I am attempting to use reflection.fieldinfo.setvalue on a structure field to modify its value, but to no avail. Why? (Code in C# and VB provided.)

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I'm attempting to use Reflection.FieldInfo.SetValue to modify a structure's integer field value. However, it doesn't get modified.

I realize, that SetValue expects an Object and that just giving it a value is working with a copy, but boxing the integer doesn't help either.

What is my mistake?

What I have tried:

Here's ready to copy and paste code in C# (further below in VB as well):

using System;
using System.Reflection;

public static class Test
{
    public struct SStruct
    {
        public int Value;
    }

    public static void Main()
    {
        // Initialize a structure record.
        SStruct rStruct = new SStruct();
        rStruct.Value = 42;

        // Reading the Value field by name:
        Type tStruct = typeof(SStruct);
        FieldInfo fValue = tStruct.GetField("Value");
        object oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read Value Before Mod: {0}", oValue);

        // Attempting to modify the Value field:
        fValue.SetValue(rStruct, 21);
        oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read Value After Mod:  {0}", oValue);
        // It didn't change.

        // SetValue is expecting an object though. Box the struct.
        object oStruct = rStruct;
        fValue.SetValue(oStruct, 21);
        oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read After Boxing:     {0}", oValue);
        // It didn't change.

        Console.Read();
    }
}

Here VB:

Imports System
Imports System.Reflection

Module Test
    Public Structure SStruct
        Public Value As Integer
    End Structure

    Public Sub Main()
        'Initialize a structure record.
        Dim rStruct As New SStruct
        rStruct.Value = 42

        'Reading the Value field by name:
        Dim tStruct As Type = GetType(SStruct)
        Dim fValue As FieldInfo = tStruct.GetField("Value")
        Dim oValue As Object = fValue.GetValue(rStruct)
        Console.WriteLine("Read Value Before Mod: {0}", oValue)

        'Attempting to modify the Value field:
        fValue.SetValue(rStruct, 21)
        oValue = fValue.GetValue(rStruct)
        Console.WriteLine("Read Value After Mod:  {0}", oValue)
        'It didn't change.

        'SetValue is expecting an object though. Box the struct.
        Dim oStruct As Object = rStruct
        fValue.SetValue(oStruct, 21)
        oValue = fValue.GetValue(rStruct)
        Console.WriteLine("Read After Boxing:     {0}", oValue)
        'It didn't change.

        Console.Read()
    End Sub
End Module

Edit:

Unboxing as suggested by Chris Copeland seems to do the trick in C#, but why does it not work in our VB enviroment? Check it out online on dotnetfiddle.net: C# produces 42,42,21 with this:

using System;
using System.Reflection;

public static class Test
{
    public struct SStruct
    {
        public int Value;
    }

    public static void Main()
    {
        // Initialize a structure record.
        SStruct rStruct = new SStruct();
        rStruct.Value = 42;

        // Reading the Value field by name:
        Type tStruct = typeof(SStruct);
        FieldInfo fValue = tStruct.GetField("Value");
        object oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read Value Before Mod: {0}", oValue);

        // Attempting to modify the Value field:
        fValue.SetValue(rStruct, 21);
        oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read Value After Mod:  {0}", oValue);
        // It didn't change.

        // SetValue is expecting an object though. Box the struct.
		object boxed = (object)rStruct;
		fValue.SetValue(boxed, 21);
		rStruct = (SStruct)boxed;
		
        oValue = fValue.GetValue(rStruct);
        Console.WriteLine("Read After Boxing:     {0}", oValue);
        // It didn't change.
   }
}

but 42,42,42 in VB:

Imports System
Imports System.Reflection

Public Module Test
    Public Structure SStruct
        Public Value As Integer
    End Structure

    Public Sub Main()
        'Initialize a structure record.
        Dim rStruct As New SStruct
        rStruct.Value = 42

        'Reading the Value field by name:
        Dim tStruct As Type = GetType(SStruct)
        Dim fValue As FieldInfo = tStruct.GetField("Value")
        Dim oValue As Object = fValue.GetValue(rStruct)
        Console.WriteLine("Read Value Before Mod: {0}", oValue)

        'Attempting to modify the Value field:
        fValue.SetValue(rStruct, 21)
        oValue = fValue.GetValue(rStruct)
        Console.WriteLine("Read Value After Mod:  {0}", oValue)
        'It didn't change.

