public async Task StartAsync(string user, decimal balance, CancellationToken cancellationToken = default)
{
if (_userStorage.Exists(user) == false)
{
throw new NoUserDefinedException($"System has no user with specified username `{user}`.");
}
if (Current is not null)
{
throw new MultipleSessionOpenedException("Session is already opened.");
}
var session = new Session
{
Started = DateTime.UtcNow,
InitialBalance = balance,
UserName = user
};
await _dbContext.Sessions.AddAsync(session, cancellationToken);
await _dbContext.SaveChangesAsync(cancellationToken);
}