Passing optional parameter to class fixture constructor in xunit

Question

I have tests that use an existing fixture, and I want to add new tests that pass a parameter to the fixture for configuration. I want to use optional parameters to avoid breaking the existing tests. Unfortunately...

Tests that don't specify the fixture with arguments get this error: Class fixture type 'ContextFixture' had one or more unresolved constructor arguments: String options

public class ContextFixture : IDisposable
{
    public ContextFixture(string? options = default) {...}
}
public class SandboxContextFixture : ContextFixture, IDisposable
{
    public SandboxContextFixture() : base(options) {...}
}
public class ServiceTests : IClassFixture<ContextFixture>
{
    public ServiceTests(ContextFixture contextFixture) {...}
}
public class RepositoryTests : IClassFixture<SandboxContextFixture>
{
    public RepositoryTests(SandboxContextFixture fixture) {...}
}

Answer 1

xUnit has some strict rules about which type of parameters can be accepted in the public constructor of a fixture class; a IMessageSink and ITestContextAccessor. See source code on GitHub. Note that exception being thrown here because the string option in your code is not supported.

To resolve, keep a default/parameterless public constructor on that ContextFixture class allowing it to be instantiated without parameters.

Add a protected constructor with parameters to that ConstextFixture and let the derived SandboxContextFixture fixture class call that one from its default/parameterless constructor.

public class ContextFixture
{
    public ContextFixture()
    { }

    protected ContextFixture(string? options)
    { }
}

public class SandboxContextFixture : ContextFixture
{
    public SandboxContextFixture() : base("some options value")
    { }
}

---

Optionally, in the ContextFixture class, the parameterless constructor can call the one with parameters.

public class ContextFixture
{
    public ContextFixture() : this(default)
    { }

    protected ContextFixture(string? options)
    {
        // ... 
    }
}