I'm writing a generic wrapper class to implement INotifyPropertyChanged for a bunch of properties within another one of my classes. I've been doing some research on the implicit conversion operator, but I'm a bit confused on how to use it within a generic class. Essentially I would like to get the internally wrapped value without needing to explicitly call the internal property. The behavior I am looking for is essentially how the Nullable<T> class/struct works where if the internal value is not null, then it will return the internally wrapped value directly. Example below:
//current behavior
MyWrapperClass<int> wrapped = new MyWrapperClass();
int startCount = wrapped.Data;
//behavior I am looking to implement
int startCount = wrapped
In the second example above wrapped will return it's internally wrapped value instead of type T instead of having to call the inner property. This is how Nullable<T> behaves.
When looking into implicit conversions it appeared that I needed to know the type before hand per this MSDN article: Using Conversion Operators
Do I need to convert on a dynamic type since the type is not known? Example:
public static implicit operator dynamic(MyWrapperClass w)
Or can I perform implicit conversion on type T as seen below? This would prevent me from making the method static, which I noticed is used in all the sample code I've seen involving both implicit and explicit conversion operators. This option seems "wrong" to me, but I could not find much information on the subject here.
public implicit operator T(MyWrapperClass w)
EDIT: This SO Question might cause this to be labeled as a dupe, but the accepted answer is not what I am looking for since they say to use the property which I am already doing.
After some testing it appears that the second option works without issue and still allows itself to be static. I used #AndersForsgren's answer to this question (not accepted answer) to figure this out. Apparently I misunderstood how the implicit operator overload works. The code snippet that corrects this is as follows:
public static implicit operator T(WrapperClass<T> input)
{
return input.Data;
}
Related
I have a generic class, MyClass<T>, and I want to be able to implicitly convert from some type, e.g. bool, to a specific version of the generic type, e.g. MyClass<string>. It seems I cannot use any of the following:
fails because "Using the generic type 'MyClass<T>' requires '1' type argument(s)":
public static implicit operator MyClass(bool value) {
return new MyClass<string>(value.ToString());
}
fails because "Unbound generic name is not valid in this context" and because "User-defined conversion must convert to or from the enclosing type":
public static implicit operator MyClass<>(bool value) {
return new MyClass<string>(value.ToString());
}
fails because "User-defined conversion must convert to or from the enclosing type":
public static implicit operator MyClass<string>(bool value) {
return new MyClass<string>(value.ToString());
}
fails because "Cannot implicitly convert type 'MyClass<string>' to 'MyClass<T>'":
public static implicit operator MyClass<T>(bool value) {
return new MyClass<string>(value.ToString());
}
Is there any way this can be achieved, or will I just have to live without it (and incur explicit calls to a conversion method everywhere)?
No, you can't do this. The C# specification is clear, your implicit operator must convert either to or from the type in which it's declared. It has to be an exact conversion, and since the declaring type is exactly MyClass<T>, the conversion has to be either to or from that.
See e.g. Can i use a generic implicit or explicit operator? C# and C# Implicit operator with generic.
At the risk of condoning or endorsing an XY Problem, here are a couple of hacky alternatives:
// Break generics by checking the type explicitly. Requires ugly casting
// and intermediate boxing, though it's possible that with some run-time
// use of Expressions, you could cache a delegate that would handle the
// conversion without the boxing. It'd still be ugly though.
class Class1<T>
{
public Class1(T t) { }
public static implicit operator Class1<T>(bool value)
{
if (typeof(T) == typeof(string))
{
return (Class1<T>)(object)(Class1OfString)value;
}
throw new InvalidOperationException("Invalid type T");
}
}
// Subclass the generic, and declare the conversion there. Of course, then
// to use the conversion, you have to reference this type explicitly. Ugly.
class Class1OfString : Class1<string>
{
public Class1OfString(string text) : base(text) { }
public static implicit operator Class1OfString(bool value)
{
return new Class1OfString(value.ToString());
}
}
class A
{
public static void M()
{
// These all compile and execute fine
Class1OfString c1 = true;
Class1<string> c2 = (Class1OfString)true;
Class1<string> c3 = true;
}
}
There are a number of variations on the themes above, but they all will involve circumventing and special casing the type in some way.
