如何为外部库中定义的模型编写ASP.NET自定义验证器

using System.ComponentModel.DataAnnotations;
using ExternalLibrary; // 外部库的命名空间

public class ExternalModelValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var externalModel = (ExternalModel)validationContext.ObjectInstance;
        // 执行自定义验证规则
        if (externalModel.Value == "SomeInvalidValue")
        {
            return new ValidationResult(ErrorMessage ?? "Invalid value.");
        }

        return ValidationResult.Success;
    }
}

public class ExternalModel
{
    [ExternalModelValidator]
    public string Value { get; set; }
}

public class MvcApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        // 注册自定义验证器
        GlobalFilters.Filters.Add(new MyCustomValidatorAttribute());
    }
}