Blazor trigger custom validation message

Question

I have the following class which is being used as an input model for an EditForm in a Blazor server side application.

public class KundeInput
{
    [ValidateComplexType]
    public List<AnsprechpartnerInput> Ansprechpartner { get; } = new List<AnsprechpartnerInput>();
  
    public string? Kundennummer { get; }
 
    [Required]
    [MaxLength(60)]
    public string Firma { get; set; } = String.Empty;
    [MaxLength(60)]
    public string? Name2 { get; set; }
    [MaxLength(60)]
    public string? Name3 { get; set; }
}

As you can see, my model contains a list of another model called AnsprechpartnerInput. Here is this model:

public class AnsprechpartnerInput
{
    public string? Kundennummer { get; set; }
    public int Nummer { get; } = -1;
    [MaxLength(60)]
    [Required]
    public string Vorname { get; set; } = String.Empty;
    [MaxLength(60)]
    [Required]
    public string Nachname { get; set; } = String.Empty;
    [MaxLength(40)]
    [Required]
    public string? Bereich { get; set; }
    / * More properties */
}

The validation works fine. However, once I have multiple invalid AnsprechpartnerInput models in my list, the ValidationSummary becomes a mess. Because it displays e.g. 5 times field xyz is invalid.

I know I can set a custom message with the ErrorMessage property but I am not able to use other attributes from my model in this message.

What I want to achive is this:

[Required(ErrorMessage = $"Vorname of {Kundennummer} is required")]
public string Vorname { get; set; } = String.Empty;

I already tried to change the message with reflection but accoridng to Microsoft this way is not recommend or supported https://github.com/dotnet/aspnetcore/issues/25611

Is there any way to get it to work? I thought of string replacement but I am not sure how I can figure out the right model for my ValidationMessage.

Also is there any way to validate the items of the list by one and get a boolean result? Let's say I want to achive this:

@foreach (var ansprechpartner in Input.Ansprechpartner)
{

    if (Input.SelectedAnsprechpartner is null)
        Input.SelectedAnsprechpartner = ansprechpartner;

    <a @onclick="() => Input.SelectedAnsprechpartner = ansprechpartner"
       class="@GetNavListClass(Input.SelectedAnsprechpartner == ansprechpartner)"
       id="list-ansprechpartner-tab-@(ansprechpartner.Nummer)"
       data-toggle="list"
       href="#list-ansprechpartner-@(ansprechpartner.Nummer)"
       role="tab"
       aria-controls="@(ansprechpartner.Nummer)">
        @((MarkupString)(ansprechpartner.Nummer < 0  ? "<span class=\"font-weight-bold\">NEU</span>" : $"({ansprechpartner.Nummer})")) @ansprechpartner.Vorname @ansprechpartner.Nachname
    </a>
    // When the model ansprechpartner is invalid, I want to display an icon
}

Thanks for any help! PS: Blazor rocks!

Answer 1

public class RequiredCollectionAttribute : ValidationAttribute
{
    private readonly string _dependentProperty;

    public RequiredCollectionAttribute(string dependentProperty)
    {
        _dependentProperty = dependentProperty;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(_dependentProperty);
        if (property == null)
        {
            return new ValidationResult($"Unknown property: {_dependentProperty}");
        }

        var dependentValue = property.GetValue(validationContext.ObjectInstance, null);
        if (dependentValue == null)
        {
            return ValidationResult.Success;
        }

        IEnumerable<string> isCollection;
        try
        {
            isCollection = (IEnumerable<string>)dependentValue;
        }
        catch (InvalidCastException)
        {
            return new ValidationResult($"Property {_dependentProperty} is not of type IEnumerable<string>.");
        }

        if (isCollection == null || (isCollection != null && isCollection.Count() == 0))
        {
            return new ValidationResult(ErrorMessage ?? "This field is required.");
        }

        return ValidationResult.Success;
    }
}


[RequiredCollection("SelectedProjectManagers", ErrorMessage = "Project Manager(s) is required.")]
public IEnumerable<string>? SelectedProjectManagers { get; set; }

Answer 2

You should use a custom validation attribute where you can explicitly add any error message you want

public class KundennummerValidationAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var model = (AnsprechpartnerInput)validationContext.ObjectInstance;
        if(string.IsNullOrEmpty((string)value))
        {
            return new ValidationResult($"Vorname of {model.Kundennummer} is required", new[] { "Kundennummer" });
        }


        return ValidationResult.Success;
    }
}

then use

[KundennummerValidation]
public string Vorname { get; set; } = String.Empty;

result : Validation summary: