[API Proposal]: ValidateRequiredMembers support for Microsoft.Extensions.Options

[lol768]

Background and motivation

Today, if I create a record with required properties for my configuration type, and bind from a configuration section to that record type, I still need to write my own IValidateOptions to do null-checks by hand, or I need to apply System.ComponentModel.DataAnnotations attributes to every member to duplicatively mark it as required.

Consider this concrete example:

public record MyConfigurationType
{
    public required string RequiredMember { get; init; }
}

services.AddOptions<MyConfigurationType>()
    .Bind(configuration.GetSection("MyConfiguration"))
    .ValidateOnStart();

{
  "MyConfiguration": {
    "foo": "bar"
  }
}

The application will start without any errors, despite RequiredMember not being set.

I can make it work by writing:

public class MyConfigurationTypeConfigurationValidation : IValidateOptions<MyConfigurationType>
{
    public ValidateOptionsResult Validate(string name, MyConfigurationType options)
    {
        if (options.RequiredMember == null)
        {
            return ValidateOptionsResult.Fail("RequiredMember must be provided");
        }

        return ValidateOptionsResult.Success;
    }
}

and then registering this IValidateOptions, but it's cumbersome to have to do this for every property.

You basically want something like this, that is generic:

public class MyConfigurationTypeConfigurationValidation : IValidateOptions<MyConfigurationType>
{
    public ValidateOptionsResult Validate(string name, MyConfigurationType options)
    {
        var type = options.GetType();
        foreach (var property in type.GetProperties()
                     .Where(p => p.GetCustomAttributes(typeof(RequiredMemberAttribute), false).Any()))
        {
            var value = property.GetValue(options);
            if (value == null)
            {
                return ValidateOptionsResult.Fail(
                    $"The property '{property.Name}' is required but was not provided.");
            }
        }

        return ValidateOptionsResult.Success;
    }
}

API Proposal

public static OptionsBuilder<TOptions> ValidateRequiredMembers<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.NonPublicProperties)] TOptions>(this OptionsBuilder<TOptions> optionsBuilder) where TOptions : class

API Usage

services.AddOptions<MyConfigurationType>()
    .Bind(configuration.GetSection("MyConfiguration"))
    .ValidateRequiredMembers() // looks for RequiredMemberAttribute
    .ValidateOnStart();

Alternative Designs

No response

Risks

No response

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Tagging subscribers to this area: @dotnet/area-extensions-configuration
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[lol768]

Quick implementation sketch:

public static OptionsBuilder<TOptions> ValidateRequiredMembers<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.NonPublicProperties)] TOptions>(this OptionsBuilder<TOptions> optionsBuilder) where TOptions : class
{
    optionsBuilder.Services.AddSingleton<IValidateOptions<TOptions>>(new RequiredMembersTypeConfigurationValidation<TOptions>());
    return optionsBuilder;
}

and

public class RequiredMembersTypeConfigurationValidation<TOptions> : IValidateOptions<TOptions> where TOptions : class
{
    public ValidateOptionsResult Validate(string? name, TOptions options)
    {
        var type = options.GetType();
        foreach (var property in type.GetProperties()
                     .Where(p => p.GetCustomAttributes(typeof(RequiredMemberAttribute), false).Any()))
        {
            var value = property.GetValue(options);
            if (value == null)
            {
                return ValidateOptionsResult.Fail(
                    $"The property '{property.Name}' is required but was not provided.");
            }
        }

        return ValidateOptionsResult.Success;
    }
}

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Tagging subscribers to this area: @dotnet/area-extensions-options
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