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.
Related
C# cannot infer a type argument in this pretty obvious case:
public void Test<T>(IEnumerable<KeyValuePair<string, T>> kvp)
{
Console.WriteLine(kvp.GetType().Name + ": KeyValues");
}
Test(new Newtonsoft.Json.Linq.JObject());
The JObject type clearly implements IEnumerable<KeyValuePair<string, JToken>>, but I get the following error:
CS0411: The type arguments for method cannot be inferred from the usage.
Why does this happen?
UPD: To the editor who marked this question as duplicate: please note how my method's signature accepts not IEnumerable<T>, but IEnumerable<KeyValuePair<string, T>>. The JObject type implements IEnumerable twice, but only one of the implementations matches this constraint - so there should be no ambiguity.
UPD: Here's a complete self-contained repro without JObject:
https://gist.github.com/impworks/2eee2cd0364815ab8245b81963934642
Here's a simpler repro:
interface I<T> {}
class X<T> {}
class Y {}
class Z {}
class C : I<X<Y>>, I<Z> {}
public class P
{
static void M<T>(I<X<T>> i) { }
public static void Main()
{
M(new C());
}
}
Type inference fails. You ask why, and why questions are always difficult to answer, so let me rephrase the question:
What line of the specification disallows this inference?
I have my copy of the C# 3 specification at hand; the line there is as follows
V is the type we are inferring to, so I<X<T>> in this case
U is the type we are inferring from, so C in this case
This will be slightly different in C# 4 because I added covariance, but we can ignore that for the purposes of this discussion.
... if V is a constructed type C<V1, … Vk> and there is a unique set of types U1, … Uk such that an implicit conversion exists from U to C<U1, … Uk> then an exact inference is made from each Ui to the corresponding Vi. Otherwise no inferences are made.
Notice the word unique in there. There is NOT a unique set of types such that C is convertible to I<Something> because both X<Y> and Z are valid.
Here's another non-why question:
What factors were considered when the design team made this decision?
You are right that in theory we could detect in your case that X<Y> is intended and Z is not. If you would care to propose a type inference algorithm that can handle non-unique situations like this that never makes a mistake -- remember, Z could be a subtype or a supertype of X<Y> or X<Something Else> and I could be covariant -- then I am sure that the C# team would be happy to consider your proposal.
We had that argument in 2005 when designing the C# 3 type inference algorithm and decided that scenarios where one class implements two of the same interface were rare, and that dealing with those rare situations created considerable complications in the language. Those complications would be expensive to design, specify, implement and test, and we had other things to spend money and effort on that would have bigger impact.
Also, we did not know when we made C# 3 whether or not we would be adding covariance in C# 4. We never want to introduce a new language feature that makes a possible future language feature impossible or difficult. It is better to put restrictions in the language now and consider removing them later than to do a lot of work for a rare scenario that makes a common scenario difficult in the next version.
The fact that I helped design this algorithm and implemented it multiple times, and completely did not remember this rule at first should tell you how often this has come up in the last 13 years. Hardly at all. It is very rare for someone to be in your particular boat, and there is an easy workaround: specify the type.
A question which you did not ask but comes to mind:
Could the error message be better?
Yep. I apologize for that. I did a lot of work making overload resolution error messages more descriptive for common LINQ scenarios, and I always wanted to go back and make the other type inference error messages more clear. I wrote the type inference and overload resolution code to maintain internal information explaining why a type had been inferred or an overload had been chosen or rejected, both for my own debugging purposes and to make better error messages, but I never got around to exposing that information to the user. There was always something higher priority.
You might consider entering an issue at the Roslyn GitHub site suggesting that this error message be improved to help users diagnose the situation more easily. Again, the fact that I didn't immediately diagnose the problem and had to go back to the spec is indicative that the message is unclear.
Problem is following.
If you don't specify type then it will try to infer automatically but if it get confuse then it throw specified exception.
In your case JObject has implemented interface IEnumerable<KeyValuePair<string, JToken>>.
Also JObject implemented JContainer and it also implemented IEnumerable< JToken>.
So when it is specified for T it is confuse between IEnumerable<JToken> and IEnumrable<KeyValuePair<string, JToken>>.
I suspect that when people who designed C# were thinking about how compiler would infer the type for a generic type parameter they thought about potential issues with that.
Consider the following scenario:
class MyClass : JObject, IEnumerable<KeyValuePair<string, int>>
{
public IEnumerator<KeyValuePair<string, int>> GetEnumerator()
{
throw new NotImplementedException();
}
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}
class Foo : MyClass { }
class FooBar : Foo { }
public void Test<T>(IEnumerable<KeyValuePair<string, T>> kvp)
{
Console.WriteLine(kvp.GetType().Name + ": KeyValues");
}
var fooBar = new FooBar();
Test(fooBar);
Which type should the T be inferred to? int, or JToken ?
