Fluent, immutable builder for ValidationProblemDetails

ValidationProblemDetailsBuilder.cs

internal class ValidationProblemDetailsBuilder
{
    private readonly List<Action<ModelStateDictionary>> _modelStateActions;
    private readonly List<Action<ValidationProblemDetails>> _problemActions;

    public ValidationProblemDetailsBuilder(): this (Array.Empty<Action<ModelStateDictionary>>(), Array.Empty<Action<ValidationProblemDetails>>())
    {
    }

    private ValidationProblemDetailsBuilder(IEnumerable<Action<ModelStateDictionary>> modelStateActions, IEnumerable<Action<ValidationProblemDetails>> problemActions)
    {
        _modelStateActions = new List<Action<ModelStateDictionary>>(modelStateActions);
        _problemActions = new List<Action<ValidationProblemDetails>>(problemActions);
    }

    public ValidationProblemDetails Build()
    {
        var modelState = new ModelStateDictionary();
        foreach (var action in _modelStateActions)
        {
            action(modelState);
        }

        var problemDetails = new ValidationProblemDetails(modelState);
        foreach (var action in _problemActions)
        {
            action(problemDetails);
        }

        return problemDetails;
    }

    public ValidationProblemDetailsBuilder WithStatusCode(HttpStatusCode statusCode) =>
        AddAction(problemDetails => problemDetails.Status = (int) statusCode);

    public ValidationProblemDetailsBuilder WithModelError(string key, string modelError) =>
        AddAction(modelState => modelState.AddModelError(key, modelError));

    // Since this is an immutable builder, concatenate the action when constructing the new builder
    private ValidationProblemDetailsBuilder AddAction(Action<ValidationProblemDetails> action) => new(_modelStateActions, _problemActions.Concat(new[] {action}));

    // Since this is an immutable builder, concatenate the action when constructing the new builder
    private ValidationProblemDetailsBuilder AddAction(Action<ModelStateDictionary> action) => new(_modelStateActions.Concat(new[] {action}), _problemActions);
}