I'm familiar with using out to pass in a simple data type for manipulation, but I can't seem to figure out how to pass in this Queue<> without causing a compile error. Any ideas?
Code:
Queue<SqlCommand> insertScriptQueue = new Queue<SqlCommand>();
private void UpdateDefaultIndicator(int newDefaultViewID,
out (Queue<SqlCommand>) insertScriptQueue)
UpdateDefaultIndicator(newViewID, out (Queue<SqlCommand>)insertScriptQueue);
You're passing in a reference type. No need to use out.
You shouldn't be initializing an out variable. If you need to modify an in-scope variable, use ref instead.
As Ed points out in his comment, "modify" may not give you the full idea of what's happening here - an out parameter of a reference type will by definition be an initialized object at the end of the function call. As most other answers have pointed out, if you want to pass in an initialized object, ref is the stronger choice.
Queue<SqlCommand> insertScriptQueue;
private void UpdateDefaultIndicator(int newDefaultViewID,
out Queue<SqlCommand> insertScriptQueue){/*body*/}
UpdateDefaultIndicator(newViewID,out insertScriptQueue);
That works fine for me... What error are you getting?
Why do you want a "out" ...here...why dont you return the type instead ? Let method return Queue<> insteasd of void..will that work for you?
The Queue is going to be passed by reference anyway, its not a value type. Just don't use 'out'. UPDATE: Pardon me, I was thinking of 'ref' - but the fact that you're passing a Queue data type in, and not just an unallocated reference, makes me think that you want to be using 'ref' anyway. Except of course that you don't need to use 'ref' because the Queue isn't a value type; its already going to be passed in 'by reference', by default.
make sure that you are assigning insertScriptQueue some kind of value inside of the UpdateDefaultIndicator method
The answer to the original question, by the way, is that you're casting to Queue, and the cast is returning an interim reference. That reference is not assignable, and so is not a legal out parameter. Erich's code implements the fix for this problem.
Related
I have an AnonymousType object that contacts two fields with their values. How can I access the value of these fields?
Ex:
SourceTypeObject { Source_Type_Id = 1, Source_Type_Name = "bibliography" }
I need to do something like : SourceTypeObject.Source_Type_Id
Is that possible?
EDIT:
Here's what I get if I tried to access the property directly:
Yes, this is the exact purpose of anonymous types. The only thing that might prevent you from doing so is if you passed the anonymous type around as a parameter with type "object". This would hide information about the anonymous type, and it would look like just any old object then.
The only recourse if this is the case is to use reflection, which is slow and awkward. Anonymous types are a meant to be a very "local" phenomenon, and if you find yourself wanting to use them elsewhere in the program, it's worth the time to promote it to a real type.
EDIT: In response to the image you posted, assuming the array is declared locally just outside of view, try to replace the object SourceTypeObject with var SourceTypeObject. This allows it to infer the anonymous type instead of being told that it's an object.
I work on developing an external API. I added a method to my public interface :
public void AddMode(TypeA mode);
public void AddMode(TypeB mode); // the new method, TypeB and TypeA are not related at all
It looked good, until one test broke that was passing a null . That made the compiler confused with ambiguous call. I fixed the test with casting the null.
However my question is :
Should I change the name just because of this?
Or should let the client do the cast as I did? (if they pass null for whatever reason)
What is the best in this case while designing APIs ?
Edit :
the call was like this AddMode(null) , not like :
TypeA vl = null;
AddMode(v1); // this doesn't cause a problem
An API should be designed so that it's easy to use correctly and hard to use incorrectly.
Your API is easy to use correctly:
AddMode(new TypeA());
does compile.
It's harder to use incorrectly:
AddMode(null);
does not compile. The user ist forced to do something like
AddMode((TypeA)null);
Which should make him think, whether this is expected usage. So I think your API is OK as it is.
I think that depends on how exceptional null as a value for the respective argument is.
Compare, for example, this ArgumentNullException constructor: It is most frequently called when an internal exception has to be set. Otherwise, this constructor, which excepts the name of the illegal argument, is passed. On odd occasions, the former has to be invoked because a custom message has to be supplied, but no internal exception is supplied (I usually do this when I'm throwing the exception for an array/collection argument that contains null, but is not null itself). So, in this case, I need the explicit cast, and I'd say it is acceptable there.
