空推算运算符或条件运算符

Question

Which coding style do you prefer:

object o = new object();
//string s1 =  o ?? "Tom"; // Cannot implicitly convert type 'object' to 'string' CS0266
string s3 = Convert.ToString(o ?? "Tom");
string s2 = (o != null) ? o.ToString() : "Tom";

s2 or s3?

Is it possible to make it shorter? s1 does not obviously work.

Answer 1

In this case, I think my preference would be:

string s1 = (string)o ?? "Tom";

Or:

string s1 = (o as string) ?? "Tom";

Depending on whether or not o is really expected to be a string or not. Either way, I prefer these because they better express what's being done, and don't pass this through an unnecessary conversion if o is already a string .

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As a general rule, I prefer whichever is clearer and/or actually works. When working with strings, I often need to write something like this instead:

string result = !string.IsNullOrEmpty(value) ? value : "Default";

...which can't really be done at all with the null-coalescing operator. On the other hand, if I'm trying to coalesce a large number of values, it's about 500 times better:

var result = firstTry ?? secondTry ?? thirdTry ?? fourthTry ?? fifthTry;

Try writing that with the ternary operator instead.

If the difference is not this dramatic, if it's just going to be a couple of characters on one line of code, it really doesn't matter, just use whatever you're comfortable with.

Answer 2

这也会起作用，尽管在字符串上调用ToString有点奇怪：

string s4 = (o ?? "Tom").ToString();

Answer 3

For this case I'd prefer using the ternary operator since it expresses the intent more clearly and avoids a redundant call to Convert.ToString() . In general I would prefer the null coalescing operator if the conversion is to an object of the same type eg

string s1 = null;
string s2 = s1 ?? string.Empty;

Answer 4

IMO, I tend to use:

var s = obj ?? "tom";

when s is the same type as obj without using the Convert or any other type of casting.

Then, I use:

var s = obj != null 
    ? obj.ToString() 
    : "tom";

when I need to cast or have some other kind of transformation on the right side before assigning to the left side...

Just my style of coding, I suppose.

Answer 5

I don't like either. Converting object to a specific type is fraught with trouble, it should never be hidden in an expression. I'd much prefer that, if it bombs, then it does so on a specific statement. And to make it absolutely obvious to a reader of the code that this conversion is being done. So, at the very minimum:

string s1 = o as string;
string s2 = s1 ?? "Tom";