That's not how that's intended to be used. Assert.SkipUnless is for use within the test, not with the attribute.

FactAttribute has a settable SkipUnless property that's intended to point to a static public property that will be read at the appropriate time. So what you want is this:

public class MyAttribute : FactAttribute
{
    public MyAttribute()
    {
        Skip = "My Reason";
        SkipUnless = nameof(ShouldRun);
    }

    public static bool ShouldRun => 1 == 2;
}

Also, you don't need a custom attribute for this. Instead, you could just do this:

public static bool ShouldRun => 1 == 2;

[Fact(Skip = "My Reason", SkipUnless = nameof(ShouldRun))]
public void My_Test_Method()
{
}

That's not how that's intended to be used. Assert.SkipUnless is for use within the test, not with the attribute.

FactAttribute has a settable SkipUnless property that's intended to point to a static public property that will be read at the appropriate time. So what you want is this:

public class MyAttribute : FactAttribute
{
    public MyAttribute()
    {
        Skip = "My Reason";
        SkipUnless = nameof(ShouldRun);
    }

    public static bool ShouldRun => 1 == 2;
}

Thanks for the answer.

Something strange happens with this code.

It seems that xUnit expects to see ShouldRun in the test class as well. This means I have to define ShouldRun twice: once in the custom attribute class and once in the test class. However, only the one in the test class will be considered!

Cannot find public static property 'ShouldRun' on type 'MyProject.MyTestClass' for dynamic skip on test method 'MyProject.MyTestClass.MyTest'

Sorry, I forgot to mention that if you're putting it in the attribute, you need to use SkipType as well (set it to the attribute type). The default for SkipType when unset is to look for the property on the test class.