If your methods really do the same, but null is still a usual value, you might want to add a parameterless overload for the null variant (i.e. the explicit cast is still possible, but users can also call the parameterless overload instead).
If your methods do something somewhat different, and yet something else for null, you can think about disallowing null altogether for the methods you've shown and adding a parameterless overload for the null case.
Update: If null is not acceptable anyway (and will result in an exception), then you should leave the method as it is. Other than for testing purposes, there should never be any whatsoever situation in which a literal null would be passed to the method, as this will invariably yield an exception. Therefore, do not change your overload names in this case.
Is null valid input to this method anyway?
Personally I'd leave it as is as long as both overloads of AddMode related, since you'd expect AddMode(X) and AddMode(Y) to be doing something related to each other.
If they are not related in any way, then maybe a method name change is in order
Well, it depends either null value is acceptable value in your API.
If not just do not accept it, by not supporting it. So even if the consumer will try to use it with null the compiler will break on ambiguity problem.
If your API accepts a null as a possible parameter value, then you have to specify it in the documentation and mention it is necessary to cast it, and write some code samples to show how.
However, if you don not want the user to use null values, you might change TypeA and TypeB as a struct instead of a class, if your class design allow it.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Casting: (NewType) vs. Object as NewType
In C#, why ever cast reference types when you can use "as"?
Casting can generate exceptions whereas "as" will evaulate to null if the casting fails.
Wouldn't "as" be easier to use with reference types in all cases?
eg:
MyObject as DataGridView
rather than,
(DataGridView)MyObject
Consider the following alternatives:
Foo(someObj as SomeClass);
and:
Foo((SomeClass)someObj);
Due to someObj being of the wrong type, the first version passes null to Foo. Some time later, this results in a NullReferenceException being thrown. How much later? Depends on what Foo does. It might store the null in a field, and then minutes later it's accessed by some code that expects it to be non-null.
But with the second version, you find the problem immediately.
Why make it harder to fix bugs?
Update
The OP asked in a comment: isn't is easier to use as and then check for null in an if statement?
If the null is unexpected and is evidence of a bug in the caller, you could say:
SomeClass c = someObj as SomeClass;
if (c == null)
{
// hmm...
}
What do you do in that if-block? There are two general solutions. One is to throw an exception, so it is the caller's responsibility to deal with their mistake. In which case it is definitely simpler to write:
SomeClass c = (SomeClass)someObj;
It simply saves you writing the if/throw logic by hand.
There is another alternative though. If you have a "stock" implementation of SomeClass that you are happy to use where nothing better is available (maybe it has methods that do nothing, or return "empty" values, etc.) then you could do this:
SomeClass c = (someObj as SomeClass) ?? _stockImpl;
This will ensure that c is never null. But is that really better? What if the caller has a bug; don't you want to help find bugs? By swapping in a default object, you disguise the bug. That sounds like an attractive idea until you waste a week of your life trying to track down a bug.
(In a way this mimics the behaviour of Objective-C, in which any attempt to use a null reference will never throw; it just silently does nothing.)
operator 'as' work with reference types only.
Sometimes, you want the exception to be thrown. Sometimes, you want to try to convert and nulls are OK. As already stated, as will not work with value types.
One definite reason is that the object is, or could be (when writing a generic method, you may not know at coding-time) being cast to a value type, in which case as isn't allowed.
One more dubious reason is that you already know that the object is of the type in question. Just how dubious depends on how you already know that. In the following case:
if(obj is MyType)
DoMyTypeStuff((MyType)obj);
else
DoMoreGeneralStuff(obj);
It's hard to justify using as here, as the only thing it really does is add a redundant check (maybe it'll be optimised away, maybe it won't). At the other extreme, if you are half-way to a trance state with the amount of information you've got in you're brain's paged-in memory and on the basis of that you are pretty sure that the object must be of the type in question, maybe it's better to add in the check.
Another good reason is that the difference between being of the wrong type and being null gets hidden by as. If it's reasonable to be passing in a string to a given method, including a null string, but it's not reasonable to pass in an int, then val as string has just made the incorrect usage look like a completely different correct usage, and you've just made the bug harder to find and potentially more damaging.
Finally, maybe if you don't know the type of the object, the calling code should. If the calling code has called yours incorrectly, they should receive an exception. To either allow the InvalidCastException to pass back, or to catch it and throw an InvalidArgument exception or similar is a reasonable and clear means of doing so.
