Label不打印导入的DLL函数字符串

Question

我在c ++中创建了一个DLL，其中包含一个只返回字符串“Hello”的函数。但是，当我在c＃应用程序中导入此dll并尝试在标签中显示此输出时，它不会显示任何内容

这是我的代码

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Data; 
using System.Drawing; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
using System.Windows.Forms; 
using System.Runtime.InteropServices; 


namespace WindowsFormsApp1
{ 
    public partial class Form1 : Form
    { 
        public Form1()
        {
            InitializeComponent();
        }

        private void close_Click(object sender, EventArgs e)
        {
            Close(); 
        }
        [DllImport("Dll1.dll", EntryPoint = "Print_Hello", CallingConvention = CallingConvention.Cdecl)]

        public static extern string Print_Hello();
        private void click_Click(object sender, EventArgs e)
        {
            string s = Print_Hello();
            Hllolbl.Text = s;
        } 
        private void Form1_Load(object sender, EventArgs e)
        {

        } 
    }
}