        'SetValue is expecting an object though. Box the struct.
        Dim oStruct As Object = CType(rStruct, Object)
        fValue.SetValue(oStruct, 21)
        rStruct = CType(oStruct, SStruct)

        oValue = fValue.GetValue(rStruct)
        Console.WriteLine("Read After Boxing:     {0}", oValue)
        'It didn't change.
    End Sub
End Module

Posted 24-Feb-21 22:44pm

Visual Herbert

Updated 24-Feb-21 23:53pm

v2

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Comments

Maciej Los 25-Feb-21 5:17am

Why do you want to change the value through the Reflection?

Visual Herbert 25-Feb-21 5:27am

We have files with records of different structures. It is known which structure comes first, but this is the only one defining by its fields which structure types follow next. Reading the structures is all fine and doable, but unfortunately, values therein need to be modified sometimes. In order to avoid writing 4 bytes at file position 12134516 it would be much preferrable to simply modify the field "MyNumber" in the structure "MyStruct".

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Chris Copeland · Accepted Answer · 2021-02-24T23:08:00

Solution 1

A struct is a pass-by-value object, which means that anytime it gets passed into a method as an argument a new instance of the struct is created, and that will receive the updates. So when you call fValue.SetValue(rStruct, 21); what this is doing is creating a new instance of SStruct, and the field is modified in that.

A solution to this is to box the structure, and then unbox afterwards, like so:

C#

object boxed = (object)rStruct;
fValue.SetValue(boxed, 21);
rStruct = (SStruct)boxed;

Posted 24-Feb-21 23:08pm

Chris Copeland

Comments

Visual Herbert 25-Feb-21 5:23am

But, isn't that exactly what I did? Please see the code block after fValue.SetValue(rStruct, 21);

It remains 42...

Chris Copeland 25-Feb-21 5:24am

You need to unbox the struct again afterwards, as this contains the reference that was passed into the method. After calling SetValue() unbox the structure again and you should see the amended field.

Visual Herbert 25-Feb-21 5:41am

Thanks Chris, it works – in C#. Output then is 42, 42, 21. Our environment is VB.Net though.

...
Dim oStruct As Object = CType(rStruct, Object)
fValue.SetValue(oStruct, 21)
rStruct = CType(oStruct, SStruct)
oValue = fValue.GetValue(rStruct)
Console.WriteLine("Read After Boxing: {0}", oValue)

still outputs 42,42,42.

I edited my question with copy-pastable source for online verification.

Chris Copeland 25-Feb-21 6:11am

Just did a quick bit of research on this, apparently you need to use ValueType instead. So to get it working I changed Dim oStruct As Object = CType(rStruct, Object) to Dim oStruct As ValueType = rStruct and that seemed to work

Visual Herbert 25-Feb-21 6:55am

Thanks kind sir, it works. I did waste my whole morning with research about this problem. Would you please be so kind to share your source?

Chris Copeland 25-Feb-21 8:14am

Sure, I just had a look at this StackOverflow post :)

Visual Herbert 25-Feb-21 8:28am

Thank you.

The link provided there is dead, but it was captured by the Wayback Machine, if someone is interested:

https://web.archive.org/web/20170619134528/http://www.tech-archive.net/Archive/DotNet/microsoft.public.dotnet.languages.vb/2006-10/msg00219.html

The essence is:

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."

Maciej Los 25-Feb-21 5:40am

5ed!

Chris Copeland 25-Feb-21 6:11am

Thanks!

huangbojun 8-Dec-22 3:43am

Great! It is so amazing for me, I have wasted so much time to test the boxed and unboxed structure in VB.net. In VB.net, should we boxed the structure to ValueType!