在c#应用程序中使用c++ dll时无法找到入口点

我写了一个c#应用程序，需要在一个按钮点击上调用c++ dll中的函数。但在点击按钮时，它抛出'EntryPointNotFound'异常。

Below is the code snippet of C#
    public class Test
    {
        [DllImport("Demo.dll", EntryPoint = "OpenFile"]
        public static extern bool OpenFile(string fileName);
    }
private void button1_Click_1(object sender, EventArgs e)
        {
            bool check = Test.OpenFile("test.txt"); // exception thrown at this point
            if (check)
            {
                // Not entering this area.. 
            }
        }

C++ Header (.h file)
__declspec(dllexport) bool OpenFile(CString fileName);
Cpp class (.cpp )
__declspec(dllexport) bool Demo::OpenFile(CString fileName)
{
        return true;
}

请帮。

extern "C" __declspec(dllexport) bool OpenFile(CString fileName);