Question

How can I multiply two 64 digit decimal numbers together in C# without overflowing?

Answer 1

If you need 64 decimal digit numbers, and if they are integers, you can use System.Numerics.BigInteger struct (needs a reference to System.Numerics.dll assembly).

BigInteger firstNumber = BigInteger.Parse("63518439492097324687235465876298368764576527346564625480");
BigInteger secondNumber = BigInteger.Parse("84890247648975285765484902890273086475254764765147643611");
BigInteger product = firstNumber * secondNumber;