How to Unit Test a Method That Return Void Type

First Create a void method with a ref parameter in your Class Library Solution.

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

namespace ClassLibrary1
{
    public class Class1
    {
        public void methodRef(ref int refvalue)
        {
           refvalue= refvalue + refvalue;
        }
    }
}

Out goal is to test this void method in unit test.

Calling the methodRef in Test Class.

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using ClassLibrary1;

namespace UnitTestProject1
{
    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMetod3()
       {
            int refvalue = 10;
            ClassLibrary1.Class1 obj = new ClassLibrary1.Class1();
            obj.methodRef(ref refvalue);
            int actualvalue = refvalue;
            Assert.AreEqual(20, actualvalue);
        }
    }
}

Build the solution and Run TestMethod3. It will give the actual Result as 20 and Expected Result as 20.