With asp.net core 3

using System.Text.Json.Serialization;

public class Startup
{
        public void ConfigureServices(IServiceCollection services)
        {
              services.AddControllers().AddJsonOptions(options =>
              {
                  options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
                  options.JsonSerializerOptions.IgnoreNullValues = true;
              });

But it seems that Swashbuckle Version 5.0.0-rc4 is not ready to support that. So we need to add a dipricated option to Swashbuckle config file until it support and reflect it.

public void ConfigureServices(IServiceCollection services)
{ 
      services.AddSwaggerGen(c =>
      {
            c.DescribeAllEnumsAsStrings();

The difference between this answer and other answers is using only the Microsoft JSON library instead of Newtonsoft.

With asp.net core 3

using System.Text.Json.Serialization;
        
public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
         services.AddControllers().AddJsonOptions(options =>
             options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter()));

But it seems that Swashbuckle Version 5.0.0-rc4 is not ready to support that. So we need to use an option(deprecated) in the Swashbuckle config file until it supports and reflects it like Newtonsoft library.

public void ConfigureServices(IServiceCollection services)
{ 
      services.AddSwaggerGen(c =>
      {
            c.DescribeAllEnumsAsStrings();

The difference between this answer and other answers is using only the Microsoft JSON library instead of Newtonsoft.