I have some code that I have no control of. This code accepts an object parameter and attempts to cast it to a type known at compile time like this:
KnownType item = (KnownType) parameter;
Is it possible in C# to design a custom class MyClass (not derived from KnownType) that can be passed as parameter to the above code and be converted to KnownType by the above code, provided that MyClass can convert itself to KnownType using its member method:
protected KnownType ConvertToKnownType()
{
// conversion code goes here
}
I have tried to implement a custom conversion operator like this:
public static implicit operator KnownType(MyClass source)
{
KnownType result;
// conversion goes here
return result;
}
but it didn't work (it was not used). Am I right to assume that the cast operator only works when the source type, target type and conversion operators are known at compile time?
Edit:
I initially didn't provide more details about the code doing the conversion because I think it is irrelevant and because I am mainly interested in the way the cast operator is implemented, i.e. does it take look at the runtime type to find an appropriate converter or is the decision made at compile time?
To clear things out, KnownType is in fact DataRowView, while MyClass is a wrapper class for DataRowView that has to derive from some other type. MyClass keeps a reference to a DataRowView. Instead of binding ComboBox.DataSource to a DataView, I bind it to an IList<MyClass> but I still need the ComboBox to be able to access the DataRowView column values as if I was binding an IList<DataRowView>. Unfortunately the cast operator works the way I was afraid it would: it only takes into account compile time type information to do conversions (it does however use runtime type information when casting between types in the same inheritance tree).
No, it is not possible. The cast provided would only succeed if you're able to derive your class from that class. Any type of conversion not based on inheritance would require the class performing the conversion to do something different than what it's doing.
Am I right to assume that the cast operator only works when the source type, target type and conversion operators are known at compile time?
Yes.
Related
I have the following class
public abstract class Result<TResult, TData> where TResult : Result<TResult, TData>
{
public virtual TData Data { get; private set; }
}
How can I use nameof on the Data property?
It tried some options but got compilation errors
nameof(Result<object, object>.Data)
Error CS0311
The type 'object' cannot be used as type parameter 'TResult' in the generic type or method
'Result<TResult, TData>'. There is no implicit reference conversion from 'object' to
'Result<object, object>'
UPDATE
To better specify:
I did not write the class in the first snippet, but I have to use it as it is.
I need to use reflection to get a PropertyInfo instance for the Data property. I can do it by calling GetProperty("Data"), but I prefer to avoid strings in case the name property is renamed with the Visual Studio Refactoring (for example from Data to Content).
I cannot use nameof(Data) and I am in a different context that doesn't know about the Data property in that class.
As such, I would call GetProperty(nameof({whatever}.Data)). Of course, if not possible I would use the string. But if there is a way, I would like to know it.
Unfortunately, the nameof operator does not work with unbound generic types. E.g. you'd like to be able to write nameof(Result<,>.Data), just like you can today write something like typeof(Result<,>). So you would need to specify type parameters when trying to get the nameof(Result<TResult, TData>.Data) value.
Which you've tried, but you provided object as the type parameter for both parameters, even though your generic type constrains TResult as being derived from Result<TResult, TData>. The type object doesn't meet the constraint, so that can't possibly work. Hence the compiler error.
Obviously, if you can provide any type that does meet the constraint, that would solve the compiler error and allow you to use the nameof operator. There's not enough information in your question to know whether that's an option in your specific scenario.
I agree with this comment that you would probably be better off asking a different question, one which takes a step back and explains how you arrived at feeling you needed this syntax in the first place. It's not clear what the broader goal you're trying to accomplish is, where you don't have known type parameters to use for this expression. Typically, code outside of the generic type that wants to make use of the generic type, would actually know the type parameters it intends to use with the generic type.
Note that in the context of the generic type itself, you can refer to the property without knowing the exact type parameters, since the property identifier does not need qualification. E.g. nameof(Data) would work, for any code that's actually in the generic class Result<TResult, TData>. Whether that helps in your specific scenario is unclear from the information provided so far.
I have an explicit operator on the class MyVO, which should be non-nullable.
public class MyVO : ValueObject<MyVO>
{
public string Value { get; } // Should never be null
private MyVO(string v) => Value = v;
public static explicit operator MyVO(string vo)
{
if (string.IsNullOrWhiteSpace(vo)) throw new Exception('...');
return new MyVO(vo);
}
However, (MyVO)null will not raise an exception. The body of the method will not be run.
var myVO = (MyVO)null; // myVO will have the null value
How to make sure it's not null?
How to make sure it's not null?
By "it" I assume you mean "the result of the cast from null to MyVO". If that is not what you mean, please clarify the question.
You cannot.
An important rule of C# is a user-defined conversion never "wins" when it conflicts with a built-in conversion. It is legal to convert null to any class type, and so a cast of MyVO on the expression null will always result in a null reference. The compiler does not even consider the user-defined conversions if a built-in conversion works. (Believe me; I wrote that code!)
As D Stanley's answer correctly points out, if the null is the value of any expression of type string then the user-defined conversion is called; there is no built-in conversion from string to MyVO so the compiler looks for an applicable user-defined conversion and finds one.
Since it hurts when you do what you're doing, you should probably stop doing what you are doing. An explicit conversion is probably not the right way to implement the desired behaviour.
I guess my question should be how to make MyVO not nullable.
Upgrade to C# 8. C# 8 supports non-nullable annotations on reference types.
