FileNotFoundException on MethodInfo.GetCustomAttribute

My goal is to display information about the methods of some classes in an uploaded dll. Loading the assembly, finding the desired classes and their methods was already successfully done. Now I was trying to show whether a method is declared "async" or not.

I found a thread that tells me how to do it: How can I tell if a C# method is async/await via reflection?

Anyway, while testing, when I call this

(AsyncStateMachineAttribute)methodInfo.GetCustomAttribute(typeof(AsyncStateMachineAttribute))

I got a System.IO.FileNotFoundException - "Could not load file or assembly '{assembly identifier}' or one of its dependencies. The system cannot find the file specified.".

I found this exception in an unanswered thread, but it did not help me: How to prevent MemberInfo.IsDefined from throwing FileNotFoundException on irrelevant attributes?

I understand that the method I am looking at has an attribute that my code does not know. I do not want to load that reference because it is only a test case and many other different attributes can be found in the same situation.

So, I need an answer to one of two questions:

1. Is there a way to get the attribute "AsyncStateMachineAttribute", if it exists, ignoring the errors on other attributes?

2. Is there another way to check whether a method (from MethodInfo) is async?

Thanks in advance! :)