用于 ？？ 如果对象不为空，则获取对象的属性

Question

Is it possible to use C# '??' operator as this :

var _var = pr.Value ?? "NA";

To check if pr is null and not if Value attribute of pr is null ?

I don't find any example that show something like this.

If yes, is it recommanded or is it better to use extension method in this case ?

Answer 1

There are 3 different scenarios here so let's go through the syntax for each:

1. Return "NA" if either pr or pr.Value is null:

    var _var = pr?.Value ?? "NA"; 

   The ?. operator will basically say that "if pr is null, return null for the whole expression and do not dereference pr.Value "

   So if pr is null, the ?. operator will make the expression evaluate to null, or if pr.Value is null, it will also evaluate to null, and in both cases, ?? "NA" ?? "NA" will evaluate the whole thing to "NA" .

   This is result-wise equivalent to this:

    string _var; if (pr is null || pr.Value is null) _var = "NA"; else _var = pr.Value; 

2. Return "NA" if pr.Value is null:

    var _var = pr.Value ?? "NA"; 

   This is what you have. This will throw a NullReferenceException if pr is null, but evaluate to "NA" if pr is non-null but pr.Value is null.

   This is result-wise equivalent to this:

    string _var; if (pr.Value is null) // can throw NullReferenceException _var = "NA"; else _var = pr.Value; 

3. Return "NA" if pr is null and ... what if pr.Value is null?

   Neither ?? nor ?. can help you here but you can use the ternary operator:

    var _var = (pr is null) ? "NA" : pr.Value; 

   This will evaluate to "NA" if pr is null, pr.Value if pr is non-null, which will thus be able to evaluate to null if pr is non-null but pr.Value is null.

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Note , the ?? operator is also evaluated lazily, just as the ternary operator, so if you have this:

var _v = pr?.Value ?? ComputeTheValue();

and that method call is expensive, it will only be evaluated if pr?.Value is actually null, if pr is non-null and pr.Value is non-null, the call will not be made.

Answer 2

To check if pr is null and not if Value attribute of pr is null ?

Seems like you only want to check if pr is null. in which case ternary operator is your option:

var _var = pr == null ? "NA" : pr.Value;

or use a simple if / else .