[API Proposal]: Introduce a code generator to handle option validation

[geeknoid]

Background and motivation

In my current project, we have many option types which need to be validated on startup. To reduce startup overhead and improve validation feature set, we've implemented a source code generator that implements the validation logic. This is a general-purpose mechanism which could benefit the broader community.

API Proposal

namespace Microsoft.Extensions.Options;

/// <summary>
/// Triggers the automatic generation of the implementation of <see cref="Microsoft.Extensions.Options.IValidateOptions{T}" /> at compile time.
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class OptionsValidatorAttribute : Attribute
{
}

/// <summary>
/// Marks a field or property to be enumerated, and each enumerated object to be validated.
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class ValidateEnumerableAttribute :  ValidationAttribute
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateEnumerableAttribute"/> class.
    /// </summary>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to
    /// generate validation for the individual members of the enumerable's type.
    /// </remarks>
    public ValidateEnumerableAttribute();

    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateEnumerableAttribute"/> class.
    /// </summary>
    /// <param name="validator">A type that implements <see cref="IValidateOptions{T}" /> for the enumerable's type.</param>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to use the given type to validate
    /// the object held by the enumerable.
    /// </remarks>
    public ValidateEnumerableAttribute(Type validator);

    /// <summary>
    /// Gets the type to use to validate the enumerable's objects.
    /// </summary>
    public Type? Validator { get; }
}

/// <summary>
/// Marks a field or property to be validated transitively.
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class ValidateTransitivelyAttribute :  ValidationAttribute
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateTransitivelyAttribute"/> class.
    /// </summary>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to
    /// generate validation for the individual members of the field/property's type.
    /// </remarks>
    public ValidateTransitivelyAttribute();

    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateTransitivelyAttribute"/> class.
    /// </summary>
    /// <param name="validator">A type that implements <see cref="IValidateOptions{T}" /> for the field/property's type.</param>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to use the given type to validate
    /// the object held by the field/property.
    /// </remarks>
    public ValidateTransitivelyAttribute(Type validator);

    /// <summary>
    /// Gets the type to use to validate a field or property.
    /// </summary>
    public Type? Validator { get; }
}

API Usage

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using Microsoft.Extensions.Options;
using Microsoft.R9.Extensions.Options.Validation;

namespace Foo;

public class MyOptions
{
    [Required]
    public string Name { get; set; } = string.Empty;

    [ValidateTransitively]
    public NestedOptions? Nested { get; set; }

    [ValidateEnumerable]
    public IList<AccountOptions> Accounts { get; set; }
}

public class NestedOptions
{
    [Range(0, 10)]
    public int Value { get; set; }
}

public class AccountOptions
{
    [Required]
    public string Username { get; set; }
}

[OptionsValidator]
internal sealed partial class MyOptionsValidator : IValidateOptions<MyOptions>
{
    // the implementation of this class is generated
}

[OptionsValidator]
internal sealed partial class NestedOptionsValidator : IValidateOptions<NestedOptions>
{
    // the implementation of this class is generated
}

[OptionsValidator]
internal sealed partial class AccountOptionsValidator : IValidateOptions<AccountOptions>
{
    // the implementation of this class is generated
}

//
// The app will need to register the IValidateOptions<MyOptions> interface to the DI to enable the generated validation. Here is some example 
//

using Microsoft.Extensions.Options;
using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.Configure<MyOptions>(builder.Configuration.GetSection(...));

builder.Services.AddSingleton<IValidateOptions<MyOptions>, MyOptionsValidator>();

With the above, the generated validator ensures that Name is specified, will perform transitive validation of the NestedOptions value, and will perform validation on all AccountOptions instances. Here's an example of the code that generator might produce:

// <auto-generated/>
#nullable enable
#pragma warning disable CS1591 // Compensate for https://github.com/dotnet/roslyn/issues/54103
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
    internal sealed partial class __AccountOptionsValidator__
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public static global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.AccountOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "AccountOptions" : name) + ".";
            var builder = global::Microsoft.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Username";
            context.DisplayName = baseName + "Username";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Username, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
    internal sealed partial class __NestedOptionsValidator__
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public static global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.NestedOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "NestedOptions" : name) + ".";
            var builder = global::Microsoft.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Value";
            context.DisplayName = baseName + "Value";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A2.GetValidationResult(options.Value, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class AccountOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.AccountOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "AccountOptions" : name) + ".";
            var builder = global::Microsoft.xtensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Username";
            context.DisplayName = baseName + "Username";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Username, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class MyOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.MyOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "MyOptions" : name) + ".";
            var builder = global::Microsoft.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Name";
            context.DisplayName = baseName + "Name";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Name, context));

            if (options.Nested != null)
            {
                builder.AddErrors(global::Foo.__NestedOptionsValidator__.Validate(baseName + "Nested", options.Nested));
            }

            if (options.Accounts != null)
            {
                var count = 0;
                foreach (var o in options.Accounts)
                {
                    if (o is not null)
                    {
                        builder.AddErrors(global::Foo.__AccountOptionsValidator__.Validate(baseName + $"Accounts[{count++}]", o));
                    }
                    else
                    {
                        builder.AddError(baseName + $"Accounts[{count++}] is null");
                    }
                }
            }