If, when you write the code to make the cast, you are sure that the cast should work, you should use (DataGridView)MyObject. This way, if the cast fails in the future, your assumption about the type of MyObject will cause an invalid cast exception at the point where you make the cast, instead of a null reference exception at some point later.
If you do want to handle the case where MyObject is not a DataGridView, then use as, and presumably check for it being null before doing anything with it.
tl;dr If your code assumes something, and that assumption is wrong at run-time, the code should throw an exception.
From MSDN (as (C# reference)):
the as operator only performs reference conversions and boxing conversions. The as operator cannot perform other conversions, such as user-defined conversions, which should instead be performed using cast expressions.
Taking into consideration all of the comments, we came across this just the other day and wondered why you would do a direct cast over using the keyword as. What if you want the cast to fail? This is sometimes the desirable effect you want from a cast if you're casting from a null object. You then push the exception up the call stack.
So, if you want something to fail, use a direct cast, if you're okay with it not failing, use the as keyword.
As is faster and doesn't throw exceptions. Therefore it is generally preferred. Reasons to use casts include:
Using as, you can only assign types that are lower in the inheritance tree to ones that are higher. For example:
object o = "abc" as object;
DataGridView d = "abc" as DataGridView // doesn't do anything
DataGridView could create a custom cast that does allow this. Casts are defined on the target type and therefore allow everything, as long as it's defined.
Another problem with as is that it doesn't always work. Consider this method:
IEnumerable<T> GetList<T>(T item)
{
(from ... select) as IEnumerable<T>
}
This code fails because T could also be a Value Type. You can't use as on those because they can never be null. This means you'll have to put a constraint on T, while it is actually unnecesary. If you don't know whether you're going to have a reference type or not, you can never use as.
Of course, you should always check for null when you use the as keyword. Don't assume no exceptions will be thrown just becase the keyword doesn't throw any. Don't put a Try {} Catch(NullReferenceException){} around it, that't unneccesary and bloat. Just assign the value to a variable and check for null before you use it. Never use it inline in a method call.
When you get Type of the variable you can check its name:
if (my_type.Name=="Int32")
however it would more elegant to write
if (my_type.Name==TypeNames.Int32)
to avoid typos. I can define such class on my own, but maybe there is already definition somewhere?
If yes, where?
Note: please avoid wondering "why would you like to get type of the variable in the first place", "it is better to use 'is'" and alike. Thank you very much!
Edit: meanwhile, I jumped into the conclusion it would be enough to ignore the type of the object (my_type variable) and check the object instead. In other words the my_type is not necessary. I forgot about null case :-( Less code, more sleep, that's what I need ;-)
Try the following
typeof(Int32).Name
If you want to compare types though doing so by name is not the best solution as it will be wrong in many cases. It's more correct to compare the types directly.
if ( m_type == typeof(Int32) ) {
...
}
The type names are not defined anywhere in a class. They are generated at runtime by the CLR (by a call to the external ConstructName function) using reflection.
Using the suggestion of JaredPar will do the job.
What is the design decision to lean towards not returning an anonymous types from a method?
You can return an instance of an anonymous type from a method - but because you can't name it, you can't declare exactly what the method will return, so you'd have to declare that it returns just object. That means the caller won't have statically typed access to the properties etc - although they could still pass the instance around, access it via reflection (or dynamic typing in C# 4).
Personally I would quite like a future version of C# to allow you to write a very brief class declaration which generates the same code (immutable properties, constructor, Equals/GetHashcode/ToString) with a name...
There is one grotty hack to go round it, called casting by example. I wouldn't recommend it though.
Because an anonymous type has no name. Therefore you cannot declare a return type for a method.
Because C# is statically typed language and in a statically typed language the return type of a method needs to be known at compile time and anonymous types have no names.
How can you use your type inside your method if the definition is only in the call of the method ?
It's not javascript.
A lot of answers heere seem to indicatie that it is not possible because of the current syntax and rule. But the question is about changing them. I think it would be possible, but a little complicated and resulting in an awkward (error-prone) syntax. Like
var f() { return new { A = "*", B = 1 }; }
var x = f();
The question is whether this adds sufficient value to the language to make it worth it.
At least up to 3.5, anonymous types are actually resolved at compile time, and this would be impossible (or quite hard) to do with anonymous method signatures.