I have a class with an implicit conversion from string defined as:
class TestClass : ITestInterface
{
public static implicit operator TestClass(string value)
{
return new TestClass(value);
}
public TestClass(string value)
{
Value = value;
}
public string Value { get; set; }
}
It implements a marker interface:
public interface ITestInterface { }
I have a method of another class defined as:
public void DoSomething(ITestInterface thing) { }
I'm getting an error trying to call that method:
public void Test()
{
TestClass a = "This works fine.";
DoSomething("Why doesn't this work?");
}
cannot convert from 'string' to 'Tests.ITestInterface'
All code is greatly simplified; my actually requirements are far more complex, but this seems to be the core of what is blocking the pattern I'd like to implement.
What is preventing this from working? (Something in the C# spec?)
Are there any changes I can make to my code to allow this type of casting to work?
You're omitting the third option that explains the issue:
//1
TestClass a = "This works fine.";
//2
ITestInterface i = "This doesn't work either!";
//3
DoSomething("Why doesn't this work?");
In (1), you've declared TestClass a. This means that the compiler knows that when you use a different type (string, in this case), that it should try to convert said value to TestClass.
In (2), you've declared ITestInterface i. This means that the compiler knows that when you use a different type (string, in this case), that it should try to convert said value to ITestInterface.
That is the source of the problem. There is no conversion defined between string and ITestInterface.
What you're currently thinking is:
Well, I know that I want this to be converted to a TestClass. Why doesn't the compiler figure out what I want it to do?
The short answer to that is that the compiler refuses to guess.
What you want to happen would lead to impossible situations. For example, what would happen if there was a second class which also implements ITestInterface?
class SecondTestClass: ITestInterface
{
public static implicit operator SecondTestClass(string url)
{
return new SecondTestClass(url);
}
public SecondTestClass(string url)
{
Value = GetValueFromTheInternet(url);
}
public string Value { get; set; }
}
Let's re-evaluate your code:
//1
TestClass a = "This works fine.";
This works. There is a conversion from string to TestClass.
//2
SecondTestClass b = "This works fine.";
This works. There is a conversion from string to SecondTestClass.
//3
ITestInterface i = "This still doesn't work!";
//4
DoSomething("This doesn't work for the same reason as //3");
This doesn't work. The compiler does not have any known conversion from string to ITestInterface.
The compiler is unable to figure out if you want this to be converted to a TestClass and then assigned to i, or if you want this to be converted to a SecondTestClass and then assigned to i.
And, as I said before, the compiler refuses to guess.
Also, just for clarity, this would work:
TestClass a = "This works fine.";
ITestInterface i1 = a;
DoSomething(a);
DoSomething(i1);
SecondTestClass b = "This works fine.";
ITestInterface i2 = b;
DoSomething(b);
DoSomething(i2);
All of these assignations work.
The crux of your problem is that the compiler wants you to explicitly state which type you want the string to be converted to. In your own example, you were already explicitly asking for a TestClass. Notice that this would not have worked if you had used var, as the compiler would not be able to figure it out in that case either.
Pretty clear from the compiler-error, isn´t it? DoSomething expects an instance of ITestInterface, which string does not implement. The fact that there is an implicit conversion from string to your class doesn´t make this conversion also applicable for any other class implementing the interface.
Imagine there was another class implementing the interface:
class AnotherTestClass : ITestInterface { }
how would the DoSomething-call now be resolved? To which class should the conversion apply? To an instance of TestClass or to AnotherTestClass? In particular if AnotherClass also has an implicit cast-operator defined. That´s why this does not work.
Or consider the other way round: when you have only the interface but no class at all that implements it (which is pretty common when you design an API), there is no conversion at all. Your design introduces some static binding from an interface to a concrete implementation of it, which is a bad thing. In fact that makes your DoSomething-method only work for instances of type TestClass, which contradicts the use of an interface as parameter. So however uses your API can use only ever provide an instance of TestClass to your method.
Apart from this I doubt a cast is a good thing here. By having an implicit concversion you imply that every string can safely be converted to your class without losing any information. E.g. is an URI a valid representation of your class? Or even your provided "Why doesn't this work?"?