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class NestedOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.NestedOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "NestedOptions" : name) + ".";
            var builder = global::Microsoft.Extensions.Options.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Value";
            context.DisplayName = baseName + "Value";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A2.GetValidationResult(options.Value, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Microsoft.Extensions.Options.ClusterMetadata.ServiceFabric
{
    partial class ServiceFabricMetadataValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Microsoft.Extensions.Options.ClusterMetadata.ServiceFabric.ServiceFabricMetadata options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "ServiceFabricMetadata" : name) + ".";
            var builder = global::Microsoft.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "ServiceName";
            context.DisplayName = baseName + "ServiceName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.ServiceName, context));

            context.MemberName = "ReplicaOrInstanceId";
            context.DisplayName = baseName + "ReplicaOrInstanceId";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A3.GetValidationResult(options.ReplicaOrInstanceId, context));

            context.MemberName = "ApplicationName";
            context.DisplayName = baseName + "ApplicationName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.ApplicationName, context));

            context.MemberName = "NodeName";
            context.DisplayName = baseName + "NodeName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.NodeName, context));

            return builder.Build();
        }
    }
}
namespace __OptionValidationStaticInstances
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
    internal static class __Attributes
    {
        internal static readonly global::System.ComponentModel.DataAnnotations.RequiredAttribute A1 = new global::System.ComponentModel.DataAnnotations.RequiredAttribute();

        internal static readonly global::System.ComponentModel.DataAnnotations.RangeAttribute A2 = new global::System.ComponentModel.DataAnnotations.RangeAttribute(
            (int)0,
            (int)10);

        internal static readonly global::System.ComponentModel.DataAnnotations.RangeAttribute A3 = new global::System.ComponentModel.DataAnnotations.RangeAttribute(
            (double)0,
            (double)9.223372036854776E+18);
    }
}
namespace __OptionValidationStaticInstances
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Extensions.Options.Generators", "1.0.0.0")]
    internal static class __Validators
    {
    }
}

Note that the generated code shown here depends on #77404

Alternative Designs

No response

Risks

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

---

Background and motivation

In my current project, we have many option types which need to be validated on startup. To reduce startup overhead and improve validation feature set, we've implemented a source code generator that implements the validation logic. This is a general-purpose mechanism which could benefit the broader community.

API Proposal

namespace Microsoft.Extensions.Options;

/// <summary>
/// Triggers the automatic generation of the implementation of <see cref="Microsoft.Extensions.Options.IValidateOptions{T}" /> at compile time.
/// </summary>
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class OptionsValidatorAttribute : Attribute
{
}

/// <summary>
/// Marks a field or property to be enumerated, and each enumerated object to be validated.
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class ValidateEnumerableAttribute : Attribute
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateEnumerableAttribute"/> class.
    /// </summary>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to
    /// generate validation for the individual members of the enumerable's type.
    /// </remarks>
    public ValidateEnumerableAttribute();

    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateEnumerableAttribute"/> class.
    /// </summary>
    /// <param name="validator">A type that implements <see cref="IValidateOptions{T}" /> for the enumerable's type.</param>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to use the given type to validate
    /// the object held by the enumerable.
    /// </remarks>
    public ValidateEnumerableAttribute(Type validator);

    /// <summary>
    /// Gets the type to use to validate the enumerable's objects.
    /// </summary>
    public Type? Validator { get; }
}

/// <summary>
/// Marks a field or property to be validated transitively.
/// </summary>
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
[Conditional("CODE_GENERATION_ATTRIBUTES")]
public sealed class ValidateTransitivelyAttribute : Attribute
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateTransitivelyAttribute"/> class.
    /// </summary>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to
    /// generate validation for the individual members of the field/property's type.
    /// </remarks>
    public ValidateTransitivelyAttribute();

