C# "internal" access modifier when doing unit testing

Internal classes need to be tested and there is an assemby attribute:

using System.Runtime.CompilerServices;

[assembly:InternalsVisibleTo("MyTests")]

Add this to the project info file, e.g. Properties\AssemblyInfo.cs.

If you want to test private methods, have a look at PrivateObject and PrivateType in the Microsoft.VisualStudio.TestTools.UnitTesting namespace. They offer easy to use wrappers around the necessary reflection code.

Docs: PrivateType, PrivateObject

For VS2017 & 2019, you can find these by downloading the MSTest.TestFramework nuget

Adding to Eric's answer, you can also configure this in the csproj file:

<ItemGroup>
    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
      <_Parameter1>MyTests</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>

Or if you have one test project per project to be tested, you could do something like this in your Directory.Build.props file:

<ItemGroup>
    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
      <_Parameter1>$(MSBuildProjectName).Test</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>

See: https://stackoverflow.com/a/49978185/1678053
Example: https://github.com/gldraphael/evlog/blob/master/Directory.Build.props#L5-L12