C# unit test project not finding my vb.net project reference

I want to call my vb.net function in a c# Unit test, so i have built my vb.net function into a .dll and referenced it in the c# unit test but it does not recognize it.

is there an easier way to call vb.net functions within c#? or how can i make my method work?

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Collections;
using TestingDateValidation;

namespace UnitTestProject2
{
[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        DateTime DT = new DateTime(2015, 1, 1);
        DateTime DT2 = new DateTime(2015, 1, 1);
        Boolean ExpectedResult = true;
        Boolean ActualResult;



        }
    }
}