    /// <summary>
    /// Initializes a new instance of the <see cref="ValidateTransitivelyAttribute"/> class.
    /// </summary>
    /// <param name="validator">A type that implements <see cref="IValidateOptions{T}" /> for the field/property's type.</param>
    /// <remarks>
    /// Using this constructor for a field/property tells the code generator to use the given type to validate
    /// the object held by the field/property.
    /// </remarks>
    public ValidateTransitivelyAttribute(Type validator);

    /// <summary>
    /// Gets the type to use to validate a field or property.
    /// </summary>
    public Type? Validator { get; }
}

API Usage

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using Microsoft.Extensions.Options;
using Microsoft.R9.Extensions.Options.Validation;

namespace Foo;

public class MyOptions
{
    [Required]
    public string Name { get; set; } = string.Empty;

    [ValidateTransitively]
    public NestedOptions? Nested { get; set; }

    [ValidateEnumerable]
    public IList<AccountOptions> Accounts { get; set; }
}

public class NestedOptions
{
    [Range(0, 10)]
    public int Value { get; set; }
}

public class AccountOptions
{
    [Required]
    public string Username { get; set; }
}

[OptionsValidator]
internal sealed partial class MyOptionsValidator : IValidateOptions<MyOptions>
{
    // the implementation of this class is generated
}

[OptionsValidator]
internal sealed partial class NestedOptionsValidator : IValidateOptions<NestedOptions>
{
    // the implementation of this class is generated
}

[OptionsValidator]
internal sealed partial class AccountOptionsValidator : IValidateOptions<AccountOptions>
{
    // the implementation of this class is generated
}

With the above, the generated validator ensures that Name is specified, will perform transitive validation of the NestedOptions value, and will perform validation on all AccountOptions instances. Here's an example of the code that generator might produce:

// <auto-generated/>
#nullable enable
#pragma warning disable CS1591 // Compensate for https://github.com/dotnet/roslyn/issues/54103
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
    internal sealed partial class __AccountOptionsValidator__
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public static global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.AccountOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "AccountOptions" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Username";
            context.DisplayName = baseName + "Username";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Username, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
    internal sealed partial class __NestedOptionsValidator__
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public static global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.NestedOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "NestedOptions" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Value";
            context.DisplayName = baseName + "Value";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A2.GetValidationResult(options.Value, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class AccountOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.AccountOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "AccountOptions" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Username";
            context.DisplayName = baseName + "Username";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Username, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class MyOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.MyOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "MyOptions" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Name";
            context.DisplayName = baseName + "Name";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.Name, context));

            if (options.Nested != null)
            {
                builder.AddErrors(global::Foo.__NestedOptionsValidator__.Validate(baseName + "Nested", options.Nested));
            }

            if (options.Accounts != null)
            {
                var count = 0;
                foreach (var o in options.Accounts)
                {
                    if (o is not null)
                    {
                        builder.AddErrors(global::Foo.__AccountOptionsValidator__.Validate(baseName + $"Accounts[{count++}]", o));
                    }
                    else
                    {
                        builder.AddError(baseName + $"Accounts[{count++}] is null");
                    }
                }
            }

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Foo
{
    partial class NestedOptionsValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Foo.NestedOptions options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "NestedOptions" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "Value";
            context.DisplayName = baseName + "Value";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A2.GetValidationResult(options.Value, context));

            return builder.Build();
        }
    }
}
#pragma warning disable CS0618 // Type or member is obsolete
namespace Microsoft.R9.Extensions.ClusterMetadata.ServiceFabric
{
    partial class ServiceFabricMetadataValidator
    {
        /// <summary>
        /// Validates a specific named options instance (or all when <paramref name="name"/> is <see langword="null" />).
        /// </summary>
        /// <param name="name">The name of the options instance being validated.</param>
        /// <param name="options">The options instance.</param>
        /// <returns>Validation result.</returns>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
        public global::Microsoft.Extensions.Options.ValidateOptionsResult Validate(string? name, global::Microsoft.R9.Extensions.ClusterMetadata.ServiceFabric.ServiceFabricMetadata options)
        {
            var baseName = (string.IsNullOrEmpty(name) ? "ServiceFabricMetadata" : name) + ".";
            var builder = global::Microsoft.R9.Extensions.Options.Validation.ValidateOptionsResultBuilder.Create();
            var context = new global::System.ComponentModel.DataAnnotations.ValidationContext(options);

            context.MemberName = "ServiceName";
            context.DisplayName = baseName + "ServiceName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.ServiceName, context));

            context.MemberName = "ReplicaOrInstanceId";
            context.DisplayName = baseName + "ReplicaOrInstanceId";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A3.GetValidationResult(options.ReplicaOrInstanceId, context));

            context.MemberName = "ApplicationName";
            context.DisplayName = baseName + "ApplicationName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.ApplicationName, context));

