Is there a shorthand for addition assignment operator for nullables that sets the value if null?

问题

I am using nullable doubles to store the sum of some values pulled from various sources. The sum could be any real value or null if no elements are present.

Currently, I use a null check and either assign or increment:

double? sum = null;
...    
if(sum == null)
    sum = someTempValue;
else
    sum += someTempValue;

I know c# has several shorthand null checks for method calls and the such. My question is if there is a shorthand notation to the addition assignment operator += when using nullables that assigns the value if null or performs the operation if not null?

回答1:

You could put ternary operator:

double? sum = null;
sum = sum == null ? someTempValue : sum + someTempValue;

sum = someTempValue + (sum == null ? 0 : sum);

sum = someTempValue + (sum ?? 0);

Credit to: Dmitry Bychenko for the last option

回答2:

Not a real shortcut, but quite compact:

sum = sum ?? 0 + someTempValue;

treat sum as 0 when sum == null. But, please, notice that ?? operator can be dangerous:

// parenthesis () are mandatory, otherwise you'll have a wrong formula
// (someTempValue + sum) ?? 0;
sum = someTempValue + (sum ?? 0);

回答3:

It's not quite clear from the post, but looks like the someTempValue is non nullable, otherwise the if logic will not work because + operator will destroy the previous sum if the someTempValue is null.

If that's true, then you can utilize the null-coalescing operator and the fact that null + value == null:

sum = (sum + someTempValue) ?? someTempValue;

If someTempValue is also nullable, then the correct logic would be:

sum = (sum + someTempValue) ?? someTempValue ?? sum;

IMO, with shorthand or not, it's better to put that logic in a method

public static class MathUtils
{
    public static double? Add(double? left, double? right)
    {
        // The avove shorthand
        return (left + right) ?? right ?? left;
        // or another one
        //return left == null ? right : right == null ? left : left + right;
    }
}

and use simple

sum = MathUtils.Add(sum, someTempValue);

or with C#6 using static feature, the even shorter

sum = Add(sum, someTempValue);

来源：https://stackoverflow.com/questions/37295114/is-there-a-shorthand-for-addition-assignment-operator-for-nullables-that-sets-th

标签

nullable