It is worth pointing out that, besides the difficulty dealing with the generic vs. specific here, the use of implicit is suspect for other reasons. The documentation states right at the top that one should use implicit only "if the conversion is guaranteed not to cause a loss of data" and implementations "should never throw exceptions". In both cases, this is "so that they can be used safely without the programmer's awareness". In other words, the very nature of implicit is that they get invoked implicitly, without the programmer necessarily even thinking about it. So they must always work, which wouldn't necessarily be the case with some of the examples above (and in one example, you have to use an explicit syntax anyway, so you might as well implement the operator as explicit anyway).
None of these options are ideal. But frankly, neither is the original scenario. It is odd for a generic type to have to deal with concrete types on a specific basis. It calls into question whether the generic type really should be generic in the first place. It is possible that you really should be doing something more like the subclassing example above, only applied further. I.e. use the generic type for whatever base behavior you need, but put all your specializations into a subclass where you know the type parameter T.
I can't offer more advice than that, given the lack of details in the question. But the basic request is shaky enough to suggest that, if only the question had included a broader problem statement and details about what led you to this actual implementation goal, a better and more applicable answer might have been provided.
This is more a question out of curiosity than necessity and came about having had to deal with Active Directory (MS) types such as SearchResultCollection (in the System.DirectoryServices namespace).
Frequently when dealing with AD in code, I find that I'm having to check values for null, Count, [0] .. whatever and convert what I get out .. all the while hoping that the underlying AD object via COM doesn't go poof etc.
After having a play about with Parallel.ForEach recently - and having to pass in an IEnumerable<T>, I thought, maybe it would be fun to see how I could cast a SearchResultCollection to an IEnumerable of my own custom type. In this type I would pull out all the values from the SearchResult object and stick them in my own, .NET managed code. Then I'd do away with the DirectoryEntry, DirectorySearcher etc. etc.
So, having already worked out that it's a good idea to do searchResultCollection.Cast() in order to supply Parallel.ForEach with it's source, I added an explicit operator for casting to my own type (let's just call it 'Item').
I tested this out within the ParallelForEach, var myItem = (Item)currentSearchResult.
Good times, my cast operator is called and it all works. I then thought, it would be nice to do something like searchResultCollection.Cast<Item>(). Sadly this didn't work, didn't hit any breakpoints in the cast operator.
I did some Googling and discovered a helpful post which Jon Skeet had answered:
IEnumerable.Cast<>
The crux of it, use .Select(...) to force the explicit cast operation. OK, but, hmmm.
I went off and perhaps disassembled System.Core -> System.Linq.Enumerable.Cast<TResult>, I noticed that this 'cast' is actually doing an 'as' keyword conversion under the hood:
public static IEnumerable<TResult> Cast<TResult>(this IEnumerable source)
{
IEnumerable<TResult> enumerable = source as IEnumerable<TResult>;
if (enumerable != null)
{
return enumerable;
}
if (source == null)
{
throw Error.ArgumentNull("source");
}
return CastIterator<TResult>(source);
}
I read some more and found this:
Implicit/Explicit conversion with respect to the "as" keyword
The top answer here states that 'as' doesn't invoke any conversion operators .. use a (cast). Semantically I find this a little weird, since the extension method is called cast.. Shouldn't it be casting? There will no doubt be a really good reason why this doesn't happen, anyone know what it is?
Even if it would have used the cast operator instead of as it still wouldn't be invoking user defined explicit operators, so it wouldn't matter. The only difference would be the type of exception thrown.
Explicit operators aren't known at all by the runtime. According to the CLR there is no way to cast your search result to Item. When the compiler notices that there is a cast that matches a given explicit operator it injects at compile time a call to that explicit operator (which is basically a static method) into the code. Once you get to runtime there is no remaining knowledge of the cast, there is simply a method call in place to handle the conversion.
Because this is how explicit operators are implemented, rather than providing knowledge to the runtime of how to do the conversion, there is no way for Cast to inject the explicit operator's call into the code. It's already been compiled. When it was compiled there was no knowledge of any explicit operator to inject, so none was injected.
