nullable

问题 I'm having trouble inserting null value from a bulk insert statement. Two columns are nullable and the Id is identity. The int nullable workd out fine, but the time doesn't. Here the bulk statement: BULK INSERT Circulation FROM '.....file.cs' WITH ( FIRSTROW = 2, MAXERRORS = 0, FIELDTERMINATOR = ',', ROWTERMINATOR = '', KEEPNULLS) Here is an extract of the csv: ID, IDStopLine, IDException, Hour, PositionHour, Day ,28, 8, 12:20, 52, 0 ,29, 163, , 1, Meaning that I'm trying to insert nulls in

问题 I was a bit surprised a few minutes ago when I tried to overload an Action in one of my Controllers I had public ActionResult Get() { return PartialView(/*return all things*/); } I added public ActionResult Get(int id) { return PartialView(/*return 1 thing*/); } .... and all of a sudden neither were working I fixed the issue by making 'id' nullable and getting rid of the other two methods public ActionResult Get(int? id) { if (id.HasValue) return PartialView(/*return 1 thing*/); else return

问题 I have some code that looks like this: f(a).and_then(|b| { g(b).and_then(|c| { h(c).map(|d| { do_something_with(a, b, c, d) }) }) }) Where f , g , and h return Option values. I need to use all the intermediate values ( a , b , c , and d ) in the do_something_with calculation. The indentation is very deep. Is there a better way to do this? Ideally it would look something like this (which of course doesn't work): try { let b = f(a); let c = g(b); let d = h(c); do_something_with(a, b, c, d) }

问题 This question already has answers here : Why cannot convert null to type parameter T in c#? (3 answers) Closed last month . A similar question, but without casting the T parameter. This does not work public T GetValueG<T>(string Query) { object value = DBNull.Value; // Read from Database which can return DBNull if (value is DBNull) return (T)null; else return (T)value; } Error CS0037 Cannot convert null to 'T' because it is a non-nullable value type But this works public T GetValueG<T>(string

问题 I have a problem updating a foreign key in an Entity Framework entity. I am using self tracking entities and have an entity with some relations where the foreign key is also present as a property (one of the new features of EF4). The key (an integer) is marked as Nullable and Concurrency Mode fixed. Specifically I have an Alarm entity with a many to 0..1 relationship to a confirming user. (a user may confirm several alarms, but an alarm can be confirmed by only zero or one users). The entity

问题 I was wondering if CsvHelper by Josh Close has anything in the configuration I am missing to translate values to null. I am a huge fan of this library, but I always thought there should be some sort of configuration to let it know what values represent NULL in your file. An example would be a column with the value "NA", "EMPTY", "NULL", etc. I am sure I could create my own TypeConverter, but I was hoping there would be an easier option to set somewhere in a config as this tends to be fairly

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Nullable type as a generic parameter possible? I came across a very weird thing with generic type constraints. I have a class like this: public SomeClass<T> where T:class { } However, I've found I can't use nullable types as I'd expect: new SomeClass<int?>(); I get an error that int? must be a reference type. Is Nullable really just a struct with syntactic sugar to make it look like a reference type? 回答1:

问题 I have a few places where I need to compare 2 (nullable) values, to see if they're the same. I think there should be something in the framework to support this, but can't find anything, so instead have the following: public static bool IsDifferentTo(this bool? x, bool? y) { return (x.HasValue != y.HasValue) ? true : x.HasValue && x.Value != y.Value; } Then, within code I have if (x.IsDifferentTo(y)) ... I then have similar methods for nullable ints, nullable doubles etc. Is there not an

问题 I'm using a linq query to output an int array. But I need to pass this into a method that only accepts int?[]. So after searching on ways to convert int[] to int?[] I found something that seemed might work here The following code is a simplified example that shows what is working and not working. using System; using System.Collections.Generic; using System.Web; using System.Linq; namespace ConsoleApp { class Program { static void Main(string[] args) { // working... int[] vids1 = new[] { "",

问题 In my service, I have a protected constructor with @Inject and one of the parameters (provider) @Nullable . Any ideas why I am getting com.google.inject.CreationException: Guice creation errors: 1) No implementation for [[service]] was bound. ? Guice is 3.0pre1, @Nullable is ours. 回答1: @Nullable isn't the same as @Inject(optional=true) ... I think if you want to inject null , you need to bind(Service.class).toProvider(Providers.<Service>of(null)) or to otherwise have some kind of provider