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[1] 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|0>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。
Unable to cast object of type 'System.Int32' to type 'System.String'.
原文：https://www.cnblogs.com/Godric/p/10111594.html