Also how complex should the algorithm be ?
Presumably that's why compiler only infers the type when there is no chance for ambiguity. It was designed this way and not without a reason.
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;
}
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.
Inspired by this question.
Short version: Why can't the compiler figure out the compile-time type of M(dynamic arg) if there is only one overload of M or all of the overloads of M have the same return type?
Per the spec, §7.6.5:
An invocation-expression is dynamically bound (§7.2.2) if at least one of the following holds:
The primary-expression has compile-time type dynamic.
At least one argument of the optional argument-list has compile-time type dynamic and the primary-expression does not have a delegate type.
It makes sense that for
class Foo {
public int M(string s) { return 0; }
public string M(int s) { return String.Empty; }
}
the compiler can't figure out the compile-time type of
dynamic d = // dynamic
var x = new Foo().M(d);
because it won't know until runtime which overload of M is invoked.
However, why can't the compiler figure out the compile-time type if M has only one overload or all of the overloads of M return the same type?
I'm looking to understand why the spec doesn't allow the compiler to type these expressions statically at compile time.
UPDATE: This question was the subject of my blog on the 22nd of October, 2012. Thanks for the great question!
Why can't the compiler figure out the compile-type type of M(dynamic_expression) if there is only one overload of M or all of the overloads of M have the same return type?
The compiler can figure out the compile-time type; the compile-time type is dynamic, and the compiler figures that out successfully.
I think the question you intended to ask is:
Why is the compile-time type of M(dynamic_expression) always dynamic, even in the rare and unlikely case that you're making a completely unnecessary dynamic call to a method M that will always be chosen regardless of the argument type?
When you phrase the question like that, it kinda answers itself. :-)
Reason one:
The cases you envision are rare; in order for the compiler to be able to make the kind of inference you describe, enough information must be known so that the compiler can do almost a full static type analysis of the expression. But if you are in that scenario then why are you using dynamic in the first place? You would do far better to simply say:
object d = whatever;
Foo foo = new Foo();
int x = (d is string) ? foo.M((string)d) : foo((int)d);
Obviously if there is only one overload of M then it is even easier: cast the object to the desired type. If it fails at runtime because the cast it bad, well, dynamic would have failed too!
There's simply no need for dynamic in the first place in these sorts of scenarios, so why would we do a lot of expensive and difficult type inference work in the compiler to enable a scenario we don't want you using dynamic for in the first place?
Reason two:
Suppose we did say that overload resolution has very special rules if the method group is statically known to contain one method. Great. Now we've just added a new kind of fragility to the language. Now adding a new overload changes the return type of a call to a completely different type -- a type which not only causes dynamic semantics, but also boxes value types. But wait, it gets worse!
// Foo corporation:
class B
{
}
// Bar corporation:
class D : B
{
public int M(int x) { return x; }
}
// Baz corporation:
dynamic dyn = whatever;
D d = new D();
var q = d.M(dyn);
Let's suppose that we implement your feature requiest and infer that q is int, by your logic. Now Foo corporation adds:
class B
{
public string M(string x) { return x; }
}
And suddenly when Baz corporation recompiles their code, suddenly the type of q quietly turns to dynamic, because we don't know at compile time that dyn is not a string. That is a bizarre and unexpected change in the static analysis! Why should a third party adding a new method to a base class cause the type of a local variable to change in an entirely different method in an entirely different class that is written at a different company, a company that does not even use B directly, but only via D?
This is a new form of the Brittle Base Class problem, and we seek to minimize Brittle Base Class problems in C#.
Or, what if instead Foo corp said:
class B
{
protected string M(string x) { return x; }
}
Now, by your logic,
var q = d.M(dyn);
gives q the type int when the code above is outside of a type that inherits from D, but
var q = this.M(dyn);
gives the type of q as dynamic when inside a type that inherits from D! As a developer I would find that quite surprising.
Reason Three:
There is too much cleverness in C# already. Our aim is not to build a logic engine that can work out all possible type restrictions on all possible values given a particular program. We prefer to have general, understandable, comprehensible rules that can be written down easily and implemented without bugs. The spec is already eight hundred pages long and writing a bug-free compiler is incredibly difficult. Let's not make it more difficult. Not to mention the expense of testing all those crazy cases.
Reason four:
Moreover: the language affords you many opportunities to avail yourself of the static type analyzer. If you are using dynamic, you are specifically asking for that analyzer to defer its action until runtime. It should not be a surprise that using the "stop doing static type analysis at compile time" feature causes static type analysis to not work very well at compile time.
An early design of the dynamic feature had support for something like this. The compiler would still do static overload resolution, and introduced a "phantom overload" that represents dynamic overload resolution only if necessary.