On the other side a constructor that expects a string is far more precise and makes it clear:
var m = new TestClass(myString);
From my experience there are only some very few cases where you really need an implicit cast. What you do far more often is to create some instance based upon some input and append some more data to that instance. In your example that means that a TestClass consists of some string-information, but may also have some more data.
public void DoSomething(ITestInterface thing) { }
Because the parameter is an interface and you want to call static implicit operator TestClass(string value) but it is impossible. The interface in C# can not have static method.
You can pass a class TestClass as a parameter only
public static void DoSomething(TestClass thing) { Console.WriteLine(thing.Value); }
Related
i know that string is like a class, and when creating a new string the string itself doesnt owe the value but only the value's pointer. but when creating a string there is no need to use the word new;
string a = "hello";
and not
string a = new string("hello");
I know that the second option is also possible but what i want to understand is why the first one?
Let's say I have a class name student which he's constructor gets a string. To create a new class I must use the saved word new.
student example = new student("Sabrina");
I tried overload oparator = but it is not possible.
How can I create a new class like a string does (without using the word new)?
student example = "Sabrina";
You can use cast operator to implicitly:
sealed class Student
{
public string Name
{
get;
private set;
}
Student()
{
}
public static implicit operator Student(string name)
{
return new Student
{
Name = name
};
}
}
Then you can do Student student = "Sabrina";.
Although you can get this done using an implicit operator, I would highly recommend not doing this at all. Strings are special animals in C# and they get special treatment - along with other special classes like int and float, you can create them without explicitly newing them up due to the way the C# compiler works:
var myInt = 0;
var myFloat = 0f;
var myString = "string";
However, this behavior is typically restricted to those special classes. Adding an implicit operator to do this is bad practice for multiple reasons:
It hides what is actually going on underneath. Are we creating a new Student under the hood when converting from a string?
It is unmaintainable. What happens when you have to add another parameter to the constructor to include the Student's ID number as well?
It eliminates the possibility of using implicitly-typed variables. You can't call var student = "name"; you must call Student student = "name".
The implicit operator paradigm breaks down very quickly. Though it's a cool thing to do, you're setting yourself up for a bad time down the road and making your code more difficult to read. I would highly advise just using new Student("name") like all other normal objects in C#.
The string "hello" in your code, in all cases, does not involve a call to a constructor. It is a constant value, created at compile time, such that all instances of the string "hello" are all the same object. Similarly, the integer 1 and decimal value 3.456, and any other "literal" value, are constants that exist before runtime, before the constructor code can have a chance to be called.
The code new string("hello"); cannot be called, as there is no constructor for string that takes a string as a value. However, if you changed it to new string("hello".ToCharArray());, you would get a string object, but it won't be the same thing as the string "hello". You've actually created a new string, at a separate memory location, from just plain "hello". It only just so happens to have the same character values contained in it as "hello".
The significance is that, if you do use the implicit type conversion trick, then one string literal converted to your type will not be the same object:
class Foo
{
private string value;
public Foo(string val)
{
this.value = val;
}
public static implicit operator Foo(string value)
{
return new Foo(value);
}
}
[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestMethod1()
{
Foo a = "asdf";
Foo b = "asdf";
Assert.AreNotEqual(a, b);
}
}
In other words, a.Equals(b) returns false. In order to make the Equals method return true, you'll have to override the Equals method in your Foo class.
public override bool Equals(object obj)
{
return obj is Foo && value.Equals(((Foo)obj).value);
}
But, as mentioned by other posters, this is an obfuscation. You'll make it difficult to use the code as well as difficult to debug it. It changes what looks like a regular assignment to a full method call. And it breaks many of the tools that Visual Studio provides for inspecting code, such as F12 to jump to a method definition.
Implicit type conversion should be used only in cases where the two types are actually quite interchangeable in use, with minor differences in their functionality. The float type can implicitly convert to double because there are no extra methods and functions for double vs float, and increasing the precision from float to double does not change the value. However, going from double to float requires an explicit cast, because the loss of precision changes the value represented.
You could have a method named createObject() and use that while having the constructors private, discourage their use.
