.NET object Constructor error (Microsoft .NET Framework Interoperability)

I have created a small .net class and build the project and following is the code. Actually this has nothing just a constructor that take a one string parameter ( I have tried this even without input parameter). 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace myDLLFile
{
    public class Class1
    {
        public Class1(string myString) 
        {
 
        }
    }
}

After that copied the .dll file to "C:\Program Files (x86)\Microsoft Dynamics NAV\71\RoleTailored Client\Add-ins" folder.

I have attached my DLL file and project file here. 

http://www.sendspace.com/filegroup/g%2FJwWjNW8iCcBryQ9k8BKA

And then i created a codeunit and create a new variable myDLL type DotNet and selected my DLL file from the Dynamics NAV tab.

It shows my class and everything was perfect to that point. 

Then i wrote my small one line of code in the run() of the code unit,

myDLL := myDLL.Class1('t');

it compile perfectly. 

But when i run the code unit it throws the following error

Cannot create an instance of the following .NET Framework object: assembly myDLLFile, Version=1.0.0.0, Culture=neutral,PublicKeyToken=acf764e249539b6a, type myDLLFIle.Class1

I have tried so many options. But still error remains same. 

Please give a small help to resolve this small issue. 

Thank You.