Failed to load API definition is displayed on the Swagger Generation page

Environment

Product	Progress® Telerik® Reporting
Project Type	ASP.NET Core

Description

"Failed to load API definition." message is displayed on the Swagger Generation page.

There may also be a short error on the page about being unable to fetch swagger.json.

Error Message

• Error message on client-side:

Fetch error

response status is 500 https://localhost:{port}/swagger/v1/swagger.json

• Server-side exception:

An unhandled exception has occurred while executing the request.
      Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Conflicting method/path combination "GET api/ReportDesignerControllerBase" for actions - Telerik.WebReportDesigner.Services.Controllers.ReportDesignerControllerBase.GetDesignerResource (Telerik.WebReportDesigner.Services),Telerik.WebReportDesigner.Services.Controllers.ReportDesignerControllerBase.GetResource (Telerik.WebReportDesigner.Services). Actions require a unique method/path combination for Swagger/OpenAPI 3.0. Use ConflictingActionsResolver as a workaround
         at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
         at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
         at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwagger(String documentName, String host, String basePath)
         at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

Cause

There is a confliction method/path in ReportDesignerController. Swagger requires actions to have unique methods/paths.

Solution

• For .NET Core 3.1 and .NET 5, configure the Swagger Generation in Startup.cs:

 public void ConfigureServices(IServiceCollection services)
        {
        ...
            services.AddSwaggerGen(c => {
                c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
                c.IgnoreObsoleteActions();
                c.IgnoreObsoleteProperties();
                c.CustomSchemaIds(type => type.FullName);
                });
                ...
        }

• For .NET 6, configure the Swagger Generation in Program.cs

builder.Services.AddSwaggerGen(c => {
    c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
    c.IgnoreObsoleteActions();
    c.IgnoreObsoleteProperties();
    c.CustomSchemaIds(type => type.FullName);
});