Add example/documentation for 'is not' with non-nullable declaration pattern

[marchy]

Please add an example/section in the 'Declaration and type patterns' section to showcase the use of a non-nullable declaration pattern with 'is not' use case, which adds the equivalent functionality of Swift's guard let, discussed in feature request here.

if (someObject.SomeProperty is not string someProperty) {
    return; // or 'continue'
}

While the positive case is covered with this example: if (greeting is string message) { .. }, the inverse is non-obvious and opens up very useful null-safe short-circuiting control flows that are not stated anywhere in the documentation. The only current example is if (input is not null) { .. } in the 'Constant pattern' and 'Logical patterns' sections, but that does not combine a Declaration pattern within it – which is what brings all the extra usefulness.

The existence of this feature is a major component of writing modern, null-safe, idiomatic C#, supported since C# 9, let's get the word out about it!

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Document Details

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• ID: 9112c305-f1cd-1aa5-4557-2c7d757e6ca0
• Version Independent ID: 312e9c81-ee2a-1d84-64d7-bcaa71d7eab8
• Content: Patterns - C# reference
• Content Source: docs/csharp/language-reference/operators/patterns.md
• Product: dotnet-csharp
• Technology: csharp-language-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

Thanks @marchy That's a great suggestion.

I've adde3d it to our list of updates for this article.