Note that the non-nullable annotation should be properly thought of as an annotation. The type system does not guarantee that the value of a variable annotated with a non-nullable annotation will never be observed to be null. Rather, it does its best to warn you when the code looks like it is wrong.
While we are looking at your code, I notice that you are using ValueObject<T>, which I assume you have obtained from something like
https://enterprisecraftsmanship.com/posts/value-object-better-implementation/
Let me take this opportunity to caution you that there are pitfalls to using this pattern; the constraint that you think or want to be applied to T is not the constraint that is applied to T. We often see things like this:
abstract class V<T> where T : V<T>
{
public void M(T t) { ... } // M must take an instance of its own type
}
If we have class Banana : V<Banana> then Banana.M takes as its argument a Banana, which is what we want. But now suppose we have class Giraffe : V<Banana>. In this scenario, Giraffe.M does not take a giraffe; it takes a banana, even though Giraffe has no relationship with Banana at all.
The constraint does not mean that M always takes an instance of its own class. If you are trying to construct a generic type with this kind of constraint in C#, you cannot; the C# type system is not rich enough to express that constraint.
null can be implicitly converted to any reference type, so the compiler is not using your explicit cast operator. try
string s = null;
o = (MyVO)s;
or just inline it
o = (MyVO)((string)s);
Have the following code which works fine.
MyType convertedItem = (MyType)item;
However I get a compiler error from
var convertedItem = item as MyType;
Cannot convert type 'OtherType' to 'MyType' via a reference
conversion, boxing conversion, unboxing conversion, wrapping
conversion, or null type conversion.
Can anyone explain why \ when this occurs. An explicit cast works fine but AS wont even compile.
** How do I get 'AS' functionality in this situation. Namely I need to do a trycast and would prefer not to invoke the exceptionhandler to accomplish it. **
as doesn't work with anything that is a struct. Logically we can understand this because a struct is non nullable by default. The suggestions of casting to object work by cheating and boxing the struct
For example, the following types would give CS0039:
class MyType
{
}
class MyOtherType
{
}
MyOtherType item = new MyOtherType();
var convertedItem = item as MyType;
In the above example, the compiler has determined that given the types participating in the cast, it's impossible to perform the requested conversion.
Here providing conversion operators would solve the problem.
EDIT: working around this error with casting to Object is not recommended, as it defeats the purpose of the type system
In developing a class that should handle various generic lambda expressions, I fell into a rather familiar hole: I had a MyClass<T> class, and I tried to cast MyClass<string> to MyClass<object>, like so:
MyClass<string> myClassString = GetMyClass(); // Returns MyClass<String>
MyClass<object> myClassObject = myClassString;
Doing so returned an compilation error saying there there's no implicit conversion between the two types, but that an explicit conversion does exist. So I added the explicit conversion:
MyClass<object> myClassObject = (MyClass<object>)myClassString;
Now the code compiled, but failed in runtime, claiming the conversion is illegal.
I am using Visual Studio 2012, and the code is part of a Portable Class Library project compiled with C# 5.
Just to make sure, I replaced MyClass IList - the same behavior appeared - an explicit conversion is allowed, but fails during run-time.
Why? Why does the compiler accept this? What's the point of the explicit conversion if it fails in runtime?
In order to allow the cast, you need to mark it as covariant. However, covariance is only allowed for interfaces (and delegates). This would look like:
interface MyInterface<out T> ...
The reason why you can compile the explicit cast is probably that the compiler assumes that the return value of GetMyClass() could be a MyClass<object>. But That's hard to say without the declaration.
To be able to cast MyClass<string> to MyClass<object> you need to fulfill the following:
MyClass<T> must be an interface, e.g. IMyClass<T>.
You need to add the out keyword to the type parameter T, making the type parameter covariant.
The type parameter T may only appear in output positions of the members of the interface.
For example:
public interface IMyClass<out T>
{
T GetItem(); // T in an output position
}
Now you can cast it:
IMyClass<string> myClassString;
IMyClass<object> myClassObject = (IMyClass<object>)myClassString;
If you have classes A and B and you want to cast B to A then one of the following must be true:
B derives/implements A or vice versa
There is an explicit cast operator defined from B to A
None of the above is true in your case so the cast is invalid.
If your class implements IEnumerable you may use an extension method
using System.Linq
...
MyClass<Object> asObject = GetMyClass().Cast<Object>();
Otherwise, write an explicit operator or function that does the conversion:
public MyClass<Base> ConvertToBase<Base, Derived>(MyClass<Derived>) where Derived : Base
{
// construct and return the appropriate object
}
use Conversion operator to implement casting of your class to another classes
http://msdn.microsoft.com/en-us/library/85w54y0a.aspx
http://msdn.microsoft.com/en-us/library/09479473(v=vs.80).aspx
Can I cast an object from type object to MyType<T> using reflection if I don't know what T is until runtime?
You can't cast to a type unknown at compile time. Casting is really only useful as a compile-time construct, as you'd need to know the type in order to use it directly.
If your goal is to work with the object via Reflection, however, that's a different scenario. In that case, you can use Type.MakeGenericType to create the correct type for your object.
This will allow you to use reflection to work upon your object.
The trick in such situations, is to use a non-generic class with generic methods.
public class MyType
{
public T GetResult<T>() {
}
}
Note, however, that this happens at compile time. Generics give you the opportunity to create different "flavors" of a type or a method at compile time; but generics are not dynamic! Generics are type-safe and type safety can only be achieved at compile time (because it's the compiler who checks the type safety).