Working with ASP.Net Core in Rider (learning it) and trying to set the Response headers.
The headers work great, no problems. But in the Rider it adds some information (as on the image below) with the tip:
Implicit conversion of "someValue" from 'string' to 'StringValues'
What does this message mean?
Thanks.
C# has a mechanism for user to define an implicit conversion from one type to another i.e. usually (since types are validated by the compiler) given two unrelated user-defined types T1 and T2 the following T1 x = new T2(); will result in compiler error, but with implicit conversion defined compiler will use it and this code will work. See more in user-defined explicit and implicit conversion operators doc.
context.Response.Headers is a IHeaderDictionary which has an indexer property of type StringValues which has an implicit conversion from string. The implicit conversion is defined like this:
public static implicit operator StringValues(string? value)
{
return new StringValues(value);
}
Implicit conversions sometimes can be tricky (for example) so it seems that Rider team decided that it worth to notify user in some way when one is happening.
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.
Why does C# implicitly convert generic types parameterized with a reference type implementing an interface to the same generic type parameterized with the implemented interface, but not perform the same implicit conversion for reference types?
Essentially, why does the first line compile but the second one fail?
IEnumerable<IComparable<Version>> x = Enumerable.Empty<Version>();
IEnumerable<IComparable<int>> y = Enumerable.Empty<int>();
Especially great would be a reference to the part of the spec that describes this behavior.
Short answer
Despite the name "implicit", implicit conversions don't apply unless the rules explicitly say they do, and the rules don't allow boxing conversions when going from IEnumerable<int> to IEnumerable<IComparable<int>>. As a simpler case, you can't go from IEnumerable<int> to IEnumerable<object> for the same reason, and that case is well documented.
Long answer
OK, first of all, why would IEnumerable<T> convert to IEnumerable<IComparable<T>> at all? This is covered in §6.1.6 (C# Language Specification 5.0):
The implicit reference conversions are:
[...]
From any reference-type to an interface or delegate type T if it has an implicit identity or reference conversion to an interface or
delegate type T0 and
T0 is variance-convertible (§13.1.3.2) to T.
And §13.1.3.2 says:
A type T<A1, …, An> is
variance-convertible to a type T<B1, …, Bn>
if T is either an interface or a delegate type
declared with the variant type parameters T<X1, …, Xn>,
and for each variant type parameter
Xi one of the following holds:
Xi is covariant and an implicit reference or identity conversion exists from Ai to
Bi
Since IEnumerable<T> is covariant in T, this means that if there is an implicit reference or identity conversion from T to IComparable<T>, then there is an implicit reference conversion from IEnumerable<T> to IEnumerable<IComparable<T>>, by virtue of these being variance-convertible.
I emphasized "reference" for a reason, of course. Since Version implements IComparable<Version>, there is an implicit reference conversion:
From any class-type S to any interface-type T, provided S
implements T.
Right, so now, why doesn't IEnumerable<int> implicitly convert to IEnumerable<IComparable<int>>? After all, int implicitly converts to IComparable<int>:
IComparable<int> x = 0; // sure
But it does so not through a reference conversion or an identity conversion, but through a boxing conversion (§6.1.7):
A boxing conversion exists from any non-nullable-value-type [...] to
any interface-type implemented by the non-nullable-value-type.
The rules of §13.1.3.2 do not allow boxing conversions in considering whether a variance conversion is possible, and there is no other rule that would enable an implicit conversion from IEnumerable<int> to IEnumerable<IComparable<int>>. Despite the name, implicit conversions are covered by explicit rules.
There is actually a much simpler illustration of this problem:
object x = 0; // sure, an int is an object
IEnumerable<object> x = new int[] { 0 }; // except when it's not
This isn't allowed for the same reason: there is no reference conversion from int to object, only a boxing conversion, and those are not considered. And in this form, there are several questions on Stack Overflow that explain why this is not allowed (like this one). To summarize: it's not that this is impossible, but to support it, the compiler would have to generate supporting code to stick the code for the boxing conversion somewhere. The C# team valued transparency in this case over ease of use and decided to allow identity-preserving conversions only.
Finally, as a matter of practical consideration, suppose you had an IEnumerable<int> and you needed an IEnumerable<IComparable<int>>, how would you get it? Well, by doing the boxing yourself:
Func<int, IComparable<int>> asComparable = i => i; // compiles to ldarg ; box ; ret
IEnumerable<IComparable<int>> x = Enumerable.Empty<int>().Select(asComparable);
Of course using Enumerable.Cast would be more practical here; I wrote it this way to highlight that an implicit conversion is involved. There is a cost to this operation, and that's just the point; the C# designers wanted this cost to be explicit.
I am basically a C# guy, but writing VB.Net code these days.
Today I came across a very different behaviour of .Net
C# Code
enum Color
{
Red,
Green,
Blue
}
class Demo
{
public static void Main()
{
System.Console.WriteLine(Color.Red);
}
}
This prints Red
But when this code is written in VB.Net it prints 0.
VB.Net Code
Module Module1
Sub Main()
System.Console.WriteLine(Color.Red)
End Sub
End Module
Enum Color
Red
Green
Blue
End Enum
Why so different?
There is no Console.WriteLine(Enum) overload so the compilers are forced to pick one of the other ones. Overload resolution rules are very arcane and the VB.NET and C# rules are not the same, but both compilers are willing to pick one when there's an implicit conversion to the target argument type and pick the one that takes the least amount of work.
Which is where another rule applies, this kind of statement in VB.NET is perfectly valid:
Dim example As Integer = Color.Red '' Fine
But the C# compiler spits at:
int example = Color.Red; // CS0266
Insisting that you apply an (int) cast. It only has an explicit conversion, not an implicit one like VB.NET.
So the C# compiler is going to ignore all the overloads that take an integral argument, none are candidates because only explicit conversions exist for them. Except one, the Console.WriteLine(Object) overload. There is an implicit conversion for that one, it takes a boxing conversion.
The VB.NET compiler sees it as well, but now the "better" conversion comes into play. A boxing conversion is a very expensive conversion, converting to Integer is very cheap. It requires no extra code. So it likes that one better.
Workarounds are simple:
System.Console.WriteLine(CObj(Color.Red)) '' or
System.Console.WriteLine(Color.Red.ToString())
C# and VB.NET have different method overload resolution rules.
C# Picks Console.WriteLine(Object), while VB.NET picks Console.WriteLine(Int32). Let's see why it does so.
VB.NET rules:
Accessibility. It eliminates any overload with an access level that prevents the calling code from calling it.
Number of Parameters. It eliminates any overload that defines a different number of parameters than are supplied in the call.
Parameter Data Types. The compiler gives instance methods preference over extension methods. If any instance method is found that requires only widening conversions to match the procedure call, all extension methods are dropped and the compiler continues with only the instance method candidates. If no such instance method is found, it continues with both instance and extension methods.
In this step, it eliminates any overload for which the data types of the calling arguments cannot be converted to the parameter types defined in the overload.
Narrowing Conversions. It eliminates any overload that requires a narrowing conversion from the calling argument types to the defined parameter types. This is true whether the type checking switch (Option Strict Statement) is On or Off.
Least Widening. The compiler considers the remaining overloads in pairs. For each pair, it compares the data types of the defined parameters. If the types in one of the overloads all widen to the corresponding types in the other, the compiler eliminates the latter. That is, it retains the overload that requires the least amount of widening.
Single Candidate. It continues considering overloads in pairs until only one overload remains, and it resolves the call to that overload. If the compiler cannot reduce the overloads to a single candidate, it generates an error.
There are a lot of overloads for WriteLine, some of them are discarded at step 3. We're basically left with the following possibilities: Object and the numeric types.
The 5th point is interesting here: Least Widening. So what do the widening rules say?
Any enumerated type (Enum) widens to its underlying integral type and any type to which the underlying type widens.
Any type widens to Object
So, your Color enum first widens to Int32 (its underlying data type) - and this is a 100% match for Console.WriteLine(Int32). It would require yet another widening conversion to go from Int32 to Object, but the rules above say to retain the overload that requires the least amount of widening.
As for C# (from the C# 5 spec at §7.5.3.2):
Given an argument list A with a set of argument expressions { E1, E2, ..., EN } and two applicable function members MP and MQ with parameter types { P1, P2, ..., PN } and { Q1, Q2, ..., QN }, MP is defined to be a better function member than MQ if
for each argument, the implicit conversion from EX to QX is not better than the implicit conversion from EX to PX, and
for at least one argument, the conversion from EX to PX is better than the conversion from EX to QX.
Ok, now how is better defined (§7.5.3.4)?
Given a conversion C1 that converts from a type S to a type T1, and a conversion C2 that converts from a type S to a type T2, C1 is a better conversion than C2 if at least one of the following holds:
An identity conversion exists from S to T1 but not from S to T2
T1 is a better conversion target than T2 (§7.5.3.5)
Let's see at §7.5.3.5:
Given two different types T1 and T2, T1 is a better conversion target than T2 if at least one of the following holds:
An implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1 exists
T1 is a signed integral type and T2 is an unsigned integral type.
So, we're converting from Color to either Object or Int32. Which one is better according to these rules?
There is an implicit conversion from Color to Object
There is no implicit conversion from Object to Color (obviously)
There is no implicit conversion from Color to Int32 (these are explicit in C#)
There is no implicit conversion from Int32 to Color (except for 0)
Spec §6.1:
The following conversions are classified as implicit conversions:
Identity conversions
Implicit numeric conversions
Implicit enumeration conversions.
Implicit nullable conversions
Null literal conversions
Implicit reference conversions
Boxing conversions
Implicit dynamic conversions
Implicit constant expression conversions
User-defined implicit conversions
Anonymous function conversions
Method group conversions
Implicit numeric conversions make no mention of enum types, and Implicit enumeration conversions deal with the other way around:
An implicit enumeration conversion permits the decimal-integer-literal 0 to be converted to any enum-type and to any nullable-type whose underlying type is an enum-type. In the latter case the conversion is evaluated by converting to the underlying enum-type and wrapping the result (§4.1.10).
Enums are handled by boxing conversions (§6.1.7):
A boxing conversion permits a value-type to be implicitly converted to a reference type. A boxing conversion exists from any non-nullable-value-type to object and dynamic, to System.ValueType and to any interface-type implemented by the non-nullable-value-type. Furthermore an enum-type can be converted to the type System.Enum.
Given the following class:
public class MyType
{
public static implicit operator MyType(Func<MyType> wrapper) {
return wrapper();
}
}
From the implicit cast of Func<MyType> to MyType, I assumed the following would be possible:
public MyType MyTypeWrapper() {
return new MyType();
}
public void MyTestMethod() {
MyType m = MyTypeWrapper; // not a call!
}
However I'm getting:
Cannot convert method group 'MyTypeWrapper' to non-delegate type 'Test.MyType'. Did you intend to invoke the method?
Which, unfortunately for me, when searched for (as I half expected) resulted in tons of questions to which the answer was:
Hey, ya dun goofed; toss () on the end of WhateverMethod!
Now, as I'm typing this, I've noticed that an explicit cast does in fact compile:
MyType m = (MyType) MyTypeWrapper;
Why is it that I cannot implicitly cast a Func<MyType> to MyType as I've described?
This is unfortunate. I'm pretty sure you've found a compiler bug, and this section of the specification is extremely difficult to read.
Section 6.4.4 of the C# 4 specification explains why your implicit conversion is illegal.
The algorithm goes like this. First look at the source type and target type. There is no source type because a method group has no type. The target type is MyType. So search MyType for user-defined implicit conversions. Now the question is: what is the set of applicable user-defined operators ... that convert from a type encompassing S? S is the source type and we have already established that there is no source type. So this is already evidence that the conversion should fail. But even if the compiler for some reason decides that your Func<MyType> conversion is applicable, the rule is a standard implicit conversion ... is performed. Method group conversions are deliberately not classified as standard conversions.
So that's why it should be illegal.
Why then is the explicit cast legal?
There's no justification for that. This appears to be a bug.
This is unfortunate; many apologies for the error. I shall report it to my former colleagues; if they have an analysis which conflicts with mine, I'll update the answer.
UPDATE: My former colleagues inform me that the spec problem whereby the source expression is assumed to have a type will be addressed by a rewording in the next release of the spec. No word yet as to whether the explicit cast behavior is a bug.
You're already using the built-in implicit conversion from method group to Func<MyType>.
The compiler won't do two implicit conversions at once.
Once you have an explicit cast to your class, the compiler knows to look for an implicit cast to any type that can be explicitly casted to your class.
Because the C# compiler isn't able to convert MyTypeWrapper into a Func<MyType>(MyTypeWrapper). There's a difference between a method group and an actual delegate.
This compiles and runs fine:
MyType m = new Func<MyType>(MyTypeWrapper);
There is an implicit conversion from a method group to a delegate type that matches that group, and there is your user defined implicit conversion from that delegate to a type. The general idea here is that the compiler is only going to use one implicit conversion in a row at a time. When it has an A and needs a C it looks for conversions from A to C, not from A to any type B and from that type to C. That algorithm goes from O(n) to O(n^2) (not to mention possibly being quite confusing for programmers).
The reason your code works when using an explicit cast to MyType is that you're no longer chaining implicit conversions.
The signature of MyTestMethod MATCHES the signature of Func<MyType> but is NOT a Func<MyType>. Func has defined some implicit casts itself to allow you to assign such methods as Funcs, but you must explicitly cast for the signature to apply, because the compiler will not chain implicit casts together for you:
MyType m = (Func<MyType>)MyTypeWrapper; // not a call!
Expression.Convert generally throws in InvalidOperationException when "No conversion operator is defined between expression.Type and type."
The return type parameter of Func<> is covariant for reference types.
// This works.
Func<SomeType> a = () => new SomeType();
Func<object> b = a;
It isn't covariant for value types.
Variance applies only to reference types; if you specify a value type for a variant type parameter, that type parameter is invariant for the resulting constructed type.
// This doesn't work!
Func<int> five = () => 5;
Func<object> fiveCovariant = five;
However, Expression.Convert believes it is possible.
Func<int> answer = () => 42;
Expression answerExpression = Expression.Constant( answer );
// No InvalidOperationException is thrown at this line.
Expression converted
= Expression.Convert( answerExpression, typeof( Func<object> ) );
No InvalidOperationException is thrown when calling Expression.Convert. The expression tree compiles correctly, but when I call the created delegate, I get an expected InvalidCastException.
Is this a bug? (I reported it as a bug on Microsoft Connect.)
How to properly check whether a type can be converted to another type? Some answers seem to refer to using Convert. I would very much prefer a method which doesn't have to use exception handling as logic.
It seems the entire variance logic isn't properly supported. It correctly complains about not being able to convert from Func<SomeType> to Func<SomeOtherType>, but it doesn't complain about converting from Func<object> to Func<string>.
Interestingly, once SomeType and SomeOtherType are in the same class hierarchy (SomeOtherType extends from SomeType), it never throws the exception. If they aren't, it does.
Is this a bug?
Yes. The expression tree library was probably not updated consistently throughout when we added covariance and contravariance. Sorry about that.
I reported it as a bug on Microsoft Connect.
Thanks! Someone will have a look at it then.
How to properly check whether a type can be converted to another type?
The question is vague. Given two type objects, do you want to know:
Does the .NET runtime think the types are assignment compatible?
Does the C# compiler think that there is an implicit conversion between the types?
Does the C# compiler think that there is an explicit conversion between the types?
"int" and "short" for example are not assignment compatible by .NET rules. Int is explicitly convertible but not implicitly convertible to short, and short is both implicitly and explicitly convertible to int, by C# rules.
It's not a bug. Expression.Convert represents a run-time type check, so an InvalidCastException at run time would be the expected behavior.
Edit: that's not entirely correct. It doesn't exactly represent a run-time type check (Here's the documentation: http://msdn.microsoft.com/en-us/library/bb292051.aspx). However, the expression tree is created at run-time, so all the type checking must happen then.
Edit: I'm using .NET 4.0 too.
By the way, Convert doesn't complain about converting from Func<object> to Func<string> because that conversion is sometimes legal. It is legal if the Func<object> is a covariant reference to an object whose runtime type is Func<string>. Example:
Func<string> sFunc = () => "S";
Func<object> oFunc = sFunc;
Func<string> anotherSFunc = (Func<string>)oFunc;
Now, Convert decides whether to throw an InvalidOperationException by checking whether one type can be coerced to another. When checking delegates for a potential reference conversion, it looks like the code does check contravariant (argument) parameters and throws an InvalidOperationException if any is a value type. It doesn't seem to do that check for the covariant (return type) parameter. So I am beginning to suspect that this is a bug, though I'm inclined to reserve judgment on that until I have a chance to look at the spec (see Eric Lippert's Maybe there's something wrong with the universe, but probably not), which I don't have time to do right now.
Eric Lippert answered part 1 of my question: it seems to be a bug. I started looking for a solution to question 2:
How to properly check whether a type can be converted to another type?
I just commit a first attempt at a Type.CanConvertTo( Type to ) method to my library. (Sources are way too complex to post here, sorry.)
if ( fromType.CanConvertTo( toType ) )
{
convertedExpression = Expression.Convert( expression, toType );
}
So far it supports checking implicit conversions for:
Simple non-generic types.
Variance for nested generic interfaces and delegates.
Invariance for generic value type parameters.
It doesn't support:
Type constraints.
Custom implicit conversion operators.
It passes all my tests for implicit conversions. Although you can specify to include explicit conversions as well (and I actually use it like that at the moment), I still need to write unit tests for all those scenarios.