I was wondering why I can not have generic property in non-generic class the way I can have generic methods. I.e.:
public interface TestClass
{
IEnumerable<T> GetAllBy<T>(); //this works
IEnumerable<T> All<T> { get; } //this does not work
}
I read #Jon Skeet's answer, but it's just a statement, which most probably is somewhere in the specifications.
My question is why actually it is that way? Was kind of problems were avoided with this limitation?
Technically, the CLR supports only generic types and methods, not properties, so the question is why it wasn’t added to the CLR. The answer to that is probably simply “it wasn’t deemed to bring enough benefit to be worth the costs”.
But more fundamentally, it was deemed to bring no benefit because it doesn’t make sense semantically to have a property parameterised by a type. A Car class might have a Weight property, but it makes no sense to have a Weight<Fruit> and a Weight<Giraffe> property.
This Generic Properties blog post from Julian Bucknall is a pretty good explanation. Essentially it's a heap allocation problem.
My guess is that it has some nasty corner cases that make the grammar ambiguous. Off-hand, this seems like it might be tricky:
foo.Bar<Baz>=3;
Should that be parsed as:
foo.Bar<Baz> = 3;
Or:
foo.Bar < Baz >= 3;
I think not using an automatic getter/setter illustrates why this isn't possible without having "T" defined at the class level.
Try coding it, the natural thing to do would be this:
IEnumerable<T> _all;
IEnumerable<T> All
{
get { return _all; }
}
Because your field uses "T", then "T" needs to be on the class the CLR knows what "T" is.
When you're using a method, you can delay definition of "T" until you actually call the method. But with the field/property, "T" needs to be declared in one place, at the class level.
Once you declare T on the class, creating a property becomes pretty easy.
public class TestClass<T>
{
IEnumerable<T> All { get; }
}
usage:
var myTestClass = new TestClass<string>();
var stuff = myTestClass.All;
And just like the "T" type parameter on a method, you can wait until you actually instantiate your TestClass to define what "T" will be.
I made somthing like that.
It type checks at run time.
public class DataPackage
{
private dynamic _list;
public List<T> GetList<T>()
{
return (List<T>)_list;
}
public void SetList<T>(List<T> list)
{
_list = list;
}
public string Name { get; set; }
}
I am trying to write out a specific case of a generic class with additional functionality, but I am not sure why the following happens.
Suppose I have a generic class:
class Generic<T>
{
protected T value;
}
If I write out a specific implementation, I can't actually use the specific type I've narrowed it down to:
EDIT: I goofed up, this doesn't work.
class Generic<float>
{
// This doesn't work
public void Add()
{
value + 1.0f;
}
}
But if I inherit from the specific version, it does work:
class Specific : Generic<float>
{
// This does work
public void Add()
{
value + 1.0f;
}
}
In case someone is still reading this, I wanted to point out that this seems to be possible with extension methods:
class Generic<T>
{
public T value;
}
static class Extension
{
public static void Add (this Generic<float> generic)
{
generic.value += 1.0f;
}
}
The downside seems to be that 'value' has to be public or internal.
Your first attempt simply isn't a valid declaration - the part that specifies a class can't specify any type arguments.
If you think about it, how would the CLR know whether or not there was a specialized type available? What would it do if there were two different specializations of the same generic type in two loaded assemblies? It would have to check all the referenced assemblies any time a particular type argument combination was used for the first time. (This couldn't be done at compile-time, as other classes may just be referring to Generic<T>.)
In many cases you can use values of the type in ways which are meaningful to the type using constraints. For example, if you constrain T with where T : IComparable<T> then you can compare any two T values using Compare. Unfortunately there's no way of representing arithmetic operators in this way, although you may want to look at Marc Gravell's generic operator work in MiscUtil.
While I feel your pain, there's simply nothing like this in .NET generics - you'll want to think of alternative designs for whatever problem you're really trying to solve.
in this case :
class Generic<float>
{
// This doesn't work
public void Add()
{
value + 1.0f;
}
}
It doesn't mean that you are working with a Generic class using float as its generic type but it means that the name of the generic type is "float" in the source (instead of using T you are using "float" as its name ) Thus there's no conversion to float.In other words you are using a generic notation as a template that can be substituted with real types later (but not in the template itself and that's why in C++ they are called templates)
In this code :
class Specific : Generic<float>
{
// This does work
public void Add()
{
value + 1.0f;
}
}
you are telling the compiler that you want the specific class be a child of a generic class while it's template type will be replaced by type float.
