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Encoding from C   to C#
extern "C" RETURNCHAR_API char* testString()
{
    return test;
}

extern "C" RETURNCHAR_API char* __cdecl testString();

public static class ImportTest
    {
        [DllImport("ReturnChar.dll", EntryPoint = "testString", CallingConvention = CallingConvention.Cdecl)]
        public static unsafe extern char* testString();
    }
    public partial class MainWindow : Window
    {
        public unsafe MainWindow()
        {
            InitializeComponent();
            try
            {

                StringBuilder sb = new StringBuilder(new string(ImportTest.testString()));
                textBox1.Text = sb.ToString();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }