Code Review: Value Objects and Error Messages

1.cs

public class AllowanceAmount : ValueObject
{
    public const int MaxLength = 50;

    public string Value { get; }

    private AllowanceAmount(string value)
    {
        Value = value;
    }

    protected override IEnumerable<object> GetEqualityComponents()
    {
        yield return Value;
    }

    public static Result<AllowanceAmount> Create(string allowance)
    {
        allowance = allowance?.Trim();

        if (string.IsNullOrEmpty(allowance))
            return Result.Ok(new AllowanceAmount(null));

        if (allowance.Length > MaxLength)
            return Result.Fail<AllowanceAmount>($"'Allowance' cannot be longer than {MaxLength} characters.");

        return Result.Ok(new AllowanceAmount(allowance));
    }
}

2.cs

var bezeichnungResult = KatalogeintragBezeichnung.Create(updateItem.Bezeichnung);
var bezeichnungWeiblichResult = kademischerGradBezeichnungWeiblich.Create(updateItem.BezeichnungWeiblich);
var typResult = AkademischerGradTyp.Create(updateItem.Type);
var rangResult = AkademischerGradRang.Create(updateItem.Rank);

3.cs

public class CustomerModel
{
    [MaxLength(50, ErrorMessage = "Allowance 1 cannot be longer than 50 characters")]
    public string Allowance1 { get; set; }

    [MaxLength(50, ErrorMessage = "Allowance 2 cannot be longer than 50 characters")]
    public string Allowance2 { get; set; }
}

4.cs

[HttpPost]
public IActionResult Create([FromBody] CustomerModel item)
{
    if (!ModelState.IsValid)
        return BadRequest(ModelState);

    // ...
}

5.cs

public class CustomerModel
{
    public string Allowance1 { get; set; }
    public string Allowance2 { get; set; }
}

[HttpPost]
public IActionResult Create([FromBody] CustomerModel item)
{
    Result<AllowanceAmount> allowance1OrError = AllowanceAmount.Create(item.Allowance1);
    Result<AllowanceAmount> allowance2OrError = AllowanceAmount.Create(item.Allowance2);

    Result result = Result.Combine(allowance1OrError, allowance2OrError);
    if (result.IsFailure)
        return BadRequest(result.Error);
    
    // ...
}

6.js

{
  "Message": "The request is invalid.",
  "ModelState": {
    "model.Allowance1": [ "Allowance 1 cannot be longer than 50 characters." ],
    "model.Allowance2": [ "Allowance 2 cannot be longer than 50 characters." ]
  }
}

7.js

{
  "Allowance1": [ "'Allowance' cannot be longer than 50 characters." ],
  "Allowance2": [ "'Allowance' cannot be longer than 50 characters." ]
}

8.cs

public class CustomerModel
{
    public string Allowance1 { get; set; }
    public string Allowance2 { get; set; }
}

[HttpPost]
public IActionResult Create([FromBody] CustomerModel item)
{
    Result<AllowanceAmount> allowance1OrError = AllowanceAmount.Create(item.Allowance1);
    Result<AllowanceAmount> allowance2OrError = AllowanceAmount.Create(item.Allowance2);

    if (Result.Combine(allowance1OrError, allowance2OrError).IsFailure)
    {
        var errors = new[]
        {
            new ErrorEntry(() => item.Allowance1, allowance1OrError),
            new ErrorEntry(() => item.Allowance2, allowance2OrError)
        };
        return BadRequest(errors);
    }

    // ...
}

public class ErrorEntry
{
    public readonly Expression<Func<object>> Expression;
    public readonly Result Result;

    public ErrorEntry(Expression<Func<object>> expression, Result result)
    {
        Expression = expression;
        Result = result;
    }
}

private BadRequestObjectResult BadRequest(ErrorEntry[] entries)
{
    foreach (ErrorEntry entry in entries)
    {
        if (entry.Result.IsSuccess)
            continue;

        ModelState.AddModelError(ConvertExpression(entry.Expression), entry.Result.Error);
    }

    return BadRequest(ModelState);
}

private string ConvertExpression(Expression<Func<object>> propertyExpression)
{
    var expression = propertyExpression.Body as MemberExpression;

    if (expression == null)
        throw new ArgumentException(propertyExpression.Body.ToString());

    return expression.Member.Name;
}