你是如何获得输入的？对于本例，我假设它是一个字符串，因为默认情况下int不能为null。但是，您可以使用int？创建一个可为空的int

private int MethodName(string input)
{
    //Validate the input is an int. You may want to support decimals here.
    //decimal.TryParse is the method. As dividing an odd number by 2 using an int will round.
    var validInput = Int32.TryParse(input, out var result);

    //You may want to return this if the input is 0 as well.
    if (!validInput)
        return 12345;

    //An even number will have remainder of 0 when divied by 2...
    var isEven = result % 2 == 0;

    if (isEven)
        return result * result;

    //Otherwise it must be odd
    return result / 2;
}