Semantically I find this a little weird, since the extension method is called cast.. Shouldn't it be casting?
It's casting each element if it needs to, within CastIterator... although using a generic cast, which won't use explicit conversion operator you've defined anyway. You should think of the explicit conversion operator as a custom method with syntactic sugar over the top, and not something the CLR cares about in most cases.
For for the as operator: that's just used to say "If this is already a sequence of the right type, we can just return." It's used on the sequence as a whole, not each element.
This can actually cause problems in some slightly bizarre situations where the C# conversion and the CLR conversions aren't aligned, although I can't remember the example I first came upon immediately.
See the Cast/OfType post within Edulinq for more details.
If I understand correctly you need a DynamicCast which I wrote sometime ago.
Runtime doesn't know about implicit and explicit casting; it is the job of the compiler to do that. but using Enumerable.Cast<> you can't get that because Enumerable.Cast involves casting from System.Object to Item where there is no conversion available(you have conversion from X to Item, and not Object to Item)
Take the advantage of dynamic in .Net4.0
public static class DynamicEnumerable
{
public static IEnumerable<T> DynamicCast<T>(this IEnumerable source)
{
foreach (dynamic current in source)
{
yield return (T)(current);
}
}
}
Use it as
var result = collection.DynamicCast<Item>();
It is casting. See the implementation of CastIterator.
static IEnumerable<TResult> CastIterator<TResult>(IEnumerable source) {
foreach (object obj in source) yield return (TResult)obj;
}
The use of the as keyword here is only to check if the entire collection can be casted to your target instead of casting item by item. If the as returns something that isn't null, then the entire collection is returned and we skip the whole iteration process to cast every item.
For example this trivial example would return a casted collection instead of iterating over every item
int?[] collection = ...;
var castedCollection = collection.Cast<int?>()
because the as works right off the bat, no need to iterate over every item.
In this example, the as gives a null result and we have to use the CastIterator to go over every object
int?[] collection = ...;
var castedCollection = collection.Cast<object>()
I'm trying to cast a generic type to a fixed one.
The following is what I expect to work, but there is a fundamental flaw in it.
public class Wrapper<T>
{
public T Value;
static public implicit operator TypeWithInt(Wrapper<int> wrapper)
{
return new TypeWithInt(wrapper.Value);
}
static public implicit operator TypeWithFloat(Wrapper<float> wrapper)
{
return new TypeWithFloat(wrapper.Value);
}
static public implicit operator TypeWithDouble(Wrapper<double> wrapper)
{
return new TypeWithDouble(wrapper.Value);
}
}
The above code doesn't compile with the following error:
User-defined conversion must convert to or from the enclosing type
As Wrapper<int> is different from Wrapper<T> it'll never work, because Wrapper<int> isn't the enclosing type.
So my question is: How can I make this casting work? Is there a way?
Your object model is a bit nonsensical because the .NET type system only considers at most 2 types at a time:
/* type 1 (int) */ int a = "abc" /* type 2 (string) */;
Yet you're trying to force another type in the middle which doesn't mesh. This isn't a limitation of the type conversion process, but instead, a limitation of the language. Every assignment (the phase at which implicit type conversions are enforced) can have at most 2 active parties, the left (int in the example above) and the right (string in the example above). Cascading implicit casts are not supported and would likely be really hard to code for.
The only possible solution I see to your problem would be to make this transition more visible to your user and add ToInt, ToFloat methods on Wrapper<T> to allow it to do its job.
Another point of interest might be the performance impact for doing this... the net result of your proposed wrapper is a boxing operation which may lead to unfortunate performance if you're working with a fair load. The alternative would be to rearchitect your application to be less type specific. Doing so would likely also eliminate the issue you're currently facing.
You could add the cast to an abstract non-generic base class and make the generic class inherit it.
A coworker and myself where discussing conversion in .NET he pointed out there are four ways to convert a type, at first I could only come up with two, implicit and explicit then he pointed out user defined conversions and using helper classes so I decided to look it up here http://msdn.microsoft.com/en-us/library/ms173105.aspx while reading that it dawned on me that I do a lot of WPF and often times use the IValueConverter interface.