            context.MemberName = "NodeName";
            context.DisplayName = baseName + "NodeName";
            builder.AddError(global::__OptionValidationStaticInstances.__Attributes.A1.GetValidationResult(options.NodeName, context));

            return builder.Build();
        }
    }
}
namespace __OptionValidationStaticInstances
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
    internal static class __Attributes
    {
        internal static readonly global::System.ComponentModel.DataAnnotations.RequiredAttribute A1 = new global::System.ComponentModel.DataAnnotations.RequiredAttribute();

        internal static readonly global::System.ComponentModel.DataAnnotations.RangeAttribute A2 = new global::System.ComponentModel.DataAnnotations.RangeAttribute(
            (int)0,
            (int)10);

        internal static readonly global::System.ComponentModel.DataAnnotations.RangeAttribute A3 = new global::System.ComponentModel.DataAnnotations.RangeAttribute(
            (double)0,
            (double)9.223372036854776E+18);
    }
}
namespace __OptionValidationStaticInstances
{
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.R9.Generators", "1.0.0.0")]
    internal static class __Validators
    {
    }
}

Note that the generated code shown here depends on #77404

Alternative Designs

No response

Risks

No response

Author:	geeknoid
Assignees:	-
Labels:	api-suggestion, untriaged, area-Extensions-Options
Milestone:	-

[eerhardt]

Would it make sense to define this source generator at the System.ComponentModel.DataAnnotations library level? That way other libraries using DataAnnotations (e.g. ASP.NET model validation) can take advantage of it as well.

[tarekgh]

Would that need to introduce a dependency on Microsoft.Extensions.Options from System.ComponentModel.DataAnnotations?

[eerhardt]

Would that need to introduce a dependency on Microsoft.Extensions.Options from System.ComponentModel.DataAnnotations?

No, that isn't the right layering. There is an Microsoft.Extensions.Options.DataAnnotations library that has a dependency on System.ComponentModel.DataAnnotations.

My thinking is that the core of this validation source generator would be written in terms of System.ComponentModel.DataAnnotations, then each API that uses DataAnnotations underneath (MS.Ext.Options, ASP.NET, etc.) could reuse this core validation source generator logic.

[DamianEdwards]

We should consider having the generator emit code that uses the Validator.TryValidateValues method, rather than using the ValidationAttribute.GetValidationResult method on each relevant attribute. This is to ensure the two-phase validation logic that first checks for RequiredAttribute is always run (which is contained in Validator).

Additionally, we should ensure the generated code is linker and native AOT friendly. This could be as simple as suppressing the warning from ValidationContext(object instance) as the generated code will root the instance type anyway, and the conditional reflection that both ValidationAttribute.GetValidationResult and Validator.TryValidateValues can end up performing to discover DisplayAttribute on validated members if ValidationContext.MemberName is set will be avoided as ValidationContext.DisplayName is being set as well which disables that path.

[tarekgh]

@geeknoid

[OptionsValidator]
internal sealed partial class MyOptionsValidator : IValidateOptions<MyOptions>
{
    // the implementation of this class is generated
}

Is it anticipated that users will ever provide an implementation within classes designated with the OptionsValidator attribute? I ask because it seems peculiar to require users to provide an empty class every time just to generate the implementation code for the IValidateOptions interface.

[geeknoid]

@tarekgh No, in general there isn't anything particularly useful you can put as implementation logic in those classes. I surveyed our source base and out of 163 uses of the attribute, only one had a single constant defined in the class, which is used elsewhere in the project.

When originally designing this stuff, I had it so it was possible to provide some hand-written validation logic that the generator code would invoke. But then it turned out to be easier to just capture that in completely different validator types, so I took that feature out of the generator.

The generator needs four bits of state:

• The type to validate
• The name of the generated type
• The namespace and/or parent type of the generated type
• The visibility modifier of the generated type

You can imagine this attribute pattern instead of the current approach:

[assembly: CreateOptionsValidator(typeof(MyOptions), Name = "Namespace1.Namespace2.MyParentType+MyOptionsValidator", Visibility="internal")]

Not great, but it could get better with some reasonable defaults:

• The generated type will default to the same namespace, parent type, and name as the type being validated, with a Validator suffix tacked on.
• The generated type will default to "internal" visibility.
• We use generic attributes.

Then you'd get:

[assembly: CreateOptionsValidator<MyOptions>]

which is indeed concise. If the validator is defined in the same assembly as the option type being validated (the 99.99% case), then you could also support annotating the option type itself:

[CreateOptionsValidator]
public class MyOptions
{
}

Which is even more concise.