Can someone explain why this is occurring? The code below was executed in the immediate window in vs2008. The prop is an Int32 property (id column) on an object created by the entity framework.
The objects entity and defaultEntity were created using Activator.CreateInstance();
Convert.ChangeType(prop.GetValue(entity, null), prop.PropertyType)
0
Convert.ChangeType(prop.GetValue(defaultEntity, null), prop.PropertyType)
0
Convert.ChangeType(prop.GetValue(entity, null), prop.PropertyType) == Convert.ChangeType(prop.GetValue(defaultEntity, null), prop.PropertyType)
false
I assume you're wondering why the third line prints false. If you want to know why the first two lines are printing 0, you'll have to post more code and tell us what you actually expected.
Convert.ChangeType returns object. Therefore when the property type is actually Int32 it will return a boxed integer.
Your final line is comparing the references of two boxed values. Effectively you're doing:
object x = 0;
object y = 0;
Console.WriteLine (x == y); // Prints False
You can use Equals instead - and the static object.Equals method handily copes with null references, should that be an issue:
object x = 0;
object y = 0;
Console.WriteLine (object.Equals(x, y)); // Prints True
Related
I'm studying recursion and I wrote this method to calculate the N° number of the Fibonacci series:
fibonacci(int n, Map memo) {
if (memo.containsKey(n)) return memo[n]; // Memo check
if (n <= 2) return 1; // base case
// calculation
memo[n] = fibonacci(n - 1, memo) + fibonacci((n - 2), memo);
return memo[n];
}
I think it doesn't need to be explained, my problem is just how to call this function from the main, avoiding providing an empty Map.
this is how I call the function now:
fibonacci(n, {});
But I would rather prefer to call it just like this:
fibonacci(n);
The canonical approach is to make memo optional, and use a fresh map if the memo argument is omitted. Because you want to change and update the map, you can't use a default value for the parameter, because default values must be constant, and constant maps are not mutable.
So, written very concisely:
int fibonacci(int n, [Map<int, int>? memo]) {
if (n <= 2) return 1;
return (memo ??= {})[n] ??= fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
The ??= operator assigns to the right-hand side if the value is null.
It's used both to initialize memo to a new map if the argument was omitted,
and to update the map if a cached value wasn't present.
I'd actually reconsider using a map. We know that the Fibonacci computation will compute a value for every prior number down to 1, so I'd just use a list instead:
int fibonacci(int n, [List<int?>? memo]) {
if (n <= 2) return 1;
return (memo ??= List<int?>.filled(n - 2))[n - 3] ??=
fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
That should work just like the map.
(I subtract 3 from n when doing the lookup because no value below 3 needs the list - it's handled by the prior if).
There are multiple ways to do it. This is my personal favorite, because it also limits the function that is only used for internal means and it doesn't have the need to check every recursion, as you already know there is a map provided:
int fibonacci(int n) {
return _fibonacci(n, {});
}
int _fibonacci(int n, Map<int, int> memo) {
if (n <= 2) return 1; // base case
final previouslyCalculated = memo[n]; // Memo check
if(previouslyCalculated != null) {
return previouslyCalculated;
}
// calculation
final next = _fibonacci(n - 1, memo) + _fibonacci((n - 2), memo);
memo[n] = next;
return next;
}
void main() {
print(fibonacci(4));
}
As Dart does not support overloading, if you actually need both versions to be publicly available (or want both private) you would have to pick different names.
Please note that I added proper types to your methods and cleaned them up a bit for everything that would not compile once proper types are used. Make sure you always use proper types and don't rely on dynamic to somehow works it's magic. The compiler can only help you, if you are explicit about what you want to do. Otherwise they can only nod and let you run into any mistake you may have made. Be smart, let your compiler help, it will catch a lot of errors for you at compile time that you would otherwise have to spent countless hours on debugging.
This is the solution I've found so far but looks very verbose and inelegant:
fibonacci(int n, [Map<int, int>? memo]) {
memo == null ? memo = {} : null; // null check
if (memo.containsKey(n)) return memo[n];
if (n <= 2) return 1;
memo[n] = fibonacci(n - 1, memo) + fibonacci((n - 2), memo);
return memo[n];
}
In this way I can call just:
fibonacci(n);
On trying to assign values to Nested Object Properties,Dart treats the Nested Object(class OperandRange) as null.
Default values have been assigned to the Nested Object Properties but the issue exists.
In the case below Nested Object Class OperandRange should be assigned minimum and maximum values but dart considers it to the Null.
How to resolve this?
Code
import 'dart:io';
//Nested Object Class
class OperandRange{
double _minValue = 0;
double _maxValue = 10;
OperandRange(this._minValue , this._maxValue);
double get minValue => _minValue;
double get maxValue => _maxValue;
set minValue(double _val){
_minValue = (_val) ;
}
set maxValue(double _val){
_maxValue = (_val) ;
}
}
class OperationData{
List<OperandRange> operandList = [];//Nested Object
List<String> operatorList = [] ;
OperationData({this.operandList, this.operatorList});
}
void main(){
int _operationCount = 2;
OperationData _operation = OperationData();
for(int _index = 0 ; _index < _operationCount ; _index++) {
stdout.write(" Operation $_index - Name(string): ");
_operation.operatorList[_index] = stdin.readLineSync();
//Null Object
stdout.write(" Operand $_index - Minimum Value (double) : ");
_operation.operandList[_index]._minValue =
double.parse(stdin.readLineSync());
stdout.write(" Operand $_index - Maximum Value (double): ");
_operation.operandList[_index]._maxValue =
double.parse(stdin.readLineSync());
}
}
Error
Operation 0 - Name(string): Add
Unhandled exception:
NoSuchMethodError: The method '[]=' was called on null.