Generic types are used for arithmetic reusability. That is, you must write something common between all possible Ts in your code.
class Generic<T>
{
protected T value; //it's valid to declare a member whose type is T
public void Add()
{
value + 1.0f; //invalid, because not all T are allowed to add
//with 0.1f by default
//consider T is the type Person
}
public void Print()
{
Type t = typeof(T); //valid, for all T we can get its type
}
}
And when you specified some T (e.g. float in your question), the compiler knows the T is float, so it's valid to add 0.1f to it.
I also looked for similar solution, I think you also got this thinking from ADA or such programming language.
But, as others wrote, making type specific class definition is not generic programming, it's specialization, so the simpliest (and only in C#) way is to create a specified class, based on generic structure:
class Generic<T>
{
protected T value;
public Generic(T val)
{
value = val;
}
}
class Generic_float : Generic<float>
{
public Generic_float(float val)
: base(val)
{
}
public void Add()
{
value = value + 1.0f;
}
}
As you can see, we created a class for the specified case, having the extra ability of extending the structure with fields and methods. This advantage is very good to refine behavior of our objects, and gives ability of implicit cast of the typed generic class to our customized one (with the notice that casting back is not possible):
public void Test()
{
Generic<float> var1 = new Generic<float>(1.5f);
Generic_float var2 = new Generic_float(2.5f);
var1 = var2; // Works, var links to var2's memory field casted as Generic<float>
var2 = var1; // cannot implicitly convert error, if want to use then have to make explicit conversion
}
Don't know if the way you expected should work in languages like C# or other managed ones, but maybe this workaround gives you what you really wanted to get.
I've got a class A with a public field b
class A
{
public static string b;
}
but now I want to make b dynamic so I call it anything. So I can make the class a DynamicObject
class A : DynamicObject
{
}
but I the compiler doesn't let me now call A.dynamicThing cos I have to instantiate A as dynamic.
How can I mangle c# further to make this work?
I don't belive you're going to find a way to make this work. It's not just the DynamicObject that makes things work. The declaration as a variable of the "dynamic" data type is what tells the compiler to actually use the DynamicObject base to resolve the member access. With static access direct to the class, you don't have that. So I really just don't think this is going to work in C# unless that changes in the future.
It's not possible right now with .NET 4
more information in this article.
I think I understand now - the closest you can get is by using an ExpandoObject :
dynamic foo = new ExpandoObject();
foo.somethinghere = "bar";
foo.dynamicThing = "baz";
Edit:
I don't think its possible to re-route the access to a static property of a class
to an expando object if the name of the property does not match - how would the compiler know that that's what you meant to do? You are getting a compile time error after all, not a runtime error.
From MSDN:
When a field, method, property, event,
operator, or constructor declaration
includes a static modifier, it
declares a static member. In addition,
a constant or type declaration
implicitly declares a static member.
Static members have the following
characteristics:
When a static member M is referenced in a member-access (Section 7.5.4)
of the form E.M, E must denote a type containing M.
...
public class FakeDynamicMethodInvoker : DynamicObject
{
// your code here
}
public class FakeDynamicWrapper<T>
{
static FakeDynamicWrapper()
{
DynamicStaticField = (dynamic)new FakeDynamicMethodInvoker();
}
public static T DynamicStaticField{ get; set; }
}
public class RealClassWithDynamicStaticField: FakeDynamicWrapper<dynamic>
{
}
somewhere in a code:
RealClassWithDynamicStaticField.DynamicStaticField.AnyMethod();
C# doesn't let you really rename variables to some dynamic name at runtime. Your question is mangled.
If you are wanting variable b to have a dynamic object at runtime, then use the dynamic keyword.
Example:
dynamic b = GetBValue();
b.SomeOperation(); // the type of "b" will be evaluated/chosen at runtime.
