Provide a way to customize the default validation message

[marcominerva]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When we get a validation error in .NET 10 Preview 3, we obtain something like this:

{
  "title": "One or more validation errors occurred.",
  "errors": {
    "Id": [
      "The field Id must be between 1 and 10."
    ]
  }
}

If a multi-language service, we can easily customize the error message for each property using resource files. However, currently we don't have a way to change the title property (One or more validation errors occurred.).

Expected Behavior

We should be able to change the value of the title property, for example to support multi-language scenarios.

.NET Version

10.0.100-preview.3.25201.16

[marcominerva]

@captainsafia, I'm thinking about an API proposal for this. What do you think about adding this property to ValidationOptions:

/// <summary>
/// Provides configuration options for the validation system.
/// </summary>
public class ValidationOptions
{
    /// <summary>
    /// Gets or sets the factory for creating the title property in the <see cref="ProblemDetails"/> validation error messages.
    /// </summary>
    /// <seealso cref="ProblemDetails"/>
    public Func<EndpointFilterInvocationContext, IDictionary<string, string[]>, string>? ValidationErrorTitleMessageFactory { get; set; }  
}

And then adding the following code in ValidationEndpointFilterFactory when creating the ProblemDetails response:

if (options.ValidationErrorTitleMessageFactory is not null)
{
    problemDetails.Title = options.ValidationErrorTitleMessageFactory(context, validateContext.ValidationErrors);
}

[captainsafia]

@marcominerva Now that we've supported the ProblemDetailsService, can we use that customization API to solve that problem?

[marcominerva]

Good catch, we can do something like this:

builder.Services.AddProblemDetails(options =>
{
    options.CustomizeProblemDetails = context =>
    {
        if (context.ProblemDetails is HttpValidationProblemDetails validationProblemDetails)
        {
            // Italian localization.
            validationProblemDetails.Title = "Si sono verificati errori di validazione sui dati";
        }
    };
});

Is this the solution you're thinking about?

[captainsafia]

@marcominerva Yep, it should also be possible to use the [ErrorMessage] attribute from data annotations to customize the error that is returned. In this case, it would modify the values that appear in the errors dictionary in the response. For localizing other problem details fields, like the title above, the ProblemDetailsService is the best way to go.

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