Receiver: null
Tried calling: []=(0, "Add")
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1 main (1.dart:41:28)
#2 _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)
#3 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
Process finished with exit code 255
Here is what's happening.
You initialize operandList with a nested list. But this never has any effect because you also initialize it in OperationData constructor. Once you mention it in constructor arguments, it will either be set to a value you pass to constructor, or set to null if you do not pass this argument to constructor.
For your purpose you may remove constructor altogether as you never pass anything to it. Then your [] defaults will stand.
Otherwise, if in some cases you need to initialize it with a custom list, you may do it like this:
class OperationData{
List<OperandRange> operandList;
List<String> operatorList;
OperationData({
List<OperandRange> operandList,
List<String>operatorList,
}) :
this.operandList = operandList ?? <OperandList>[],
this.operatorList = operatorList ?? <String>[]
;
}
The same goes for your OperandRange class. 0 and 10 defaults will never be used as the constructor requires explicit values. By the way, I do not see OperandRange creation at all. The list stays empty. You will catch a next error when trying to access an index out of bounds when you fix the first error.
Also you should upgrade to Dart 2.12 if possible. It introduced null-safety that would show you this error at compile time.
According to the documentation, the function identical checks whether two references are to the same object.
With that in mind, I don't understand why the following is the case:
int a = 1;
int b = 1;
print(identical(a, b)); // prints 'true'
Map c = { 1: 'y' };
Map d = { 1: 'y' };
print(identical(c, d)); // prints 'false'
I'd expect both calls to return 'false'.
identical compares references. a and b are references to a compile time literal 1. Thus they are identical.
I having an issue when trying to update a SF stateful service after updating a child object of the service state. The upgrade fails to pass the 1st upgrade domain with a 'package activation' error. Digging around in the event viewer on the offending node gives the below exception:
Errormsg=TStore.OnApplyAddAsync: Unexpected exception System.NullReferenceException: Object reference not set to an instance of an object.
at xxx.DataChildObject.GetHashCode(IEqualityComparer comp)
at xxx.Data.GetHashCode(IEqualityComparer comp)
at xxx.Data.GetHashCode() in C:\Users\xxx\Source\Repos\xxx\xxx\xxx.Core\Domain.fs:line 17
at System.Fabric.Store.TStore`5.OnApplyAdd(TransactionBase txn, MetadataOperationData metadataOperationData, RedoUndoOperationData operationRedoUndo, Boolean isIdempotent, String applyType) Assert=System.Exception: at System.Environment.GetStackTrace(Exception e, Boolean needFileInfo)
The change to the 'DataChildObject' was to add a new field that is a tuple of 2 doubles.
I understand that F# is automatically generating equality methods and these must be triggered during SF state and that due to datacontact serialisation these are null during the GetHashCode check.
I don't understand exactly when this check is being performed or why though?
As a test i tried overriding the GetHasCode method on my DataChildObject, but this didn't change the error i get when trying to upgrade my service.
[<DataContract>]
type DataChildObject =
class
[<DataMember(IsRequired=false,Name="Value1")>] val mutable Value1 : float * float
[<DataMember(IsRequired=false,Name="Value2")>] val mutable Value2 : float * float
[<DataMember(IsRequired=false,Name="NewValue")>] val mutable NewValue : float * float
new (v1,v2) = {Value1=v1;Value2=v2;NewValue=1.0,1.0}
override this.GetHashCode() =
let value1 = if (box this.Value1 = null) then 1 else this.Value1.GetHashCode()
let value2 = if (box this.Value2 = null) then 1 else this.Value2.GetHashCode()
let newValue = if (box this.NewValue = null) then 1 else (this.NewValue).GetHashCode()
value1+value2+newValue
end
I expect these domain objects to change more in the near future so any help in understanding exact how to get past or avoid this issue is helpful.
Thanks
Below code is throwing an IndexOutOfBoundsException at line Field f = getField(counter);
Why is it being thrown ? Surely the field exists because I am looping based on fieldcount. Or is the list fields in the manager not gauranteed to be sequential? If this is the case how should I delete fields from a screen that are of type - MyButtonField
Thanks
int fieldCount = getFieldCount() - 1;
if(fieldCount > 1){
for(int counter = 0; counter <= fieldCount ; ++counter){
Field f = getField(counter);
if(f instanceof MyButtonField){
delete(f);
}
}
}
You haven't specified what delete(f) does, but if it removes it from the list of fields, then your "valid count" will effectively decrease.
To rewrite this somewhat and fix the problem:
for (int index = getFieldCount() - 1; index >= 0; index--){
Field f = getField(index);
if (f instanceof MyButtonField) {
delete(f);
}
}
This will go from the end of the fields instead of the start, so it doesn't matter if you remove an entry and everything shuffles up - the items which shuffle up will be the ones you've already looked at.
The best way is to use Iterator for iteration then call the method remove().
Example:
for(Iterator it = getFields().iterator();it.hasNext()){
Field f = (Field) it.next();
if(f instanceof MyButtonField){
it.remove();
}
}
The method getFields() has to return a collection of Field elements.