我在哪里可以找到“可空友好模式”的文档？

Question

In https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/nullable-reference-types-specification.md#element-access the following code is used, which I don't understand and I haven't found any documentation:

// Nullable friendly pattern
if (array[0] is { } o)
{
    Console.WriteLine(o.ToString());
}

My incomprehension refers to the empty curly brackets. Where is documented how to understand / use this. Is it possible to use this scheme "{ } var-name" in another place or is it bound to the usage with the is-operator?

Answer 1

It is part of the C# pattern matching language feature, although not named explicitly as such.

The documentation about recursive pattern matching mentions it in the property pattern section, showing the ways to check for not null giving an example for a string .

Note that a null-checking pattern falls out of a trivial property pattern. To check if the string s is non-null, you can write any of the following forms

if (s is object o) ... // o is of type object
if (s is string x) ... // x is of type string
if (s is {} x) ... // x is of type string
if (s is {}) ...

---

A tutorial gives an other example where being used in a switch statement and specifies

The { } case matches any non-null object that didn't match an earlier arm.

public decimal CalculateToll(object vehicle) => vehicle switch
{
    Car c           => 2.00m,
    Taxi t          => 3.50m,
    Bus b           => 5.00m,
    DeliveryTruck t => 10.00m,
    { }             => throw new ArgumentException(message: "Not a known vehicle type", paramName: nameof(vehicle)),
    null            => throw new ArgumentNullException(nameof(vehicle))
};

---

What happens is that if (array[0] is { } o) gets translated into if (array[0] != null) .
You can see it at https://sharplab.io