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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ClassLibrary1
{
   public class Class1
   {
      public string Test( string param ) 
      {
         Form1 TestForm = new Form1();
         TestForm.ShowDialog();
         return param;
      }        
   }
}					

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...
Type TFNOneFunction = function(param: String): string; stdcall;
...
Var
  myfunction : TFNOneFunction;
  myDLLhandle : Thandle;
begin
  myDLLhandle:=LoadLibrary('ClassLibrary1.dll');
  myfunction := nil;
  If myDLLhandle <> 0  then
  begin
    @myfunction := GetProcAddress(myDLLhandle,'ClassLibrary1.Class1.Test'); // hier auch schon nur mit Test versucht
    If @myfunction <> nil then
      myfunction('test')
    else
      showmessage('error');

    FreeLibrary(myDLLhandle);
  end
  else
    showmessage('error');
  end;					

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using RGiesecke.DllExport;
using System.Runtime.InteropServices;

namespace ClassLibrary1
{
   public class Class1
   {
      [DllExport( "Test", CallingConvention = CallingConvention.StdCall )]
      public static string Test( string param ) 
      {
         Form1 TestForm = new Form1();
         TestForm.ShowDialog();
         return param;
      }