A very simple database context file for a Todo API.

TodoContext.cs

public class TodoContext : DbContext
{
    private readonly IConfiguration config;

    public TodoContext(DbContextOptions options, IConfiguration config) : base(options)
    {
        this.config = config;
    }

    public DbSet<Todo> Todos { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        var envVariableName = config.GetValue<string>("EnvKeys:TodoDbConn");
        var connectionString = Environment.GetEnvironmentVariable(envVariableName);

        if (string.IsNullOrEmpty(connectionString))
        {
            throw new Exception($"The environment variable {envVariableName} is not set.");
        }

        optionsBuilder.UseSqlServer(connectionString);
    }
}