vb.net - Reflection on structure differs from class - but only in code

Question

Code snippet:

Dim target As Object
' target gets properly set to something of the desired type
Dim field As FieldInfo = target.GetType.GetField("fieldName", _
  BindingFlags.Instance Or BindingFlags.Public Or BindingFlags.NonPublic)
field.SetValue(target,newValue)

This snippet works perfectly IF target is set to an instance of a CLASS.

However, if target is set to an instance of a STRUCTURE, the code does not actually change the value of the field. No error, but the value remains unchanged.

And, oddly, if I'm stepping through code, watch the SetValue fail to do anything, and immediately go to the Immediate window and type exactly the same SetValue operation, that works.

Any suggestions on what's going on and how to actually change the field IN CODE?

Edit:

Per request from Jon Skeet, actual code:

Private Shared Function XmlDeserializeObject(ByVal objectType As Type, _
        ByVal deserializedID As String) As Object
    Dim result As Object
    result = CreateObject(objectType)

    mXmlR.ReadStartElement()
    Do While mXmlR.IsStartElement _
    AndAlso mXmlR.Name <> elementItem
        Dim field As FieldInfo = result.GetType.GetField(FullName, _
            BindingFlags.Instance Or BindingFlags.Public Or BindingFlags.NonPublic)
        field.SetValue(result, XmlDeserialize(field.FieldType))
    Loop

    Return result
End Function

External variables and called routines:
* mXmlR is an XmlTextReader, and is properly initialized and positioned (else this would not work on classes)
* CreateObject works (ditto)
* XmlDeserialize mostly works, and at the point in question is handling an integer just fine. The only known problem is with structures.

As for how I'm checking the value, I'm mostly looking at the Locals window, but I've also used print statements in the Immediate window, and I'm running an NUnit test which is failing because of this problem - while the equivalent test with a class, rather than a structure, passes.

Here's the test.

<Serializable()> Private Structure SimpleStructure
    Public MemberOne As Integer
End Structure

<Test()> Sub A016_SimpleStructure()
    Dim input As New SimpleStructure
    input.MemberOne = 3
    Dim st As String = Serialize(input)
    Debug.Print(st)
    Dim retObject As Object = Deserialize(st)
    Assert.IsNotNull(retObject)
    Assert.IsInstanceOfType(GetType(SimpleStructure), retObject)
    Assert.AreEqual(input.MemberOne, DirectCast(retObject, SimpleStructure).MemberOne)
End Sub

Answer 1

Working with your original sample, I agree that it works in C# but not in VB! If you use Reflector or ILDasm you will see that the call to Field.SetValue(target, ...) is actually compiled (in VB) as:

field.SetValue(RuntimeHelpers.GetObjectValue(target), ...)

GetObjectValue "Returns a boxed copy of obj if it is a value class; otherwise obj itself is returned." I.e. the value is being set on a copy of your struct!

This link gives the explanation (such as it is). The workaround is to declare target as System.ValueType instead of Object. I'm not sure if that actually helps in your real-life code: you may need a messy type test to be able to handle value types separately from reference types.