FluentAssertions: How to compare two collections using a custom comparison on each pair of elements?

Question

Given the following inputs:

var customers = new[] {
    new Customer { Name = "John", Age = 42 },
    new Customer { Name = "Mary", Age = 43 }
};
var employees = new[] {
    new Employee { FirstName = "John", Age = 42 },
    new Employee { FirstName = "Mary", Age = 43 }
};

What would be the best way to compare these lists using FluentAssertions?

My only approach at the moment looks like this -- quite similar to Enumerable.SequenceEqual:

using (var customerEnumerator = customers.GetEnumerator())
using (var employeeEnumerator = employees.GetEnumerator())
{
    while (customerEnumerator.MoveNext())
    {
        employeeEnumerator.MoveNext().Should().BeTrue();
        var (customer, employee) = (customerEnumerator.Current, employee.Current);

        customer.Name.Should().BeEquivalentTo(employee.FirstName);
        customer.Age.Should().Be(employee.Age);
    }
    employeeEnumerator.MoveNext().Should().BeFalse();
}

Of course, this is neither easy to read nor providing diagnostic outputs of FA's usual quality. Is there any FluentAssertions-built-in method to make this assertion?

score 7 · Accepted Answer · answered Apr 29 '19 at 07:13

One way to improve the assertion would be to extract the comparison into a custom IEquivalencyStep to guide how a Customer and an Employee should be compared.

It consist of two parts:

CanHandle which determines when this comparison is applicable, and
Handle to perform the custom comparisons.

public class CustomerEmployeeComparer : IEquivalencyStep
{
    public bool CanHandle(IEquivalencyValidationContext context,
        IEquivalencyAssertionOptions config)
    {
        return context.Subject is Customer
            && context.Expectation is Employee;
    }

    public bool Handle(IEquivalencyValidationContext context, IEquivalencyValidator
        parent, IEquivalencyAssertionOptions config)
    {
        var customer = (Customer)context.Subject;
        var employee = (Employee)context.Expectation;

        customer.Name.Should().Be(employee.FirstName, context.Because, context.BecauseArgs);
        customer.Age.Should().Be(employee.Age, context.Because, context.BecauseArgs);

        return true;
    }
}

To use CustomerEmployeeComparer in an assertion, add it by invoking Using(new CustomerEmployeeComparer()) on the EquivalencyAssertionOptions config parameter of BeEquivalentTo.

Note: As your example requires the two lists to be compared in order, I've added WithStrictOrdering() to the example below.

[TestMethod]
public void CompareCustomersAndEmployeesWithCustomEquivalencyStep()
{
    // Arrange
    var customers = new[] {
        new Customer { Name = "John", Age = 42 },
        new Customer { Name = "Mary", Age = 43 }
    };

    var employees = new[] {
        new Employee { FirstName = "John", Age = 42 },
        new Employee { FirstName = "Mary", Age = 43 }
    };

    // Act / Assert
    customers.Should().BeEquivalentTo(employees, opt => opt
        .Using(new CustomerEmployeeComparer())
        .WithStrictOrdering());
}

public class Employee
{
    public string FirstName { get; set; }
    public int Age { get; set; }
}

public class Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
}

Changing the name of the first Employee to Jonathan, now gives this failure message:

Message: Expected item[0] to be "Jonathan" with a length of 8, but "John" has a length of 4, differs near "hn" (index 2).

With configuration:
- Use declared types and members
- Compare enums by value
- Include all non-private properties
- Include all non-private fields
- Match member by name (or throw)
- Without automatic conversion.
- UnitTestProject15.CustomerEmployeeComparer
- Without automatic conversion.
- Always be strict about the collection order

For anyone interested, there is a related open issue about overriding which properties to compare. https://github.com/fluentassertions/fluentassertions/issues/535

FluentAssertions: How to compare two collections using a custom comparison on each pair of elements?

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