Refactoring multiple if-else

Question

I'm performing a business logic but the way I have implemented it, looks a bit convoluted and makes unit testing harder. How do I break down this logic into easier unit-testable tiny methods?

public async Task<ResultTypes> ValidatePinAsync(int id, string pin)
{
    ICustomer customer = await _repository.GetCustomer(id);

    if (customer.PinDisabled)
        return ResultTypes.PinDisabled;

    if (string.IsNullOrEmpty(customer.PinNumber))
    {
        customer.PinNumber = customer.DoB.ToString();

        //perform repository call to update PIN here?

        if (customer.PinNumber == pin)
        {
            return ResultTypes.Success;
        }
        else
        {
            //perform repository call to update failure count and then return disabled or invalid
            return ResultTypes.InvalidPinEntered;
        }
    }

    if (customer.PinNumber == pin)
    {
        return ResultTypes.Success;
    }
    else
    {
        //perform repository call to update failure count and then return disabled or invalid
    }

    return ResultTypes.Success;
}

Answer 1

I refactored your code a little (as per @canton7's suggestion) to leaf only 3 if statements, none of wich are nested. The logic inside the if (string.IsNullOrEmpty(customer.PinNumber)) could be moved to it's own method.

public async Task<ResultTypes> ValidatePinAsync(int id, string pin)
{
    ICustomer customer = await _repository.GetCustomer(id);

    if (customer.PinDisabled)
        return ResultTypes.PinDisabled;

    if (string.IsNullOrEmpty(customer.PinNumber))
    {
        customer.PinNumber = customer.DoB.ToString();
        //perform repository call to update PIN here?
    }

    if (customer.PinNumber != pin)
    {
        //perform repository call to update failure count and then return disabled or invalid
        return ResultTypes.InvalidPinEntered
    }

    return ResultTypes.Success;
}