Getfield.SetValue doesn't work

Question

in my project i'm currently working on i stumbled upon this problem: I want to create a Instance of the Class "ApiID". I got the Code from Reflector, as you can see the .dll (not my Project) imports is from ummanaged code, which i don't have access to. ``` [StructLayout(LayoutKind.Sequential, Size=0x10), CLSCompliant(false), NativeCppClass, DebugInfoInPDB, MiscellaneousBits(0x40)] public struct ApiId { private long ; public static unsafe void (ApiId* idPtr1, ApiId* idPtr2) { ApiId.{ctor}(idPtr1, (ApiId modopt(IsConst)* modopt(IsImplicitlyDereferenced)) idPtr2); } public static unsafe void (ApiId* idPtr1) { ApiId.{dtor}(idPtr1); } } ``` My C#-Code looks like this: ``` var _apiId = new ApiId(); long vlue = 0x0000000041403070; typeof(ApiId).GetField("", BindingFlags.Instance | BindingFlags.NonPublic).SetValue(_apiId, vlue); ``` The Code Runs succesfully, but the Field does not change and stays 0... what am I doing wrong? gr…

Accepted Answer

The problem here is boxing.

This line:

```
typeof(ApiId).GetField("<alignment member>", ...).SetValue(_apiId, vlue);
```

will box the struct value in `_apiId` and set the field value of the boxed copy. It will not change the original.

FieldInfo.SetValue is defined like this:

```
public void SetValue(
    Object obj,
    Object value
)
```

So the first parameter, your `_apiId` will be boxed. Boxing will make a copy of the struct value. SetValue will thus change the boxed copy and not the original.

Instead, you can box the object yourself:

```
object apiId = new ApiId();
long vlue = ...
typeof(ApiId)...
_apiId = (ApiId)apiId;
```

Here's a .NET Fiddle that demonstrates. Try changing which line is commented out to see the difference.

Getfield.SetValue doesn't work

Solution

Problem