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Specifying your OpenAPI description

To find out which endpoint swagger is using, open the file Startup.cs and search for .UseSwaggerUI. You should find this block of code:

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
    app.UseSwagger();
    app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "WebAPI v1"));
}

Use the endpoint you find and run the tool like this:

httprepl http://localhost:5000 --openapi /swagger/v1/swagger.json

If you do not find any references to swagger, then see below for how to install and configure swagger for your project. Search for None of the above worked, swagger isn't installed.

Ignoring your environment

If that doesn't work, then you are not running your Web API in a development environment. So either use a development environment, or uncomment the if-statement while testing (to setup your environment for development - see below):

//if (env.IsDevelopment())
//{
    app.UseDeveloperExceptionPage();
    app.UseSwagger();
    app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "WebAPI v1"));
//}

Remember to restore the code you uncommented, if any, before you deploy to production.

Changing your environment

The profile your Web API is using, is specified in the file Properties\launchSettings.json. Open the file and search for ASPNETCORE_ENVIRONMENT. Then change the instances you find to:

"ASPNETCORE_ENVIRONMENT": "Development"

If this doesn't work, or the instances were already set to "Development", it means that you are not using any of the profiles specified in your launch settings. If no profile is used, ASPNETCORE_ENVIRONMENT defaults to "Production". When using the dotnet run command, the --launch-profile parameter lets you specify which profile to use:

dotnet run --launch-profile "name_of_profile"

As a last resort you can set the environment variable ASPNETCORE_ENVIRONMENT in the shell you are using, before you run the command dotnet run:

Bash
export ASPNETCORE_ENVIRONMENT=Development

CMD
set ASPNETCORE_ENVIRONMENT=Development

PowerShell
$env:ASPNETCORE_ENVIRONMENT='Development'

None of the above worked, swagger isn't installed

I none of the above worked, it means you don't have swagger installed. Install swagger for your project and when done, try again.

Package Installation

Open your project in VS Code and run the following command from the Integrated Terminal and replace WebAPI.csproj with the name of your own project file

dotnet add WebAPI.csproj package Swashbuckle.AspNetCore -v 5.6.3

Add and configure Swagger middleware

Add the Swagger generator to the services collection in the Startup.ConfigureServices method, as the last statement in the method:

public void ConfigureServices(IServiceCollection services)
{
    [... other code here ...]

    // Register the Swagger generator, defining 1 or more Swagger documents
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "WebAPI", Version = "v1" });
    });

}

In the Startup.Configure method, enable the middleware for serving the generated JSON document and the Swagger UI, at the top of the method:

public void Configure(IApplicationBuilder app)
{
    // Enable middleware to serve generated Swagger as a JSON endpoint.
    app.UseSwagger();

    // Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.),
    // specifying the Swagger JSON endpoint.
    app.UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
    });

    [... other code here for setting up routing and the like ...]
}

To learn more about setting up swagger, profiles and the environment

Get started with Swashbuckle and ASP.NET Core
Managing Production and Development Settings in ASP.NET Core
Use multiple environments in ASP.NET Core
ASP.NET Core web API documentation with Swagger / OpenAPI