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

时间：2023-12-29 08:47:44

最近在研究.netcore，尝试把前后端完全分离。但是在写接口的时候，Post参数是FromBody的时候报错了

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[]

      An unhandled exception has occurred while executing the request.

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

   at System.ComponentModel.DataAnnotations.StringLengthAttribute.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.Internal.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.Internal.ControllerBinderDelegateProvider.<>c__DisplayClass0_0.<<CreateBinderDelegate>g__Bind|>d.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()

   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()

   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)

   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)

   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()

   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()

   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)

   at Microsoft.AspNetCore.Cors.Infrastructure.CorsMiddleware.Invoke(HttpContext context)

   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

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>

        /// ID

        /// </summary>

        [Key]

        [StringLength(20)]

	[DatabaseGenerated(DatabaseGeneratedOption.Identity)]

	public override int ID { get; set; }

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

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