nullable

来源： https://stackoverflow.com/questions/63862777/foreign-key-to-one-to-all-relationship

来源： https://stackoverflow.com/questions/33736697/bool-compare-with-bool-vs-getvalueordefault-vs-operator

来源： https://stackoverflow.com/questions/33736697/bool-compare-with-bool-vs-getvalueordefault-vs-operator

问题 Consider the following code in C#. public int Foo(int a) { // ... } // in some other method int? x = 0; x = Foo(x); The last line will return a compilation error cannot convert from 'int?' to 'int' which is fair enough. However, for example in Haskell there is Maybe which is a counterpart to Nullable in C#. Since Maybe is a Functor I would be able to apply Foo to x using fmap . Does C# have a similar mechanism? 回答1: We can implement such functionality ourselves: public static class FuncUtils

问题 Consider the following code in C#. public int Foo(int a) { // ... } // in some other method int? x = 0; x = Foo(x); The last line will return a compilation error cannot convert from 'int?' to 'int' which is fair enough. However, for example in Haskell there is Maybe which is a counterpart to Nullable in C#. Since Maybe is a Functor I would be able to apply Foo to x using fmap . Does C# have a similar mechanism? 回答1: We can implement such functionality ourselves: public static class FuncUtils

问题 Is there a way to make the analyzer understand that the variable Bar has a value for the following case? #nullable enable class Foo { bool GenerateArray => Bar.HasValue; int? Bar { get; set; } void FooBar() { var data = (GenerateArray) ? new int[Bar.Value] : null; } } There is the warning "Nullable value type may be null." for Bar.Value but it obviously can't be. I am aware of two ways to avoid the warning. Both have disadvantages: Using Bar.HasValue directly instead of the property

问题 In a C# 8 project with nullable reference types enabled, I have the following code which I think should give me a warning about a possible null dereference, but doesn't: public class ExampleClassMember { public int Value { get; } } public struct ExampleStruct { public ExampleClassMember Member { get; } } public class Program { public static void Main(string[] args) { var instance = new ExampleStruct(); Console.WriteLine(instance.Member.Value); // expected warning here about possible null

问题 Kotlin has a feature called string templates. Is it safe to use nullable variables inside a string? override fun onMessageReceived(messageEvent: MessageEvent?) { Log.v(TAG, "onMessageReceived: $messageEvent") } Will the above code throw NullPointerException if messageEvent is null ? 回答1: You can always make a tiny project on try.kotlinlang.org and see for yourself: fun main(args: Array<String>) { test(null) } fun test(a: String?) { print("result: $a") } This code compiles fine and prints null

问题 Kotlin has a feature called string templates. Is it safe to use nullable variables inside a string? override fun onMessageReceived(messageEvent: MessageEvent?) { Log.v(TAG, "onMessageReceived: $messageEvent") } Will the above code throw NullPointerException if messageEvent is null ? 回答1: You can always make a tiny project on try.kotlinlang.org and see for yourself: fun main(args: Array<String>) { test(null) } fun test(a: String?) { print("result: $a") } This code compiles fine and prints null

问题 I have code like this: IEnumerable<string?> items = new [] { "test", null, "this" }; var nonNullItems = items.Where(item => item != null); //inferred as IEnumerable<string?> var lengths = nonNullItems.Select(item => item.Length); //nullability warning here Console.WriteLine(lengths.Max()); How can I write this code in a convenient way such that: There is no nullability warning, because the type nonNullItems is inferred as IEnumerable<string> . I don't need to add unchecked non-nullability