[OptionsValidator] Source Generator for [Range] Ignores ErrorMessage and DisplayName

Description

I have the below

    #pragma warning disable ASP0029 
    [ValidatableType]
    #pragma warning restore ASP0029
    public class WeatherForecast
    {
        public int? Id { get; set; }
        public required DateTime Date { get; set; }


        [Range(-20, 55, ErrorMessage = "Temperature must be between -20 and 55.")]
        public required int TemperatureC { get; set; }

        [Required(ErrorMessage = "Summary is required.")]
        public string? Summary { get; set; }

        public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);

        [ValidateObjectMembers]
        public SubWeatherForecast SubWeather { get; set; } = new();
    }

    #pragma warning disable ASP0029
    [ValidatableType]
    #pragma warning restore ASP0029
    public class SubWeatherForecast
    {
        [Required(ErrorMessage = "Name is required.")]
       public string? Name { get; set; }   
    }  

    [OptionsValidator]
    public partial class WeatherForecastValidator : IValidateOptions<WeatherForecast>
    {
    }

    [OptionsValidator]
    public partial class SubWeatherForecastValidator : IValidateOptions<SubWeatherForecast>
    {
    }

When this is triggered through Minimal API I get the correct error message of Temperature must be between -20 and 55.

However, when I execute through:

            var validator = new WeatherForecastValidator();

            var result = validator.Validate(null, weatherForecast);

The error message reverts to default "The field WeatherForecast.TemperatureC must be between -20 and 55."

When I do the same thing with [Required(ErrorMessage = "Summary is required.")] it works correctly.

In addition when validating with Minimal API default the key on the sub class is correctly SubWeather.Name but when processing with validator.Validate the key is cut off and it is only "Name"

Reproduction Steps

Use the Dot Net 10 template, use minimal api end point, include builder.Services.Validation(). On the end point .DisableValidation(), then

            var validator = new WeatherForecastValidator();
            var result = validator.Validate(null, weatherForecast);

Expected behavior

validator.Validate(null, weatherForecast); is processed in the same way that Minimal API is processed.

Actual behavior

The ErrorMessage in [Range] is not respected with validator.Validate and sub classes cut off the name of the key.

Regression?

No response

Known Workarounds

Don't use validator.Validate.

Configuration

No response

Other information

No response

Tagging subscribers to this area: @dotnet/area-system-componentmodel-dataannotations
See info in area-owners.md if you want to be subscribed.

Repro

#:sdk Microsoft.NET.Sdk.Web
#:property PublishAot=false

using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Options;
using Microsoft.Extensions.Validation;
using System.ComponentModel.DataAnnotations;

var builder = WebApplication.CreateBuilder();

builder.Services.AddValidation();

var app = builder.Build();

app.MapPost("", ([FromForm] WeatherForecast weatherForecast) =>
{
    return $"Forecast is '{weatherForecast.Summary}' with temp {weatherForecast.TemperatureC}C.";
}).DisableAntiforgery();

app.MapPost("validator", ([FromForm] WeatherForecast weatherForecast) =>
{
    var validator = new WeatherForecastValidator();
    var result = validator.Validate(null, weatherForecast);

    if (result.Succeeded)
    {
        return $"Forecast is '{weatherForecast.Summary}' with temp {weatherForecast.TemperatureC}C.";
    }
    else
    {
        return result.FailureMessage;
    }
}).DisableAntiforgery().DisableValidation();

app.Run();

#pragma warning disable ASP0029
[ValidatableType]
#pragma warning restore ASP0029
public class WeatherForecast
{
    public int? Id { get; set; }
    public required DateTime Date { get; set; }

    [Range(-20, 55, ErrorMessage = "Temperature must be between -20 and 55.")]
    public required int TemperatureC { get; set; }

    [Required(ErrorMessage = "Summary is required.")]
    public string? Summary { get; set; }

    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);

    [ValidateObjectMembers]
    public SubWeatherForecast SubWeather { get; set; } = new();
}

#pragma warning disable ASP0029
[ValidatableType]
#pragma warning restore ASP0029
public class SubWeatherForecast
{
    [Required(ErrorMessage = "Name is required.")]
    public string? Name { get; set; }
}

[OptionsValidator]
public partial class WeatherForecastValidator : IValidateOptions<WeatherForecast>
{
}

