Unable to cast object of type 'System.Int32' to type 'System.String'.

最近在研究telerik，本来好好的，添加了一个可为空的Int32字段后总是报错，最初以为空字段需要单独处理些什么，但后来翻箱倒柜，终于从：https://www.cnblogs.com/Godric/p/10111594.html发现原来是model中写法错了，跟参考博客一样

System.InvalidCastException: The field of type System.Int32 must be a string, array or ICollection type.
   at System.ComponentModel.DataAnnotations.MaxLengthAttribute.IsValid(Object value)
   at System.ComponentModel.DataAnnotations.ValidationAttribute.IsValid(Object value, ValidationContext validationContext)
   at System.ComponentModel.DataAnnotations.ValidationAttribute.GetValidationResult(Object value, ValidationContext validationContext)
   at Microsoft.AspNetCore.Mvc.DataAnnotations.DataAnnotationsModelValidator.Validate(ModelValidationContext validationContext)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.ValidateNode()
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.VisitSimpleType()
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.Visit(ModelMetadata metadata, String key, Object model)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.VisitChildren(IValidationStrategy strategy)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.VisitComplexType(IValidationStrategy defaultStrategy)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.Visit(ModelMetadata metadata, String key, Object model)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Validation.ValidationVisitor.Validate(ModelMetadata metadata, String key, Object model, Boolean alwaysValidateAtTopLevel)
   at Microsoft.AspNetCore.Mvc.ModelBinding.ObjectModelValidator.Validate(ActionContext actionContext, ValidationStateDictionary validationState, String prefix, Object model, ModelMetadata metadata)
   at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.EnforceBindRequiredAndValidate(ObjectModelValidator baseObjectValidator, ActionContext actionContext, ParameterDescriptor parameter, ModelMetadata metadata, ModelBindingContext modelBindingContext, ModelBindingResult modelBindingResult)
   at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.BindModelAsync(ActionContext actionContext, IModelBinder modelBinder, IValueProvider valueProvider, ParameterDescriptor parameter, ModelMetadata metadata, Object value)
   at Microsoft.AspNetCore.Mvc.Controllers.ControllerBinderDelegateProvider.<>c__DisplayClass0_0.<<CreateBinderDelegate>g__Bind|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

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　关键错误点Unable to cast object of type 'System.Int32' to type 'System.String'.提示是无法将“System.Int32”的对象强制转换为“System.String”。于是我就找了所有使用Int的地方，终于发现

        /// <summary>
        /// 排序数字
        /// </summary>
        //[MaxLength(10)]这个不能加，自己做的代码生成器添加的
        public Int32? SortId {get;set;}

在一个基类中有个Int类型的ID，不小心加上了MaxLength属性。int类型是不能添加MaxLength、StringLength属性的，去掉之后就可以正常访问了。