C#: Declaring a local variable after null check

Question

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast#type-testing-with-pattern-matching

Is and As can assign local variable to be used subsequently.

Is there a similar way to declare a local variable to be used after a null check?

if(Factory() != null factoryResult)
{
  factoryResult.UsingResult();
}

What are you expecting from this? It doesn't make sense to me. There is nothing assigned here and no pattern matched. — Fildor, Apr 27 '23 at 08:17
^^ `if( Factory() is Product product ) product.UsingResult();` <- like that? — Fildor, Apr 27 '23 at 08:19
You can still use `is`. check this example out: https://stackoverflow.com/questions/811614/c-sharp-is-keyword-and-checking-for-not — MC LinkTimeError, Apr 27 '23 at 08:21
The question's own link shows the answer. `if (iBoxed is int a)` defines a new variable whose value isn't null. The docs call this a [declaration pattern](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/patterns#declaration-and-type-patterns). You can use `if(Factory() is SomeType x)` — Panagiotis Kanavos, Apr 27 '23 at 08:23
Maybe you are confused, because `null is SomeType` is false => https://dotnetfiddle.net/aJM5I5 — Fildor, Apr 27 '23 at 08:24
If the ONLY thing your are doing with `factoryResult` is what you posted, then you could just write: `Factory()?.UsingResult();` — Matthew Watson, Apr 27 '23 at 08:51

Irakli · Accepted Answer · 2023-04-27T08:38:16.963

Depending on which version of c# you're using, it is possible to use pattern matching using is keyword (is not, if you want the opposite of what has been stated) to check not only for null but also for specific properties, and declare in one go. You could also use the declared variable for additional checks in the same if statement

As per your example, { } would mean that it's an object of a class that is not null:

if(Factory() is { } factoryResult)
{
  factoryResult.UsingResult();
}

or

if(Factory() is FactoryResult factoryResult)
{
  factoryResult.UsingResult();
}

assuming that FactoryResult is a class that is being returned from Factory()

or suppose you have to check also for IsUsable boolean that has to be true at the same time:

if(Factory() is FactoryResult { IsUsable: true } factoryResult)
{
  factoryResult.UsingResult();
}

C#: Declaring a local variable after null check

1 Answers1