Nullable.GetUnderlyingType用于通过Nullable<T>结构表示的nullable value types。Nullable reference types执行某种类型擦除，在运行时不表示为类型。您可以尝试使用NullabilityInfoContext获取此信息：

NullabilityInfoContext context = new();
var nullabilityInfo = context.Create(methodInfo.GetParameters().First());
Console.WriteLine(nullabilityInfo.ReadState);    // Nullable
Console.WriteLine(nullabilityInfo.WriteState);   // Nullable

public interface IMyInterface
{
    Task GetMyModel(MyModel? p);
}

public class MyModel
{
}