Add functionality to access vba with c# (or c++/cli)

Okay so I finally found a way myself.

In this post calling managed c# functions from unmanaged c++ there is the NuGet Package Unmanaged Exports linked. So I created a new C# DLL Project, inserted for testing purpose following code:

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

namespace ClassLibrary1
{
    internal static class Class1
    {
        [DllExport("square", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall)]
        static Double Square(Double dateValue)
        {
            return dateValue * dateValue;
        }

        [DllExport("test", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall)]
        static String Test()
        {
            return " ok";
        }

        [DllExport("mytest2", CallingConvention = System.Runtime.InteropServices.CallingConvention.StdCall)]
        static void Test4(String doSomething)
        {
            // Manipulate (and obisously create new) String
            doSomething += " ok";
        }
    }
}

Compiled this with x64 switch on, and created an VBA Sheet in excel with:

Option Explicit

Private Declare PtrSafe Function square Lib "C:\Users\<Name>\documents\visual studio 2013\Projects\ClassLibrary1\ClassLibrary1\bin\x64\Release\ClassLibrary1.dll" (ByVal dateValue As Double) As Double
Private Declare PtrSafe Function test Lib "C:\Users\<Name>\documents\visual studio 2013\Projects\ClassLibrary1\ClassLibrary1\bin\x64\Release\ClassLibrary1.dll" () As String
Private Declare PtrSafe Function mytest2 Lib "C:\Users\<Name>\documents\visual studio 2013\Projects\ClassLibrary1\ClassLibrary1\bin\x64\Release\ClassLibrary1.dll" (ByRef dateValue As String)

Private Sub CommandButton1_Click()
    Dim zahl As Double
    zahl = CInt(TextBox1.text)
    zahl = square(zahl)
    Label1.Caption = CStr(zahl)

    Dim text As String
    text = "This is"
    mytest2 (text)
    MsgBox text
End Sub

The only error I still get is when calling the test function. Returning an string seems to break excel. (Program instantly quits) But I think that should be solved in another question.