Read and write files and command lines

A value object implements ISpanParsable<T> and ISpanFormattable, and carries a TypeConverter, which is what a library that reads text without knowing the type looks for. Where a library finds one of them, it works as is; where it does not, the recipe is to write the underlying values and parse them back cell by cell with TryParse, which says which rule a value broke.

Sep

Sep 0.17.1 works as is, through ISpanParsable<T> and ISpanFormattable:

var iban = row["Iban"].Parse<Iban>();   // throws ValueObjectException on a value the type rejects

if (!Quantity.TryParse(row["Quantity"].Span, CultureInfo.InvariantCulture, out var quantity, out var validation))
{
    // validation.ErrorCode names the rule
}

Spectre.Console.Cli

Spectre.Console.Cli binds a value object through its TypeConverter, but a rejection prints only Error: Failed to convert '500' to Nullable`1., and the rule is lost. Unwrap it in an exception handler:

app.Configure(config => config.SetExceptionHandler((exception, _) =>
{
    if (exception.InnerException is ValueObjectException rejected)
    {
        AnsiConsole.MarkupLineInterpolated($"[red]Error:[/] {exception.Message} {rejected.Message} ({rejected.ErrorCode})");
        return 2;
    }

    AnsiConsole.WriteException(exception);
    return -1;
}));

An option left out of the command line leaves a non-nullable value-object option default. Declare options T?, or make the value a required <arg>.

ClosedXML, MiniExcel and EPPlus

All three write a value-object property as a text cell, through ToString(), so a number or a date lands "stored as text". Given a sequence of value objects directly, ClosedXML writes their public properties as columns, Value among them, and EPPlus a Value column. The probe also found an IsDefault column in both, a public property then, and no longer is (inferred, not re-run). Reading back fails: MiniExcel's Query<Order>, EPPlus's ToCollection<Order>() and ClosedXML's GetValue<Iban>() all throw.

Export a projection of the underlying values:

worksheet.Cell(1, 1).InsertTable(orders.Select(order => new
{
    Id = order.Id.Value,
    Iban = order.Iban.Value,
    Quantity = order.Quantity.Value,
    Due = order.Due.Value.ToDateTime(TimeOnly.MinValue),
}));

The cells then hold numbers and dates: the probe read them back as 7 (Number) | 12,5 (Number) | 03/10/2026 00:00:00 (DateTime), printed in its own culture. Import cell by cell with TryParse, and report the address of the cell with validation.ErrorCode, so the person who filled the sheet knows which cell broke which rule.

EPPlus 5 and later is under the Polyform Noncommercial licence: commercial use needs a licence.