Swagger使用的时候报错：Failed to load API definition

NuGet添加Swashbuckle.AspNetCore，在Startup.cs添加和启用中间件Swagger

public void ConfigureServices(IServiceCollection services)
        {
            #region Swagger
            services.AddSwaggerGen(c =>
            {
                c.SwaggerDoc("v1", new Info
                {
                    Version = "v1.0.0",
                    Title = "My Web API",
                    Description = "说明文档",
                    TermsOfService = "None",
                    Contact = new Swashbuckle.AspNetCore.Swagger.Contact { Name = "My.Web", Email = "My.Web@xxx.com", Url = "https://www.cnblogs.com/Zev_Fung/" }
                });
            });
            #endregion
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            #region Swagger
            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", "My Web API V1");
            });
            #endregion
        }

使用自带的Kestrel web调试，输入地址：http://localhost:<port>/swagger，默认跳转到https://localhost:<port>/swagger/index.html

Swagger提示报错:

Failed to load API definition.
Errors
Fetch errorInternal Server Error /swagger/v1/swagger.json

打开http://localhost:<port>/swagger/v1/swagger.json，提示错误

An unhandled exception occurred while processing the request.
NotSupportedException: Ambiguous HTTP method for action - xxxxx.Controllers.BooksController.Post (xxxxx). Actions require an explicit HttpMethod binding for Swagger 2.0
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.CreatePathItem(IEnumerable<ApiDescription> apiDescriptions, ISchemaRegistry schemaRegistry)

Stack Query Cookies Headers
NotSupportedException: Ambiguous HTTP method for action - xxxxx.Controllers.BooksController.Post (xxxxx). Actions require an explicit HttpMethod binding for Swagger 2.0
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.CreatePathItem(IEnumerable<ApiDescription> apiDescriptions, ISchemaRegistry schemaRegistry)
System.Linq.Enumerable.ToDictionary<TSource, TKey, TElement>(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.CreatePathItems(IEnumerable<ApiDescription> apiDescriptions, ISchemaRegistry schemaRegistry)
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwagger(string documentName, string host, string basePath, string[] schemes)
Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Show raw exception details

或者在控制台查看输出错误

大概意思是：不支持的异常:操作的HTTP方法不明确，方法需要必须指明请求方式

在方法加上 [HttpGet]、[HttpPost] 等，就可以通过 Swagger UI 查看 API

上面只是其中一种引起的Fetch errorInternal Server Error /swagger/v1/swagger.json原因，其他也是通过查看错误信息解决