Добавьте файлы проекта.

Base64encoderTests/Base64encoderTests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>.NETCoreApp,Version=v5.0</TargetFramework>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
+    <PlatformTarget>x64</PlatformTarget>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.2.7" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.2.7" />
+    <PackageReference Include="coverlet.collector" Version="3.1.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Base64encoder\Base64encoder.csproj" />
+  </ItemGroup>
+
+</Project>

Base64encoderTests/Encoder64Tests.cs

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+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Ru.Pfr.Crypto.Base64encoder;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Ru.Pfr.Crypto.Base64encoder.Tests
+{
+    [TestClass()]
+    public class Encoder64Tests
+    {
+        [TestMethod()]
+        public void EncodeTo64Test()
+        {
+            String result = Encoder64.EncodeTo64("MyPassword");
+            Console.WriteLine(result);
+            
+        }
+    }
+}