在dll'dll.dll'中找不到名为'joinstring'的入口点。在C中#

我在C中创建了一个dll,您可以用这个方法看到:

namespace Dll
{
    public class Check
    {

        public string JoinString(string fristName, string Lastname)
        {

            return fristName + " " + Lastname;
        }
    }
}

我构建了这个dll并想在另一个应用程序中使用它 DLLImport 如你所见:

 [DllImport("Dll.dll", EntryPoint = "JoinString")]
    public static extern string JoinString (string fristName, string Lastname);
    private void button1_Click(object sender, EventArgs e)
    {
        string s = JoinString("aaa", "bbbb");

        MessageBox.Show(s.ToString());


    }

但我有个错误:

An unhandled exception of type 'System.EntryPointNotFoundException' occurred in WindowsFormsApplication3.exe

Additional information: Unable to find an entry point named 'JoinString' in DLL 'Dll.dll'.