Dependency Injection

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddScoped<IMySvc, MySvc>();
var app = builder.Build();

app.MapGet("/", (IMySvc svc) => svc.Hello());

app.Run();

public interface IMySvc { string Hello(); }
public class MySvc : IMySvc { public string Hello() => "hi"; }

Notes

Prefer constructor injection; avoid service locator pattern.