Value Conversions

Value conversions let you transform property values during mapping — for example, formatting a DateTime as a string or converting between custom types.

Defining conversions

Use the Convert and ConvertBack parameters on [MapFrom]:

[Facette(typeof(Employee), "SocialSecurityNumber")]
public partial record EmployeeDto
{
    [MapFrom("HireDate", Convert = nameof(FormatDate), ConvertBack = nameof(ParseDate))]
    public string HiredOn { get; init; } = "";

    public static string FormatDate(DateTime dt) => dt.ToString("yyyy-MM-dd");
    public static DateTime ParseDate(string s) => DateTime.Parse(s);
}

Method requirements

• Convert — used in FromSource and Projection. Must be a static method on the DTO type that takes the source property type and returns the DTO property type.
• ConvertBack — used in ToSource. Must be a static method on the DTO type that takes the DTO property type and returns the source property type.

You can specify one without the other:

• Convert only — ToSource will skip this property (or you can disable GenerateToSource)
• ConvertBack only — useful when FromSource can map directly but the reverse needs transformation

Generated output

// FromSource
HiredOn = EmployeeDto.FormatDate(source.HireDate)

// ToSource
HireDate = EmployeeDto.ParseDate(this.HiredOn)

// Projection
HiredOn = EmployeeDto.FormatDate(source.HireDate)

Diagnostics

If the referenced method doesn't exist or isn't a static method on the DTO type:

• FCT008 — Convert method not found
• FCT009 — ConvertBack method not found

Tips

• Keep conversion methods simple — they're called per-property, per-mapping operation
• Remember that Projection conversions must be translatable by your LINQ provider if used with EF Core. Method calls in projections may not translate to SQL
• For enum↔string/int conversions, see Enum Conversion — Facette handles those automatically without custom converters