<Object> (Object, String)
IL_00A1	pop	
IL_00A2	ldloc.0	   // localStruct
IL_00A3	box	Size
IL_00A8	stloc.2	   // boxedAsValueType
IL_00A9	ldloc.2	   // boxedAsValueType
IL_00AA	ldc.i4.s	0A  // 10
IL_00AC	box	Int32
IL_00B1	callvirt	PropertyInfo.SetValue (Object, Object)
IL_00B6	ldloc.2	   // boxedAsValueType
IL_00B7	ldstr	"Will have Height = 10: "
IL_00BC	ldloc.2	   // boxedAsValueType
IL_00BD	callvirt	ValueType.ToString ()
IL_00C2	call	String.Concat (String, String)
IL_00C7	call	Extensions.Dump<ValueType> (ValueType, String)
IL_00CC	pop	
IL_00CD	ret	

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

IL_0000	ldtoken	Size
IL_0005	call	Type.GetTypeFromHandle (RuntimeTypeHandle)
IL_000A	ldstr	"Height"
IL_000F	ldc.i4.s	14  // 20
IL_0011	call	Type.GetProperty (String, BindingFlags)
IL_0016	ldloca.s	00    // localStruct
IL_0018	initobj	Size
IL_001E	ldloca.s	00    // localStruct
IL_0020	ldc.i4.5	
IL_0021	call	Size.set_Width (Int32)
IL_0026	ldloc.0	   // localStruct
IL_0027	ldstr	"Should have Width 5"
IL_002C	call	Extensions.Dump<Size> (Size, String)
IL_0031	pop	
IL_0032	dup	
IL_0033	ldloc.0	   // localStruct
IL_0034	box	Size
IL_0039	ldc.i4.s	0A  // 10
IL_003B	box	Int32
IL_0040	callvirt	PropertyInfo.SetValue (Object, Object)
IL_0045	ldloc.0	   // localStruct
IL_0046	ldstr	"Should have Height 10"
IL_004B	call	Extensions.Dump<Size> (Size, String)
IL_0050	pop	
IL_0051	ldloc.0	   // localStruct
IL_0052	box	Size
IL_0057	stloc.1	   // boxedAsObject
IL_0058	dup	
IL_0059	ldloc.1	   // boxedAsObject
IL_005A	ldc.i4.s	0A  // 10
IL_005C	box	Int32
IL_0061	callvirt	PropertyInfo.SetValue (Object, Object)
IL_0066	ldloc.1	   // boxedAsObject
IL_0067	ldstr	"boxedAsObject"
IL_006C	call	Extensions.Dump<Object> (Object, String)
IL_0071	pop	
IL_0072	ldloc.0	   // localStruct
IL_0073	box	Size
IL_0078	stloc.2	   // boxedAsValueType
IL_0079	ldloc.2	   // boxedAsValueType
IL_007A	ldc.i4.s	0A  // 10
IL_007C	box	Int32
IL_0081	callvirt	PropertyInfo.SetValue (Object, Object)
IL_0086	ldloc.2	   // boxedAsValueType
IL_0087	ldstr	"boxedAsValueType"
IL_008C	call	Extensions.Dump<ValueType> (ValueType, String)
IL_0091	pop	
IL_0092	ret