nullable

问题 It happened to me that I accidentally did this: @javax.annotation.Nullable @javax.validation.constraints.NotNull The compiler didn't complain and everything seemed to work well. Now I am wondering what is internally happening when two or more annotations contradict each other. Furthermore, does the order matter? Is this @Nullable @NotNull the same than this? @NotNull @Nullable How is this working for annotations in general? Who wins? EDIT: I am looking for a general answer, not necessarily

问题 I am curious about specific scenario in SQL when using EF Core. For example there is a column that allows null and at the same time has defalut value, it is unusual situation but that is not the issue here. Question is about technical possibility. [SomeId] [uniqueidentifier] NULL DEFAULT (newsequentialid()) And in Application there is Entity(Code First) which has appropriate Property public Guid? SomeId { get; set; } The problem is how to Insert this Entity into DB so that SomeId would have

问题 Using Entity Framework version 4.0 (or any other version that is compatible with .NET 4.0), I want to map this existing relational database schema: to this logical object model: which I have tried setting up as follows: (I hope the German captions won't be too disorienting.) Entity Framework gives me this error: Error 3031: Problem in mapping fragments …: Non-nullable column FooBs.B in table FooBs is mapped to a nullable entity property. In the logical model, B ought to be nullable. However,

问题 I have a table with 75+ columns in it. Almost all of the columns have the NOT NULL constraint. If do a giant alter table modify statement (with every column in there), I get an error saying something along the lines of "You can't set this field to NULL, because it already is NULL" I have to do this for several tables, and so would prefer to have a dynamic solution. Can I dynamically find all of the columns that are NOT NULL, and set them to NULL? I've seen several similar questions like this,

问题 I have this call: public IObservable<T> Subscribe(object topic, string service, string connectionString, string query) { try { this.connection.ConnectionString = connectionString; this.connection.Open(); this.connection.Query<T>(query, new { transactionid = topic }).ToObservable().Subscribe(message => this.subject.OnNext(message)); return this.subject; } catch (Exception e) { this.subject.OnError(e); return this.subject; } finally { this.subject.OnCompleted(); this.connection.Close(); } }

问题 I'm trying this (and all PoST var are treated before user send it, no SQL Injection worries): $stmt = $con->prepare($sql); $stmt->bindParam(":1", $this->getPes_cdpessoa()); $stmt->bindParam(":2", $this->getPdf_nupessoa_def()); When any of those vars are NULL, PDO cries and don't let execute my statement, and on my Table, i DO allow these fields beign nullables. Is there any way to check if the values are empty, pdo just bind NULL to then (and i mean, a smart way instead if(empty($_POST[

问题 Do you know if there is a shortcut for: if (x == null) null else f(x) For Java Optional you can just do: x.map(SomeClass::f) 回答1: Kotlin utilizes its own approach to the idea of Option , but there're map , filter , orElse equivalents: val x: Int? = 7 // ofNullable() val result = x ?.let(SomeClass.Companion::f) // map() ?.takeIf { it != 0 } // filter() ?: 42 // orElseGet() I ended up writing a full comparison here: 回答2: You can use let in this case, like this: fun f(x : Int) : Int{ return x+1

问题 This question already has answers here : Nullable types and the ternary operator: why is `? 10 : null` forbidden? [duplicate] (9 answers) Closed 6 years ago . Why do we need explicit cast in second statement? bool? a = null; bool b = false; bool c = true; 1.) if(b || c) a = b; else a = null; 2.) a = (b || c)?(Nullable<bool>)b:null; 回答1: The conditional operator is an expression, thus it needs a return type - also both cases have to have the same return type. In your case, there is no way of

问题 Is it possible to detect a Nullable type (cast into an object) when it is null? Since Nullable<T> is really a struct I think it should be possible. double? d = null; var s = GetValue(d); //I want this to return "0" rather than "" public string GetValue(object o) { if(o is double? && !((double?)o).HasValue) //Not working with null return "0"; if(o == null) return ""; return o.ToString(); } 回答1: http://msdn.microsoft.com/en-us/library/ms228597(v=vs.80).aspx Objects based on nullable types are

问题 Some time ago I thought that Nullable<> value types are classes, encapsulating value types and a bool to HasValue. With some implicit cast operador for null, just implemented at BCL. But being a struct, how this can be achieved? Nullable<> struct is "special" for CLR? 回答1: Nullable<T> is defined as a normal struct, but there's special hooks within the CLR to box/unbox an instance of [mscorlib]System.Nullable`1 to null according to the HasValue property. There's more details on this here 回答2: