Xunit test fails with NU1701 - unit-testing

Question

I try to get my unit tests for Xamarin to run, but fail to get it working. I added the following nuget packages to my project:
MSTest.TestAdapter
MSTest.TestFramework
xunit
I can see my unit tests in the list of tests. But when I start to run a single test I get the following message:
No test matches the given testcase filter FullyQualifiedName= ...
After researching a bit I found in the Build section this message:
Warning NU1701: Package 'MSTest.TestAdapter 1.4.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project.
How can I get my tests running?
EDIT: I uninstalled the MSTest Nuget from the project and got the following:
No test matches the given testcase filter
EDIT 2: The test looks like this:
using System;
using System.Collections.Generic;
using System.Text;
using App.Services;
using Xunit;
namespace App.Tests.Services
{
public class HelperTest
{
[Fact]
public void Encrypt_password_Test()
{
string password = "Helloworld";
string expectedResult = "";
string result = Helpers.Encrypt_password(password);
Assert.Equal(expectedResult, result);
}
}
}
I expect that the test will fail, because of the empty string.
The csproj file:
<?xml version="1.0" encoding="utf-8"?>
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<NeutralLanguage>en-GB</NeutralLanguage>
<AssemblyName>App</AssemblyName>
<RootNamespace>App</RootNamespace>
</PropertyGroup>
<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
<DebugType>pdbonly</DebugType>
<DebugSymbols>true</DebugSymbols>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
<PackageReference Include="SharpZipLib" Version="1.1.0" />
<PackageReference Include="sqlite-net-pcl" Version="1.5.231" />
<PackageReference Include="System.Numerics.Vectors" Version="4.5.0" />
<PackageReference Include="Xamarin.Essentials" Version="1.2.0" />
<PackageReference Include="Xamarin.Forms" Version="4.1.0.581479" />
<PackageReference Include="Xamarin.Plugin.FilePicker" Version="2.1.18" />
<PackageReference Include="xunit" Version="2.4.1" />
</ItemGroup>
<ItemGroup>
<XliffResource Include="MultilingualResources\App.de.xlf" />
</ItemGroup>
<ItemGroup>
<Compile Update="Localizations\AppResources.Designer.cs">
<DesignTime>True</DesignTime>
<AutoGen>True</AutoGen>
<DependentUpon>AppResources.resx</DependentUpon>
</Compile>
<Compile Update="Views\Login\NewProfilePage.xaml.cs">
<DependentUpon>NewProfilePage.xaml</DependentUpon>
</Compile>
</ItemGroup>
<ItemGroup>
<EmbeddedResource Update="Localizations\AppResources.resx">
<Generator>ResXFileCodeGenerator</Generator>
<LastGenOutput>AppResources.Designer.cs</LastGenOutput>
</EmbeddedResource>
<EmbeddedResource Update="Views\Settings\AppSettings.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\Login\NewProfilePage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\Settings\ProfilelistPage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\Settings\ProfilePage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\ProjectPage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\Settings\SensorListPage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\SensorTestPage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
<EmbeddedResource Update="Views\Settings\SettingsPage.xaml">
<Generator>MSBuild:UpdateDesignTimeXaml</Generator>
</EmbeddedResource>
</ItemGroup>
<ItemGroup>
<Folder Include="Tests\Projectlist\" />
<Folder Include="Tests\Settings\" />
</ItemGroup>
</Project>
I also created a example xunit test project. There I noticed that it uses netcoreapp2.1 as TargetFramework. When I change the TargetFramework in my app it makes it incompatible with Xamarin.

Answer 1

You nearly found the solution yourself. Indeed the reason the tests are not running is because netstandard target frameworks are not executable. netstandard is a contract, but it needs a runtime such as netcoreapp, the .NET Framework (net472 for example), Xamarin, or others. Tests were being discovered by static analysis, not by execution.
The usual way people write tests for their code is to put tests in a separate project. This also means when you ship your app to customers, the dlls they install don't have test code that will never run and just make the download larger for no reason.
So, create a xunit test project. Ideally it targets the same runtime that your app is going to run on, but I've never done any Xamarin development so I don't know if that's feasible, so maybe netcoreapp2.1 or netcoreapp2.2 is good enough. If you're using Visual Studio, add a project reference, otherwise edit the csproj and add <ProjectReference Include="../path/to/project.csproj" />.
If your production code has internal classes that you don't want to make public, then add the InternalsVisibleTo attribute to your production assembly, and it will be available just to your test assembly.