Tests

curl -d "Date=2000-01-01&TemperatureC=10&Summary=Cool" http://localhost:5000
Forecast is 'Cool' with temp 10C.

curl -d "Date=2000-01-01&TemperatureC=100&Summary=Cool" http://localhost:5000
{"title":"One or more validation errors occurred.","errors":{"TemperatureC":["Temperature must be between -20 and 55."]}}

curl -d "Date=2000-01-01&TemperatureC=10&Summary=Cool&SubWeather.Name=" http://localhost:5000
{"title":"One or more validation errors occurred.","errors":{"SubWeather.Name":["Name is required."]}}

curl -d "Date=2000-01-01&TemperatureC=10&Summary=Cool" http://localhost:5000/validator
Forecast is 'Cool' with temp 10C.

curl -d "Date=2000-01-01&TemperatureC=100&Summary=Cool&SubWeather.Name=Test" http://localhost:5000/validator
TemperatureC: The field WeatherForecast.TemperatureC must be between -20 and 55.

curl -d "Date=2000-01-01&TemperatureC=10&Summary=Cool&SubWeather.Name=" http://localhost:5000/validator
Name: Name is required.

Tagging subscribers to this area: @dotnet/area-extensions-options
See info in area-owners.md if you want to be subscribed.

For some reason, the source generator synthesizes its own version of the Range attribute, rather than relying on the one from System.ComponentModel.Annotations. The synthesized attribute ignores any custom error message, and always returns a default message:

namespace __OptionValidationGeneratedAttributes
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.SourceGeneration", "10.0.14.15411")]
    [global::System.AttributeUsage(global::System.AttributeTargets.Property | global::System.AttributeTargets.Field | global::System.AttributeTargets.Parameter, AllowMultiple = false)]
    file class __SourceGen__RangeAttribute : global::System.ComponentModel.DataAnnotations.ValidationAttribute
    {
        ...
        
        public override string FormatErrorMessage(string name) =>
            string.Format(global::System.Globalization.CultureInfo.CurrentCulture, GetValidationErrorMessage(), name, Minimum, Maximum);
        
        ...
        private string GetValidationErrorMessage()
        {
            return (MinimumIsExclusive, MaximumIsExclusive) switch
            {
                (false, false) => "The field {0} must be between {1} and {2}.",
                (true, false) => "The field {0} must be between {1} exclusive and {2}.",
                (false, true) => "The field {0} must be between {1} and {2} exclusive.",
                (true, true) => "The field {0} must be between {1} exclusive and {2} exclusive.",
            };
        }
        
        ...
    }
}

Searching this repo for __SourceGen__ suggests the problem also affects:

LengthAttribute
MinLengthAttribute
MaxLengthAttribute
CompareAttribute

It may affect other attributes as well, but those are the ones that come up in the test projects in this repo.

The documentation explains why the attributes are replaced - although it makes no mention of the CompareAttribute.

I'm guessing the custom error message is ignored because it could potentially end up using reflection - specifically, if you use the ErrorMessageResourceType, the SetResourceAccessorByPropertyLookup method has to use reflection to locate the property or method used to return the error message.

But it's slightly annoying that the synthesized attributes ignore hard-coded, non-localized error messages. Particularly when other validation attributes respect the custom error messages, and could even end up triggering the reflection code path.

I'd like to take the ErrorMessage part of this.

The generator replaces [Range] with a synthesized __SourceGen__RangeAttribute, and its FormatErrorMessage always uses the built-in message. So ErrorMessage and ErrorMessageResourceName/ErrorMessageResourceType are ignored. The fix is to use the built-in message only when none of them is set, as RangeAttribute does.

MaxLength, MinLength, Length and Compare already use ErrorMessageString, so only Range is affected.

The nested member name (SubWeather.Name reported as Name) is a separate change, and I'm leaving it out.

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[OptionsValidator] Source Generator for [Range] Ignores ErrorMessage and DisplayName #123137

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[OptionsValidator] Source Generator for [Range] Ignores ErrorMessage and DisplayName #123137

Description

Description

Reproduction Steps

Expected behavior

Actual behavior

Regression?

Known Workarounds

Configuration

Other information

Activity

dotnet-policy-service commented on Jan 13, 2026

mguinness commented on Jan 27, 2026

dotnet-policy-service commented on Mar 26, 2026

RichardD2 commented on Mar 26, 2026

RichardD2 commented on Mar 26, 2026

Laurianti commented on Oct 16, 2026

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