Conditional Mapping

The [MapWhen] attribute lets you conditionally include or exclude a property during mapping based on a runtime condition.

Usage

[Facette(typeof(Employee), "SocialSecurityNumber",
    NestedDtos = new[] { typeof(AddressDto) })]
public partial record ConditionalEmployeeDto
{
    private static bool _includeSalary = true;

    [MapWhen(nameof(ShouldIncludeSalary))]
    public decimal Salary { get; init; }

    public static bool ShouldIncludeSalary() => _includeSalary;
    public static void SetIncludeSalary(bool value) => _includeSalary = value;
}

Generated code

The generator wraps the property assignment in a conditional:

// Generated FromSource
Salary = ConditionalEmployeeDto.ShouldIncludeSalary()
    ? source.Salary
    : default

When the condition returns false, the property gets the default value for its type (0 for decimal, null for reference types, etc.).

Method requirements

The condition method must be:

• static — no instance required
• Parameterless — takes no arguments
• Returns bool — true to include, false to skip

If the method doesn't meet these requirements, Facette emits FCT013.

Runtime toggling

Because the condition is evaluated at mapping time (not compile time), you can change behavior dynamically:

// Include salary for authorized users
ConditionalEmployeeDto.SetIncludeSalary(true);
var withSalary = ConditionalEmployeeDto.FromSource(employee);
Console.WriteLine(withSalary.Salary); // 95000

// Hide salary for unauthorized users
ConditionalEmployeeDto.SetIncludeSalary(false);
var withoutSalary = ConditionalEmployeeDto.FromSource(employee);
Console.WriteLine(withoutSalary.Salary); // 0

Use cases

• Role-based field visibility — hide sensitive fields based on user role
• Feature flags — conditionally include new fields during rollout
• Environment-based — include debug fields only in development