我已经在.Net5和.Net core2.1项目中测试了你的方法，它工作得很好。
提示：* 请将“env.isdevelopment”中的“swagger”语句改为“outsite”。* 这是容易被忽略的地方。

.网络版5.0

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Server.HttpSys;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.OpenApi.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using webapi.Middleware;
using System.Reflection;
using Swashbuckle.AspNetCore.SwaggerGen;
using Microsoft.Extensions.Options;
using Microsoft.AspNetCore.Mvc.ApiExplorer;

namespace webapi
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            //services.AddApplicationInsightsTelemetry();
            services.AddLogging();
            services.AddControllers().AddNewtonsoftJson();

            services.AddApiVersioning(
                options =>
                {
                    // reporting api versions will return the headers "api-supported-versions" and "api-deprecated-versions"
                    options.ReportApiVersions = true;
                });
            services.AddVersionedApiExplorer(
                options =>
                {
                    // add the versioned api explorer, which also adds IApiVersionDescriptionProvider service
                    // note: the specified format code will format the version as "'v'major[.minor][-status]"
                    options.GroupNameFormat = "'v'VVV";

                    // note: this option is only necessary when versioning by url segment. the SubstitutionFormat
                    // can also be used to control the format of the API version in route templates
                    options.SubstituteApiVersionInUrl = true;
                });
            services.AddTransient<IConfigureOptions<SwaggerGenOptions>, ConfigureSwaggerOptions>();
            if (this.Configuration["EnableSwagger"] == "true")
            {
                //services.AddSwaggerGen(opt =>
                //opt.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1" });

                //// Set the comments path for the Swagger JSON and UI.
                //var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                //var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
                //opt.IncludeXmlComments(xmlPath);
                services.AddSwaggerGen(opt =>
                {
                    opt.AddSecurityDefinition("Bearer", new OpenApiSecurityScheme
                    {
                        Name = "Authorization",
                        Type = SecuritySchemeType.ApiKey,
                        Scheme = "bearer",
                        BearerFormat = "JWT",
                        In = ParameterLocation.Header,
                        Description = "JWT Authorization header using the Bearer scheme."
                    });
                    opt.AddSecurityRequirement(new OpenApiSecurityRequirement
                    {
                        {
                          new OpenApiSecurityScheme
                            {
                                Reference = new OpenApiReference
                                {
                                    Type = ReferenceType.SecurityScheme,
                                    Id = "Bearer"
                                }
                            },
                            Array.Empty<string>()
                        }
                    });
                });
            }

            services.AddDistributedMemoryCache();
            services.AddSession();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env,
            IApiVersionDescriptionProvider provider)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                foreach (var description in provider.ApiVersionDescriptions.OrderByDescending(o => o.GroupName))
                {
                    c.SwaggerEndpoint(
                        $"/swagger/{description.GroupName}/swagger.json",
                        description.GroupName.ToUpperInvariant());
                }
            });
            app.UseHttpsRedirection();
            app.UseRouting();

            app.UseAuthorization();
            //app.UseCustomMiddleware();
            app.UseSession();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });

        }
    }
    public class ConfigureSwaggerOptions : IConfigureOptions<SwaggerGenOptions>
    {
        readonly IApiVersionDescriptionProvider _apiVerProvider;

        public ConfigureSwaggerOptions(IApiVersionDescriptionProvider apiVerProvider) => _apiVerProvider = apiVerProvider;

        public void Configure(SwaggerGenOptions options)
        {
            foreach (var description in _apiVerProvider.ApiVersionDescriptions)
            {
                options.SwaggerDoc(description.GroupName, GetSwaggerDocInfo(description));
            }
        }

        static OpenApiInfo GetSwaggerDocInfo(ApiVersionDescription description)
        {
            var info = new OpenApiInfo
            {
                Title = $"WebAPI {description.ApiVersion}",
                Version = description.GroupName,
                Description = "Web API Template",
                Contact = new OpenApiContact()
                {
                    Name = "Web API service"
                },
                License = new OpenApiLicense()
                {
                    Name = "MIT"
                }
            };

            if (description.IsDeprecated)
            {
                info.Description += $" {description.ApiVersion} API version is deprecated.";
            }
            return info;
        }
    }
}

.网络版2.1

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.ApiExplorer;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;
using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Swagger21
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            //services.AddApplicationInsightsTelemetry();
            services.AddLogging();
            services.AddMvcCore().AddVersionedApiExplorer();
            services.AddApiVersioning(o =>
            {
                o.AssumeDefaultVersionWhenUnspecified = true;
                o.DefaultApiVersion = new ApiVersion(1, 0);

            });

            services.AddTransient<IConfigureOptions<SwaggerGenOptions>, ConfigureSwaggerOptions>();
            if (this.Configuration["EnableSwagger"] == "true")
            {
                //services.AddSwaggerGen(opt =>
                //opt.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1" });

                //// Set the comments path for the Swagger JSON and UI.
                //var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                //var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
                //opt.IncludeXmlComments(xmlPath);
                services.AddSwaggerGen(opt =>
                {
                    opt.AddSecurityDefinition("Bearer", new OpenApiSecurityScheme
                    {
                        Name = "Authorization",
                        Type = SecuritySchemeType.ApiKey,
                        Scheme = "bearer",
                        BearerFormat = "JWT",
                        In = ParameterLocation.Header,
                        Description = "JWT Authorization header using the Bearer scheme."
                    });
                    opt.AddSecurityRequirement(new OpenApiSecurityRequirement
                        {
                            {
                              new OpenApiSecurityScheme
                                {
                                    Reference = new OpenApiReference
                                    {
                                        Type = ReferenceType.SecurityScheme,
                                        Id = "Bearer"
                                    }
                                },
                                Array.Empty<string>()
                            }
                        });
                });
            }

            services.AddDistributedMemoryCache();
            services.AddSession();
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app , IHostingEnvironment env,
            IApiVersionDescriptionProvider provider)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                foreach (var description in provider.ApiVersionDescriptions.OrderByDescending(o => o.GroupName))
                {
                    c.SwaggerEndpoint(
                        $"/swagger/{description.GroupName}/swagger.json",
                        description.GroupName.ToUpperInvariant());
                }
            });
            app.UseHttpsRedirection();
            app.UseSession();
            app.UseMvc();

        }
    }
    public class ConfigureSwaggerOptions : IConfigureOptions<SwaggerGenOptions>
    {
        readonly IApiVersionDescriptionProvider _apiVerProvider;

        public ConfigureSwaggerOptions(IApiVersionDescriptionProvider apiVerProvider) => _apiVerProvider = apiVerProvider;

        public void Configure(SwaggerGenOptions options)
        {
            foreach (var description in _apiVerProvider.ApiVersionDescriptions)
            {
                options.SwaggerDoc(description.GroupName, GetSwaggerDocInfo(description));
            }
        }

        static OpenApiInfo GetSwaggerDocInfo(ApiVersionDescription description)
        {
            var info = new OpenApiInfo
            {
                Title = $"WebAPI {description.ApiVersion}",
                Version = description.GroupName,
                Description = "Web API Template",
                Contact = new OpenApiContact()
                {
                    Name = "Web API service"
                },
                License = new OpenApiLicense()
                {
                    Name = "MIT"
                }
            };

            if (description.IsDeprecated)
            {
                info.Description += $" {description.ApiVersion} API version is deprecated.";
            }
            return info;
        }
    }
}

你失踪了

app.UseSwagger();
   app.UseSwaggerUI();