Csharp 带有 in 修饰符的事件参数的 Observable.FromEvent 导致异常

我正在使用一个第三方库，它有一个事件\事件处理程序，我想将其转换为

系统ArgumentException-无法绑定到目标方法，因为其签名与委托类型的签名不兼容.

从…起

System.Private.CoreLib.dll!System.Reflection.RuntimeMethodInfo.CreateDelegateInternal(System.Type delegateType, object firstArgument, System.DelegateBindingFlags bindingFlags) Line 556    C#
    System.Private.CoreLib.dll!System.Reflection.RuntimeMethodInfo.CreateDelegate(System.Type delegateType, object target) Line 534 C#
    System.Reactive.dll!System.Reactive.ReflectionUtils.CreateDelegate<Test.TheDelegate>(object o, System.Reflection.MethodInfo method) Line 18 C#

If I remove the in 从…起 the arguments, It works as expected. I'm at a loss to understand why? Is even possible to do?

using System;
using System.Diagnostics;
using System.Reactive.Linq;

Test test = new();

Start();
test.TriggerEvent();
test.TriggerEvent();

void Start()
{
    Debug.WriteLine("Start");
    var o = Observable.FromEvent<Test.TheDelegate, SomeStruct>
        (h => test.TheEvent += h, 
         h => test.TheEvent -= h)
                      .Subscribe(OnNext);
}

void OnNext(SomeStruct s) => Debug.WriteLine("OnNext");

public class Test
{
    public delegate void TheDelegate(in SomeStruct o);

    // Without the in (or ref) keyword, no exception
    //public delegate void TheDelegate(SomeStruct o);

    public event TheDelegate? TheEvent;

    public void TriggerEvent()
    {
        SomeStruct s = new();
        TheEvent?.Invoke(s);
        Debug.WriteLine("Event Fired");
    }
}

public struct SomeStruct
{ }