What does this error mean, and how do I rectify it?

Question

system.overflow exception: value too small or too large

I'm busy with a loan calculator app. In the code below, I'm trying to calculate the loan payment (paymentD; payment) to push into a loan object. The user values which are used to calculate the loan are: Loan Amount (amountD), Loan Rate (monthlyRate;rateD), and Loan Term (term).

In the values mentioned above, the Loan Amount and Loan Rate are decimals, and the term is of type int. As such, I had to convert the decimal type to the double type in order to calculate paymentD, before reconverting the answer back to a decimal (payment).

However when I run the code, it only works when I enter a whole number into the rate value. When I try to use a fraction value (say 3.5), it throws this error. When debugging for the 3.5, payment returns NaN.

How do I fix this? integer value added to the rate

Code:

private decimal CalcMonthlyPayments(decimal amount, int term, decimal rate)
{
    
    var monthlyRate = CalcMonthlyRate(rate);
    var rateD = Convert.ToDouble(monthlyRate);

    var amountD = Convert.ToDouble(amount);

    var paymentD = (amountD * rateD) / (1 - Math.Pow(1 + rateD, -term));
    var payment = Convert.ToDecimal(paymentD);

    return payment;
}

private decimal CalcMonthlyRate(decimal rate)
{
    return rate / 1200;
}

private decimal CalcMonthlyInterest(decimal balance, decimal monthlyRate)
{
    return balance * monthlyRate;
}

Answer 1

You have a value that is too big or too small for the data-type being used.

Here is an example (of a different data type) that shows how you can "pre check" on ~some values.

https://docs.microsoft.com/en-us/dotnet/api/system.int16.maxvalue?view=net-6.0#examples

But if you have something like this:

Int16 result = Int16.MaxValue + 100;

you can see the issue. The code is trying to go 100 "beyond" the supported max.