C# Validate object recursively. Manual validation of property object.

CompositeValidationResult.cs

public class CompositeValidationResult : ValidationResult
{
    private readonly List<ValidationResult> _results = new();

    public CompositeValidationResult(string errorMessage) : base(errorMessage)
    {
    }

    public CompositeValidationResult(string errorMessage, IEnumerable<string> memberNames) : base(errorMessage,
        memberNames)
    {
    }

    protected CompositeValidationResult(ValidationResult validationResult) : base(validationResult)
    {
    }

    public IEnumerable<ValidationResult> Results => _results;

    public void AddResult(ValidationResult validationResult)
    {
        _results.Add(validationResult);
    }
}

ValidateObjectAttribute.cs

public class ValidateObjectAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null) return ValidationResult.Success;
        var results = new List<ValidationResult>();

        if (value is not IEnumerable<object> list)
        {
            var context = new ValidationContext(value, null, null);
            Validator.TryValidateObject(value, context, results, true);
        }
        else
        {
            foreach (var o in list)
            {
                var context = new ValidationContext(o, null, null);
                Validator.TryValidateObject(o, context, results, true);
            }
        }

        if (results.Count == 0) return ValidationResult.Success;

        var compositeResults =
            new CompositeValidationResult(
                $"{validationContext.DisplayName} {string.Join("; ", results.Select(r => r.ErrorMessage))}");
        results.ForEach(compositeResults.AddResult);

        return compositeResults;
    }
}