Blog post introducing phantom methods
Details on phantom methods
As you can see in the second post, this approach introduces a lot of complexity (the second article talks about how type inference would need to be modified to make the approach work out). I'm not surprised that the C# team decided to go with the simpler idea of always using dynamic overload resolution when dynamic is involved.
However, why can't the compiler figure out the compile-time type if M has only one overload or all of the overloads of M return the same type?
The compiler could potentially do this, but the language team decided not to have it work this way.
The entire purpose of dynamic is to have all expressions using dynamic execute with "their resolution is deferred until the program is run" (C# spec, 4.2.3). The compiler explicitly does not perform static binding (which would be required to get the behavior you want here) for dynamic expressions.
Having a fallback to static binding if there was only a single binding option would force the compiler to check this case - which was not added in. As for why the language team didn't want to do it, I suspect Eric Lippert's response here applies:
I am asked "why doesn't C# implement feature X?" all the time. The answer is always the same: because no one ever designed, specified, implemented, tested, documented and shipped that feature.
I think the case of being able to statically determine the only possible return type of a dynamic method resolution is so narrow that it would be more confusing and inconsistent if the C# compiler did it, rather than having across the board behavior.
Even with your example, what if Foo is part of a different dll, Foo could be a newer version at runtime from a binding redirect with additional M's that have a different return type, and then the compiler would have guessed wrong because the runtime resolution would return a different type.
What if Foo is an IDynamicMetaObjectProvider d might not match any of the static arguments and thus it would fall back on it's dynamic behavior which could possibly return a different type.
First let me explain how I currently handle validation, say, for an IPv4 address:
public struct IPv4Address {
private string value;
private IPv4Address(string value) {
this.value = value;
}
private static IPv4Address CheckSyntax(string value) {
// If everything's fine...
return new IPv4Address(value);
// If something's wrong with the syntax...
throw new ApplicationException("message");
}
public static implicit operator IPv4Address(string value) {
return CheckSyntax(value);
}
public static implicit operator string(IPv4Address address) {
return address.value;
}
}
I have a bunch of structs like this one.
They often have additional private members to handle things but not methods are exposed publicly.
Here is a sample usage:
IPv4Address a;
IPv4Address b = "1.2.3.4";
a = b;
b = "5.6.7.8";
string address = a;
// c contains "1.2.3.4"
IPv4Address c = address;
// See if it's valid
try {
IPv4Address d = "111.222.333.444";
}
catch (ApplicationException e) {
// Handle the exception...
}
I can feel that there's something very disturbing with this, hence I'm pondering about switching to a static class with methods like IsIPv4Address and so on.
Now, here's what I think is wrong with the approach above:
New team members will have to wrap their heads around this
It might impede integration with 3rd party code
Exceptions are expensive
Never seen anything like this and I am a conservative type at heart :)
And then what I like about it:
Very close to having a lot of specialized primitives since you have value types.
In practice they can be often used like primitive types would, for example it isn't a problem passing the struct above to a method that accepts a string.
And, my favourite, you can pass these structs between objects and be sure that they contain a syntactically valid value. This also avoids having to always check for correctness, which can be expensive if done unnecessarily and even forgotten.
I can't find a fatal flaw to the approach above (just a beginner here), what do you think?
Edit: as you can infer from the first line this is only an example, I'm not asking for a way to validate an IP address.
First of all, you should read posts about implicit casting, and when to use it (and why it's bad to use it in your scenario), you can start here.
If you need to have checking methods, they should be public static, instead of such strange constructs, beside this, having such methods allows you to choose if you want to throw exceptions (like .Parse() methods do), or signal by returning some value that should be checked (like .TryParse() methods).
Beside this, having static methods for creating valid objects doesn't mean you can't use value type (struct) instead of a class, if you really want to. Also, remember that structs have implicit empty constructor, that you can't "hide", so even your construct can be used like this:
IPv4Address a = new IPv4Address();
which will give you invalid struct (value will be null).
I also very much like the concept of mimicking the primitive patterns in the framework, so I would follow that through and lean toward IpV4Address.Parse("1.2.3.4") (along with TryParse) rather than implicit cast.
Seems like a whole lot of complexity with no benefit. Just because you can doesn't mean you should.
Instead of a CheckSyntax(string value) that returns a IP (This method is poorly worded btw) I would just have something like a
bool IsIP(string)
Then you could put this in a utility class, or a base class, or a separate abstraction someplace.
From the MSDN topic on implicit conversions
The pre-defined implicit conversions
always succeed and never cause
exceptions to be thrown. Properly
designed user-defined implicit
conversions should exhibit these
characteristics as well
Microsoft itself has not always followed this recommendation. The following use of the implicit operator of System.Xml.Linq.XName throws an XmlException:
XName xname = ":";
Maybe the designers of System.Xml.Linq got away with it by not documenting the exception :-)