使用带有自定义验证器的数据库

但是我的ApplicationDbContext总是空的！

您可以在此处参阅官方文件。已经说过ValidationContext.GetService为空。不支持在IsValid方法中注入用于验证的服务。

对于您的场景，您需要首先阅读答案，学习如何将IServiceProvider传递给ValidationContext。

详细演示：

1. 自定义DataAnnotationsValidator

   public class DIDataAnnotationsValidator: DataAnnotationsValidator
   {
   [CascadingParameter] EditContext DICurrentEditContext { get; set; }
   [Inject]
   protected IServiceProvider ServiceProvider { get; set; }
   protected override void OnInitialized()
   {
   if (DICurrentEditContext == null)
   {
   throw new InvalidOperationException($"{nameof(DataAnnotationsValidator)} requires a cascading " +
   $"parameter of type {nameof(EditContext)}. For example, you can use {nameof(DataAnnotationsValidator)} " +
   $"inside an EditForm.");
   }
   DICurrentEditContext.AddDataAnnotationsValidationWithDI(ServiceProvider);
   }
   }
   

2. 自定义EditContextDataAnnotationsExtensions

   public static class EditContextDataAnnotationsExtensions
   {
   private static ConcurrentDictionary<(Type ModelType, string FieldName), PropertyInfo> _propertyInfoCache
   = new ConcurrentDictionary<(Type, string), PropertyInfo>();
   public static EditContext AddDataAnnotationsValidationWithDI(this EditContext editContext, IServiceProvider serviceProvider)
   {
   if (editContext == null)
   {
   throw new ArgumentNullException(nameof(editContext));
   }
   var messages = new ValidationMessageStore(editContext);
   // Perform object-level validation on request
   editContext.OnValidationRequested +=
   (sender, eventArgs) => ValidateModel((EditContext)sender, serviceProvider, messages);
   // Perform per-field validation on each field edit
   editContext.OnFieldChanged +=
   (sender, eventArgs) => ValidateField(editContext, serviceProvider, messages, eventArgs.FieldIdentifier);
   return editContext;
   }
   private static void ValidateModel(EditContext editContext, IServiceProvider serviceProvider,ValidationMessageStore messages)
   {
   var validationContext = new ValidationContext(editContext.Model, serviceProvider, null);
   var validationResults = new List<ValidationResult>();
   Validator.TryValidateObject(editContext.Model, validationContext, validationResults, true);
   // Transfer results to the ValidationMessageStore
   messages.Clear();
   foreach (var validationResult in validationResults)
   {
   foreach (var memberName in validationResult.MemberNames)
   {
   messages.Add(editContext.Field(memberName), validationResult.ErrorMessage);
   }
   }
   editContext.NotifyValidationStateChanged();
   }
   private static void ValidateField(EditContext editContext, IServiceProvider serviceProvider, ValidationMessageStore messages, in FieldIdentifier fieldIdentifier)
   {
   if (TryGetValidatableProperty(fieldIdentifier, out var propertyInfo))
   {
   var propertyValue = propertyInfo.GetValue(fieldIdentifier.Model);
   var validationContext = new ValidationContext(fieldIdentifier.Model, serviceProvider, null)
   {
   MemberName = propertyInfo.Name
   };
   var results = new List<ValidationResult>();
   Validator.TryValidateProperty(propertyValue, validationContext, results);
   messages.Clear(fieldIdentifier);
   messages.Add(fieldIdentifier, results.Select(result => result.ErrorMessage));
   // We have to notify even if there were no messages before and are still no messages now,
   // because the "state" that changed might be the completion of some async validation task
   editContext.NotifyValidationStateChanged();
   }
   }
   private static bool TryGetValidatableProperty(in FieldIdentifier fieldIdentifier, out PropertyInfo propertyInfo)
   {
   var cacheKey = (ModelType: fieldIdentifier.Model.GetType(), fieldIdentifier.FieldName);
   if (!_propertyInfoCache.TryGetValue(cacheKey, out propertyInfo))
   {
   // DataAnnotations only validates public properties, so that's all we'll look for
   // If we can't find it, cache 'null' so we don't have to try again next time
   propertyInfo = cacheKey.ModelType.GetProperty(cacheKey.FieldName);
   // No need to lock, because it doesn't matter if we write the same value twice
   _propertyInfoCache[cacheKey] = propertyInfo;
   }
   return propertyInfo != null;
   }
   }
   

3. 用DIDataAnnotationsValidator替换DataAnnotationsValidator

   <EditForm Model="@book" >
   <DIDataAnnotationsValidator />   //change here
   <ValidationSummary />
   <div class="row content">
   <div class="col-md-2"><label for="Name">Name</label></div>
   <div class="col-md-3"><InputText id="name" @bind-Value="book.UserName" /></div>
   <ValidationMessage For=" (() => book.UserName)" />
   </div>  
   <div class="row content">
   <button type="submit">Submit</button>
   </div>
   </EditForm>
   @code {
   Booking book= new Booking();
   }
   

4. 然后您可以使用自定义的验证属性：

   public class LandlordNameIsUniqueValidator : ValidationAttribute
   {
   protected override ValidationResult IsValid(object value, ValidationContext validationContext)
   {
   //This is always null
   var context = (LykosqlContext)validationContext.GetService(typeof(LykosqlContext));
   var checkName = new LandlordData(context);
   var name = value.ToString();
   var nameExists = checkName.CheckNameIsUnique(name);
   
   return new ValidationResult(ErrorMessage, new[] { validationContext.MemberName });
   }
   }
   

5. 型号设计：

   public class Booking
   {
   public int Id { get; set; }
   [LandlordNameIsUniqueValidator]
   public string UserName { get; set; }
   }