c# CSarpCodeProvider 动态编译生成类后，用反射调用目标发生异常

动态生成的类如下：
using System;
using System.Runtime.InteropServices;
using System.Threading.Tasks;
namespace Testtoolproject
{
public class DynamicClass
{
[DllImport("Win32MutlOld(极致汇仪1).dll", EntryPoint = "Mutil", CallingConvention = CallingConvention.Cdecl)]
public static extern double Mutil(double x, double y);
}
}

错误提示如下：