Old question but worth to answer :)
Static constructors are the answer to these problems.
https://msdn.microsoft.com/en-us/library/k9x6w0hc.aspx
public class MyClass
{
public static dynamic StaticDynamicObject;
static MyClass()
{
StaticDynamicObject = new ExpandoObject();
StaticDynamicObject.Prop = "woohoo!";
}
}
I'm using a 3rd party's set of webservices, and I've hit a small snag. Before I manually make a method copying each property from the source to the destination, I thought I'd ask here for a better solution.
I've got 2 objects, one of type Customer.CustomerParty and one of type Appointment.CustomerParty. The CustomerParty objects are actually property and sub-oject exactly the same. But I can't cast from 1 to the other.
So, I need to find a certain person from the webservice. I can do that by calling Customer.FindCustomer(customerID) and it returns a Customer.CustomerParty object.
I need to take that person that I found and then use them a few lines down in a "CreateAppointment" request. Appointment.CreateAppointment takes an appointment object, and the appointment object contains a CustomerParty object.
However, the CustomerParty object it wants is really Appointment.CustomerParty. I've got a Customer.CustomerParty.
See what I mean? Any suggestions?
Why don't you use AutoMapper? Then you can do:
TheirCustomerPartyClass source = WebService.ItsPartyTime();
YourCustomerPartyClass converted =
Mapper.Map<TheirCustomerPartyClass, YourCustomerPartyClass>(source);
TheirCustomerPartyClass original =
Mapper.Map<YourCustomerPartyClass, TheirCustomerPartyClass>(converted);
As long as the properties are identical, you can create a really simple map like this:
Mapper.CreateMap<TheirCustomerPartyClass, YourCustomerPartyClass>();
Mapper.CreateMap<YourCustomerPartyClass, TheirCustomerPartyClass>();
This scenario is common when writing domain patterns. You essentially need to write a domain translator between the two objects. You can do this several ways, but I recommend having an overridden constructor (or a static method) in the target type that takes the service type and performs the mapping. Since they are two CLR types, you cannot directly cast from one to the other. You need to copy member-by-member.
public class ClientType
{
public string FieldOne { get; set; }
public string FieldTwo { get; set; }
public ClientType()
{
}
public ClientType( ServiceType serviceType )
{
this.FieldOne = serviceType.FieldOne;
this.FieldTwo = serviceType.FieldTwo;
}
}
Or
public static class DomainTranslator
{
public static ServiceType Translate( ClientType type )
{
return new ServiceType { FieldOne = type.FieldOne, FieldTwo = type.FieldTwo };
}
}
I'm using a 3rd party's set of
webservices...
Assuming you can't modify the classes, I'm not aware of any way you can change the casting behavior. At least, no way that isn't far, far more complicated than just writing a CustomerToAppointmentPartyTranslator() mapping function... :)
Assuming you're on a recent version of C# (3.5, I believe?), this might be a good candidate for an extension method.
Have you looked at adding a conversion operator to one of the domain classes to define an explicit cast. See the msdn documentation here.
Enjoy!
A simple and very fast way of mapping the types is using the PropertyCopy<TTarget>.CopyFrom<TSource>(TSource source)
method from the MiscUtil library as described here:
using MiscUtil.Reflection;
class A
{
public int Foo { get; set; }
}
class B
{
public int Foo { get; set; }
}
class Program
{
static void Main()
{
A a = new A();
a.Foo = 17;
B b = PropertyCopy<B>.CopyFrom(a);
bool success = b.Foo == 17; // success is true;
}
}
Two classes with exactly the same signature, in two different namespaces, are two different classes. You will not be able to implicitly convert between them if they do not explicitly state how they can be converted from one to the other using implicit or explicit operators.
There are some things you may be able to do with serialization. WCF DataContract classes on one side do not have to be the exact same type as the DataContract on the other side; they just have to have the same signature and be decorated identically. If this is true for your two objects, you can use a DataContractSerializer to "convert" the types through their DataContract decoration.
If you have control over the implementation of one class or the other, you can also define an implicit or explicit operator that will define how the other class can be converted to yours. This will probably simply return a new reference of a deep copy of the other object in your type. Because this is the case, I would define it as explicit, to make sure the conversion is only performed when you NEED it (it will be used in cases when you explicitly cast, such as myAppCustomer = (Appointment.CustomerParty)myCustCustomer;).
Even if you don't control either class, you can write an extension method, or a third class, that will perform this conversion.