自定义Unique校验属性中ValidationContext始终为Null的解决方法

问题原因 #

validationContext为Null通常是因为验证触发时未传入依赖注入上下文：要么是手动调用验证方法时省略了ValidationContext参数，要么是验证流程未在ASP.NET Core的依赖注入容器环境中执行。即使你设置了RequiresValidationContext = true，如果验证触发方式不正确，框架也不会传入上下文。

核心修复步骤 #

1. 完善UniqueAttribute的实现（确保能正确获取上下文与服务）

修改自定义Attribute，通过validationContext解析DbContext实现唯一性校验，同时明确指定要校验的实体和属性：

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
public class UniqueAttribute : ValidationAttribute
{
    private readonly Type _dbContextType;
    private readonly string _entitySetName;
    private readonly string _targetPropertyName;

    // 构造函数：指定DbContext类型、实体集合名称、要校验的属性名
    public UniqueAttribute(Type dbContextType, string entitySetName, string targetPropertyName)
    {
        _dbContextType = dbContextType;
        _entitySetName = entitySetName;
        _targetPropertyName = targetPropertyName;
        ErrorMessage = $"该{_targetPropertyName}已被占用";
    }

    public override bool RequiresValidationContext => true;

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null)
            return ValidationResult.Success;

        // 从validationContext获取DbContext实例
        var dbContext = validationContext.GetService(_dbContextType);
        if (dbContext == null)
            return new ValidationResult("无法获取数据库上下文");

        // 获取对应的实体集合
        var dbSet = dbContext.GetType().GetProperty(_entitySetName)?.GetValue(dbContext) as IQueryable;
        if (dbSet == null)
            return new ValidationResult($"找不到实体集合{_entitySetName}");

        // 通过表达式树构建查询，检查值是否存在
        var entityType = dbSet.ElementType;
        var parameter = Expression.Parameter(entityType, "entity");
        var property = Expression.Property(parameter, _targetPropertyName);
        var constantValue = Expression.Constant(value);
        var equalsExpr = Expression.Equal(property, constantValue);
        var lambda = Expression.Lambda(equalsExpr, parameter);

        var anyMethod = typeof(Queryable).GetMethods()
            .First(m => m.Name == "Any" && m.GetParameters().Length == 2)
            .MakeGenericMethod(entityType);

        var exists = (bool)anyMethod.Invoke(null, new object[] { dbSet, lambda });

        return exists ? new ValidationResult(ErrorMessage) : ValidationResult.Success;
    }
}

2. 正确使用Attribute

在模型类中指定DbContext、实体和属性信息：

public class Category
{
    // 传入ApplicationDbContext类型、实体集合名、要校验的属性名
    [Unique(typeof(ApplicationDbContext), nameof(Category), nameof(Name))]
    public string Name { get; set; }
    public string Description { get; set; }
}

3. 确保验证在依赖注入环境中触发

• MVC/API场景：确保控制器标记了[ApiController]属性，ASP.NET Core会自动在模型绑定后触发验证，并传入完整的ValidationContext。
• 手动验证场景：如果需要手动验证模型，必须手动创建ValidationContext并传入服务提供者：

  // 从依赖注入容器获取IServiceProvider
  var serviceProvider = ...; 
  var category = new Category { Name = "测试分类" };
  var validationContext = new ValidationContext(category, serviceProvider, null);
  var validationResults = new List<ValidationResult>();
  var isValid = Validator.TryValidateObject(category, validationContext, validationResults, validateAllProperties: true);
  

• EF Core保存时验证：如果要在SaveChanges时触发验证，需要在DbContext中手动创建ValidationContext：

  public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
  {
      var serviceProvider = this.GetService<IServiceProvider>();
      foreach (var entry in ChangeTracker.Entries()
          .Where(e => e.State is EntityState.Added or EntityState.Modified))
      {
          var validationContext = new ValidationContext(entry.Entity, serviceProvider, null);
          Validator.ValidateObject(entry.Entity, validationContext, validateAllProperties: true);
      }
      return await base.SaveChangesAsync(cancellationToken);
  }
  

内容的提问来源于stack exchange，提问作者Mahesh Kantariya