Fix options source gen with non-accessible validation attributes

[tarekgh]

The Options source generator creates code for validating an options class. This options class may be inherited from a base class located in a different assembly. It is also possible for the base class to have properties marked with the internal validation attribute, which means they should not be accessible outside the base class assembly. In such situations, the source generator should exclude the validation of these properties since it cannot generate code to access internal properties. Here's an example code illustrating this problem:

Assembly 1

        using System;
        using System.ComponentModel.DataAnnotations;

        #nullable enable

        namespace ValidationTest;

        public class BaseOptions
        {
            [Timeout] // internal attribute not visible outside the assembly
            public int Prop1 { get; set; }

            [Required]
            public string Prop2 { get; set; }
        }

        [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
        internal sealed class TimeoutAttribute : ValidationAttribute
        {
            protected override ValidationResult IsValid(object? value, ValidationContext? validationContext)
            {
                return ValidationResult.Success!;
            }
        }

Assembly 2 reference Assembly 1 and uses the source gen

        using Microsoft.Extensions.Options;
        using System.ComponentModel.DataAnnotations;

        #nullable enable
        #pragma warning disable CS1591

        namespace ValidationTest;
   
        public class ExtOptions : BaseOptions
        {
            [Range(0, 10)]
            public int Prop3 { get; set; }
         }

        [OptionsValidator]
        internal sealed partial class ExtOptionsValidator : IValidateOptions<ExtOptions>
        {
        }
    }

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Issue Details

---

The Options source generator creates code for validating an options class. This options class may be inherited from a base class located in a different assembly. It is also possible for the base class to have properties marked with the internal validation attribute, which means they should not be accessible outside the base class assembly. In such situations, the source generator should exclude the validation of these properties since it cannot generate code to access internal properties. Here's an example code illustrating this problem:

Assembly 1

            using System;
            using System.ComponentModel.DataAnnotations;

            #nullable enable

            namespace ValidationTest;

            public class BaseOptions
            {
                [Timeout] // internal attribute not visible outside the assembly
                public int Prop1 { get; set; }

                [Required]
                public string Prop2 { get; set; }
            }

            [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
            internal sealed class TimeoutAttribute : ValidationAttribute
            {
                protected override ValidationResult IsValid(object? value, ValidationContext? validationContext)
                {
                    return ValidationResult.Success!;
                }
            }

Assembly 2 reference Assembly 1 and uses the source gen

            using Microsoft.Extensions.Options;
            using System.ComponentModel.DataAnnotations;

            namespace ValidationTest;

            #nullable enable
            #pragma warning disable CS1591

            public class ExtOptions : BaseOptions
            {
                [Range(0, 10)]
                public int Prop3 { get; set; }
             }

            [OptionsValidator]
            internal sealed partial class ExtOptionsValidator : IValidateOptions<ExtOptions>
            {
            }
        }

Author:	tarekgh
Assignees:	tarekgh
Labels:	area-Extensions-Options
Milestone:	-

[tarekgh]

@xakep139 @geeknoid could you please have a look? Thanks!