.net 6 how can I get Configuration before the app is build

I am changing my project from .net 5 to .net 6 and have experienced an issue in the program.cs file with the current error of

Cannot modify ServiceCollection after application is built

I am needing to read the configuration before the app is built but the only current way that I know of getting the configuration is after the app is built . This is part of my code:

var builder = WebApplication.CreateBuilder(args);


// Add builder.Services to the container.
builder.Services.AddControllersWithViews();
builder.Services.AddCors();

var app = builder.Build();

IConfiguration configuration = app.Configuration;

var appSettings = new AppSettings(configuration);
var key = Encoding.ASCII.GetBytes(appSettings.Secret);
builder.Services.AddAuthentication(x => // error happens here we have already used builder.Build()
{
    x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
    x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
})
            .AddJwtBearer(x =>
            {
                x.RequireHttpsMetadata = false;
                x.SaveToken = true;
                x.TokenValidationParameters = new TokenValidationParameters
                {
                    ValidateIssuerSigningKey = true,
                    IssuerSigningKey = new SymmetricSecurityKey(key),
                    ValidateIssuer = true,
                    ValidateAudience = true,
                    ValidAudience = appSettings.Target,
                    ValidIssuer = appSettings.Target
                };
            });

Is there a way of getting IConfiguration before we call builder.Build() ? Since I need the configuration to pass into the class AppSettings which then gets a secret key and allows me to do Authentication on JWT token .

>Solution :

move var app = builder.Build() after authentication configuration.

the .Build() method essentially takes your configuration and gives you a WebApplication that’s built upon the WebApplicationBuilder so you cannot modify the app after it’s been built