InvestmentController.cs

 /// <summary>
    /// Create an NGN investment
    /// </summary>
    [HttpPost("ngn")]
    [ProducesResponseType(typeof(InvestmentExtendedResponse), 201)]
    [ProducesResponseType(typeof(GenericResponse), 400)]
    [ProducesResponseType(typeof(GenericResponse), 404)]
    public async Task<IActionResult> CreateNgn([FromBody] CreateNgnInvestmentRequest req)
    {
        var user = await _userService.FindById(req.OwnerId);

        var staffId = User.GetCallerId();
        var staff = await _staffService.FindById(staffId);

        if (staff is null)
            return NotFound("Staff account not found.");

        if (user is null)
            return NotFound("User account not found.");

        Investment investment;

        if (req.SourceInvestmentId.HasValue)
        {
            var sourceInvestment = await _investmentService.FindById(req.SourceInvestmentId.Value);

            if (sourceInvestment is null)
                return NotFound("Source investment does not exist");

            if (sourceInvestment.Status != InvestmentStatus.Liquidated)
                return BadRequest("You can only reinvest a liquidated investment");

            if (req.BusinessId.HasValue)
            {
                var business = await _businessService.FindById(req.BusinessId.Value);

                if (business is null)
                    return NotFound("Business profile not found.");

                if (business.OwnerId != req.OwnerId)
                    return BadRequest("Business profile does not match the user.");

                investment = await _investmentService.CreateCorporateNgn(user.Id, user.FullName(), business.Id,
                    business.BusinessName, req.PrincipalAmount, req.PayoutFrequency, req.TenorInDays, req.InterestRate, sourceInvestment, staff);
            }
            else
            {
                investment = await _investmentService.CreatePersonalNgn(user.Id, user.FullName(), req.PrincipalAmount,
                    req.PayoutFrequency, req.TenorInDays, req.InterestRate, sourceInvestment, staff);
            }
        }
        else
        {
            if (req.BusinessId.HasValue)
            {
                var business = await _businessService.FindById(req.BusinessId.Value);

                if (business is null)
                    return NotFound("Business profile not found.");

                if (business.OwnerId != req.OwnerId)
                    return BadRequest("Business profile does not match the user.");

                investment = await _investmentService.CreateCorporateNgn(user.Id, user.FullName(), business.Id,
                    business.BusinessName, req.PrincipalAmount, req.PayoutFrequency, req.TenorInDays, req.InterestRate);
            }
            else
            {
                investment = await _investmentService.CreatePersonalNgn(user.Id, user.FullName(), req.PrincipalAmount,
                    req.PayoutFrequency, req.TenorInDays, req.InterestRate);
            }
        }


        return Created(_mapper.Map<InvestmentExtendedResponse>(investment));
    }

Yeah, the logic for creating investment is a bit repetitive. We can extract it into a separate method.

[HttpPost("ngn")]
[ProducesResponseType(typeof(InvestmentExtendedResponse), 201)]
[ProducesResponseType(typeof(GenericResponse), 400)]
[ProducesResponseType(typeof(GenericResponse), 404)]
public async Task CreateNgn([FromBody] CreateNgnInvestmentRequest req)
{
var user = await _userService.FindById(req.OwnerId);
var staffId = User.GetCallerId();
var staff = await _staffService.FindById(staffId);

if (staff is null)
    return NotFound("Staff account not found.");

if (user is null)
    return NotFound("User account not found.");

Investment investment;

if (req.SourceInvestmentId.HasValue)
{
    var sourceInvestment = await _investmentService.FindById(req.SourceInvestmentId.Value);

    if (sourceInvestment is null)
        return NotFound("Source investment does not exist");

    if (sourceInvestment.Status != InvestmentStatus.Liquidated)
        return BadRequest("You can only reinvest a liquidated investment");

    investment = await CreateInvestment(user, req, sourceInvestment, staff);
}
else
{
    investment = await CreateInvestment(user, req, null, staff);
}

return Created(_mapper.Map<InvestmentExtendedResponse>(investment));

}

private async Task CreateInvestment(User user, CreateNgnInvestmentRequest req, Investment sourceInvestment, Staff staff)
{
Investment investment;

if (req.BusinessId.HasValue)
{
    var business = await _businessService.FindById(req.BusinessId.Value);

    if (business is null)
        return NotFound("Business profile not found.");

    if (business.OwnerId != req.OwnerId)
        return BadRequest("Business profile does not match the user.");

    investment = await _investmentService.CreateCorporateNgn(user.Id, user.FullName(), business.Id,
        business.BusinessName, req.PrincipalAmount, req.PayoutFrequency, req.TenorInDays, req.InterestRate, sourceInvestment, staff);
}
else
{
    investment = await _investmentService.CreatePersonalNgn(user.Id, user.FullName(), req.PrincipalAmount,
        req.PayoutFrequency, req.TenorInDays, req.InterestRate, sourceInvestment, staff);
}

return investment;

}