My Question started as - isn't IValueConverter just an example of a conversion with a helper class (when you implement it of course)? Then I was like well, what is the real difference between user-defined conversions and conversions with a helper class? If you follow the links from the above mentioned MSDN page the documentation is sorda slim. Like this is the example from the conversion operators page.
class SampleClass
{
public static explicit operator SampleClass(int i)
{
SampleClass temp = new SampleClass();
// code to convert from int to SampleClass...
return temp;
}
}
It doesn't really make it so clear. To me it looks like a static class that needs an int in the ctor?
Anyway - Hopefully some C# ninja can illuminate this subject. One final thought is, I generally try and stay away from converting things unless there is a real good reason (i.e. parsing a spreadsheet) in regular everyday code I tend to think of it as a code smell, is that considered best practice?
Thanks
This isn't a full answer to your questions, but that code snippet overrides the "explicit cast" of the class, which isn't really intuitive fromt eh method signature. Basically it would allow you to do:
int one = 1;
SampleClass x = (SampleClass)one;
Common sense says that cast should fail, because and int isn't a SampleClass but the code snippet in your question comes into play, and makes the cast possible.
The other complementing method is:
public static implicit operator SampleClass(int i)
Note the keyword here is implicit instead of explicit, and this version would allow for implicit casting, so this would work:
int one = 1;
SampleClass x = one;
Note that you no longer have to specify the cast.
Say I have a simple object which supports implicit casting to System.String
public sealed class CompanyCode
{
public CompanyCode(String value)
{
{ Regex validation on value format }
_value = value;
}
readonly String _value;
public override String ToString() => _value;
static public implicit operator String(CompanyCode code) =>
code?.ToString();
}
Now lets say in another part of my program I perform a comparison with a string:
var companyCode = { some company code object }
if (companyCode == "MSFTUKCAMBS")
// do something...
What is the compiler doing with the == operator? Is it implicitly casting companyCode to a string and running the System.String == implementation? Is it using the System.Object == implementation? Or will the compiler just complain at me? (I don't have a compiler to check this right now).
As far as I can see I have a couple of other options.
Implement an ==(String x) operator on CompanyCode.
Implement the IEquatable<String> interface on CompanyCode.
Would any (or both) of these options be preferable?
Say I have a simple object which supports implicit casting to System.String
I would question this design decision to start with. The fact that it's brought up this question of operator overloading suggests that your colleagues will be asking the same sort of questions. I don't even know the answer (in terms of what the compiler will do) off the top of my head.
I definitely wouldn't suggest implementing IEquatable<string>, as then x.Equals(y) won't be symmetric with y.Equals(x). You could implement two overloads of == in your class, both ways round... but then it wouldn't be consistent with Equals.
I would suggest just having a property called Value or Code of type string, then you can use:
if (companyCode.Value == "MSFTUKCAMBS")
and it will be immediately clear what that means.
Basically, I think the situations where implicit conversions are appropriate are very few and far between.
From the Design Guidelines for Class Library Developers
Do not provide a conversion operator if such conversion is not clearly expected by the end users.
Is there such a clear expectation here?
It will implicitly cast to a string and check equality using the string's == operator.
For the case you show - every way you offered is suitable, but every way has a different purpose and meaning.
Implicitly conversion should usually be avoided.
Implementing the == is to allow comparing with a string,
and IEquatable is simply to allow using the class as type IEquatable, for outside code references. The IEquatable may very well just return the == result.
For this case, I would choose the == operator overloading, unless you have any other purpose for using implicit conversion.
Also, if you use implicit conversions, it will be a bit uglier code-wise but smarter robust-wise to overload an EXPLICIT conversion, and not an implicit one, so whenever one wants to convert your class to a string, he will have to cast it using (string)obj, which is also a good reminder for whats really happening in the code.
I personally would suggest do not use operator overloading, in these cases. It's kind of confusing looking on the code, to understand what is going on.
It's a much better, imo, having some function that esplicitly manifests comparison operation, or, like Jon suggests, use a property.
In short make it clear for the reader of your code what you're gonna to compare.