How To Unit Test A Class With Options In C# & .NET

One of the abilities offered by the .NET platform is a flexible and robust configuration and settings management engine.

I have discussed this at some length in this post, “Loading & Using Application Settings In .NET” and the subsequent post, “Flexible Leveraging of appsettings.json in .NET Applications”.

Suppose we have a Settings class that looks like this:

public class SystemSettings
{
    public string ForegroundColour { get; set; }
    public string BackgroundColour { get; set; }
}

And we are using it in a Printer class like this:

public class Printer
{
    private readonly SystemSettings _systemSettings;
    public string ForegroundColour => _systemSettings.ForegroundColour;
    public string BackgroundColour => _systemSettings.BackgroundColour;

    public Printer(IOptions<SystemSettings> settings, SystemSettings systemSettings)
    {
        _systemSettings = systemSettings;
    }
}

Suppose we need to unit test this?

Note that this is a unit test, not an integration test.

This means we don’t have access to the underlying platform, such as ASP.NET, to do the heavy lifting for us, which is covered in this post, “Dependency Injection In C# & .NET Part 7 - Integration Testing”.

There is a way around this - using the Create method of the Options class.

The code would look like this:

using AwesomeAssertions;
using Microsoft.Extensions.Options;
using SystemSettings;

namespace SystemSettingsTests;

public class PrinterTests
{
    [Fact]
    public void Printer_Loads_Correctly()
    {
      	// Create the settings
        var settings = new SystemSettings
        {
            ForegroundColour = "Red",
            BackgroundColour = "White"
        };

      	// Create the options
        var options = Options.Create(settings);

      	// Pass the option where neeed
        var printer = new Printer(options);

        printer.ForegroundColour.Should().Be("Red");
        printer.BackgroundColour.Should().Be("White");
    }
}

If we run this code, it should run successfully.

TLDR

The Options class has a Create method that allows you to unit test types that have settings passed to them through the .NET settings & configuration infrastructure.

The code is in my GitHub.

Happy hacking!