Eventual assertions for FluentAssertions

FluentAssertions.Eventual is an extension that allows to wait for FluentAssertions checks to pass which can be useful when writing end-to-end tests.

Basic usage

Any FluentAssertions checks can be placed under a special foreach loop, which will implement the waiting and retry logic.

foreach (var _ in EventualAssertions.Attempts(4.Seconds(), 400.Milliseconds()))
{
    button.Should().BeVisible();
}

Source generator usage

When having a custom assertion class for a dynamic by nature subject the class can be decorated by the [GenerateEventual] attribute to get a special waiting wrapper generated.

Your code (simplified):

[GenerateEventual]
public class ButtonAssertions
{
    [CustomAssertion]
    public AndConstraint<ButtonAssertions> BeVisible(string? because = null, params object[] becauseArgs)
    {
        // implementation
    }
}

What gets generated (simplified):

public static class ButtonAssertions_Eventual_Extensions
{
    public static ButtonAssertions_Eventual Eventually(this ButtonAssertions underlying) { /* ... */ }
    // more generated extensions
}

public class ButtonAssertions_Eventual
{
    // constructor, fields

    [CustomAssertion]
    public AndConstraint<ButtonAssertions> BeVisible(string? because = null, params object[] becauseArgs)
    {
        AndConstraint<CurrentDateTimeAssertions> result = default !;
        foreach (var _  in EventualAssertions.Attempts(timeout, delay))
            result = underlying.BeVisible(because, becauseArgs);
        return result;
    }
}

Which allows for the following syntax:

button.Should().Eventually().BeVisible();
button.Should().Eventually(4.Seconds(), 400.Milliseconds()).BeVisible();
button.Should().EventuallyLong().BeVisible();

More info

Complete interactive README in mybinder:

Also in nbviewer: