[NETCore 3.1]

ConfigureSwaggerOptions.cs

using Microsoft.AspNetCore.Mvc.ApiExplorer;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;
using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;

namespace SwaggerVersioning
{
    // <summary>
    /// Configures the Swagger generation options.
    /// </summary>
    /// <remarks>
    /// <para>
    /// This allows API versioning to define a Swagger document per API version after the
    /// <see cref="IApiVersionDescriptionProvider" /> service has been resolved from the service container.
    /// </para>
    /// <para>Taken from https://github.com/microsoft/aspnet-api-versioning.</para>
    /// </remarks>
    public class ConfigureSwaggerOptions : IConfigureOptions<SwaggerGenOptions>
    {
        #region member vars

        private readonly IApiVersionDescriptionProvider _provider;

        #endregion

        #region constructors and destructors

        /// <summary>
        /// Initializes a new instance of the <see cref="ConfigureSwaggerOptions" /> class.
        /// </summary>
        /// <param name="provider">
        /// The <see cref="IApiVersionDescriptionProvider">provider</see> used to generate Swagger
        /// documents.
        /// </param>
        public ConfigureSwaggerOptions(IApiVersionDescriptionProvider provider)
        {
            _provider = provider;
        }

        #endregion

        #region explicit interfaces

        /// <inheritdoc />
        public void Configure(SwaggerGenOptions options)
        {
            // add a swagger document for each discovered API version
            // note: you might choose to skip or document deprecated API versions differently
            foreach (var description in _provider.ApiVersionDescriptions)
            {
                options.SwaggerDoc(description.GroupName, CreateInfoForApiVersion(description));
            }
        }

        #endregion

        #region methods

        /// <summary>
        /// Internal implementation for building the Swagger basic config.
        /// </summary>
        /// <param name="description">The description object containing the.</param>
        /// <returns>The generated Open API info.</returns>
        private static OpenApiInfo CreateInfoForApiVersion(ApiVersionDescription description)
        {
            var info = new OpenApiInfo
            {
                Title = "Title",
                Version = description.ApiVersion.ToString(),
                Description = @"",

            };
            if (description.IsDeprecated)
            {
                info.Description += @"<p><strong><span style=""color:white;background-color:red"">VERSION IS DEPRECATED</span></strong></p>";
            }
            return info;
        }

        #endregion
    }
    
        // Add security lock for individual methods that require authorization
    public class SecurityRequirementsOperationFilter : IOperationFilter
    {
        public void Apply(OpenApiOperation operation, OperationFilterContext context)
        {
            var authAttributes = context.MethodInfo.DeclaringType.GetCustomAttributes(true)
                .Union(context.MethodInfo.GetCustomAttributes(true))
                .OfType<AuthorizeAttribute>();

            var allowAnonymousAttributes = context.MethodInfo.DeclaringType.GetCustomAttributes(true)
                .Union(context.MethodInfo.GetCustomAttributes(true))
                .OfType<AllowAnonymousAttribute>();


            if (authAttributes.Any() && !allowAnonymousAttributes.Any())
            {
                operation.Security.Add(new OpenApiSecurityRequirement()
                {
                    {
                        new OpenApiSecurityScheme
                        {
                            Reference = new OpenApiReference
                            {
                                Type = ReferenceType.SecurityScheme,
                                Id = "Bearer"
                            },
                            Scheme = "OAuth2",
                            Name = "Bearer",
                            In = ParameterLocation.Header,
                        }, new List<string>()
                    }
                });


                //operation.Responses.Add("401", new OpenApiResponse { Description = "Unauthorized" });
                //operation.Responses.Add("403", new OpenApiResponse { Description = "Forbidden" });
            }
        }
    }
}

SVStartupConfig

        public void ConfigureServices(IServiceCollection services)
        {
            #region API Versioning
            services.AddApiVersioning(o =>
            {
                o.AssumeDefaultVersionWhenUnspecified = true;
                o.DefaultApiVersion = new ApiVersion(1, 0);
                // reporting api versions will return the headers "api-supported-versions" and "api-deprecated-versions"
                o.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;
            });
            #endregion
          
            #region API Documentation (Swagger)
                services.AddTransient<IConfigureOptions<SwaggerGenOptions>, ConfigureSwaggerOptions>();
                services.AddSwaggerGen(
                    options =>
                    {
                        options.OperationFilter<SecurityRequirementsOperationFilter>();
                        
                        // integrate xml comments
                        var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                        var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
                        options.IncludeXmlComments(xmlPath);
                    });
            #endregion
        }
        
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", "API V1");
                c.SwaggerEndpoint("/swagger/v2/swagger.json", "API V2");
                c.RoutePrefix = "docs";
                c.InjectStylesheet("/swagger/ui/custom.css");
            });

        }

SwaggerVersioning

using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;
using System.Linq;


namespace SwaggerVersioning
{
    public class SwaggerVersioning
    {
        public class RemoveVersionFromParameter : IOperationFilter
        {
            public void Apply(OpenApiOperation operation, OperationFilterContext context)
            {
                var versionParameter = operation.Parameters.Single(p => p.Name == "version");
                operation.Parameters.Remove(versionParameter);
            }
        }

        public class ReplaceVersionWithExactValueInPath : IDocumentFilter
        {
            // https://stackoverflow.com/questions/60563647/migrate-code-2-0-to-3-1-core-code-then-swagger-api-versing-not-working
            public void Apply(OpenApiDocument swaggerDoc, DocumentFilterContext context)
            {
                var paths = new OpenApiPaths();
                foreach (var path in swaggerDoc.Paths)
                {
                    paths.Add(path.Key.Replace("v{version}", swaggerDoc.Info.Version), path.Value);
                }
                swaggerDoc.Paths = paths;
            }
        }
    }
}