How to set a base member generic List using derived specific List
Change List inEdges to List inEdges and it will work. The problem is that Segments is known as a List Look up variance/covariance/contravariance if you're interested. ).
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I have a compiler error when trying to set a generic base collection class member in this derived class. Error CS0029: Cannot implicitly convert type 'System.Collections.Generic. List' to 'System.Collections.Generic.
List' Here's the outline of my generic collection public class Path : IEnumerable where T : Segment { private List segments = new List(); public List Segments { set { segments = value; } get { return this. Segments; } } public Path() { this. Segments = new List(); } public Path(List s) { this.
Segments = s; } } A derived generic class of this collection is then defined for a derived class IntSegment of Segment (for which the base collection is defined) public class IntersectionClosedPath : Path, IEnumerable where T : IntSegment { public IntersectionClosedPath(List inEdges) : base() { Segments = inEdges; } } I can't understand why this assignment is not allowed. (I don't need to make a deep copy of the incoming List). C# generics generic-list link|improve this question edited Mar 16 at 17:17phoog11k921 asked Mar 16 at 15:32gwizardry835 85% accept rate.
Change List inEdges to List inEdges and it will work. The problem is that Segments is known as a List, where T : IntSegment, while inEdges is a List. (For reasons I'll not go into here unless you ask, such an assignment is not allowed.
Look up variance/covariance/contravariance if you're interested. ).
1 that's a good alternative solution – Andras Zoltan Mar 16 at 15:43 Thanks this works...i'll bear the reading material in mind for later. I guess the only risk is that I have a collection which is supposed to contain the derived members but doesn't - if I accidently send in a List of the base. I'm not worried about this now, so no problem.
– gwizardry Mar 16 at 15:50 Very new to generics....but enjoying the possibilities achieveable with them! – gwizardry Mar 16 at 15:52 1 With the generics restrictions that exist, you won't be able to compile something that accidentally sends in a List of the base. What you send in will always have to be a List, and T will always have to match exactly.
E.g. This (assume class ExtendedIntSegment : IntSegment exists) won't even compile: new IntersectionClosedPath(new List()); Because ExtendedIntSegment and IntSegment aren't exactly the same. If you make those two types the same, either of those, it'll compile and work just fine, with great type safety.
– Tim S. Mar 16 at 16:01.
A classic problem. A List cannot be implicitly converted to a List. You can cast the items in a List and create a List like this: listOfBase = listOfDerived.Cast().ToList().
1 I'm being a pedant, but this won't quite work - Cast() returns an IEnumerable, not List – Andras Zoltan Mar 16 at 15:40 Grr! Forgot the ToList. Fixed now.
– Kendall Frey Mar 16 at 15:42 I don't quite understand this as i'm not trying to convert from base to derived but the other way round. – gwizardry Mar 16 at 15:49 D'oh! I must be still sleeping.
– Kendall Frey Mar 16 at 15:50.
A List is not equivalent to a List where T : TBase. Why? Because List doesn't inherit from List, only the generic type parameters are related.
You can instead do this in the constructor: Segments = inEdges.Cast().ToList() I would also change the constructor parameter to IEnumerable Equally, it might be that @Tim S. Has the best solution based on what you want to achieve. Personally I believe he's probably nailed it.
– gwizardry Mar 16 at 16:04 @gwizardry - In IntersectionClosedPath yes. – Andras Zoltan Mar 16 at 16:39.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.
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Change List inEdges to List inEdges and it will work. The problem is that Segments is known as a List Look up variance/covariance/contravariance if you're interested. ).
Up vote 0 down vote favorite share g+ share fb share tw.
I have a compiler error when trying to set a generic base collection class member in this derived class. Error CS0029: Cannot implicitly convert type 'System.Collections.Generic. List' to 'System.Collections.Generic.
List' Here's the outline of my generic collection public class Path : IEnumerable where T : Segment { private List segments = new List(); public List Segments { set { segments = value; } get { return this. Segments; } } public Path() { this. Segments = new List(); } public Path(List s) { this.
Segments = s; } } A derived generic class of this collection is then defined for a derived class IntSegment of Segment (for which the base collection is defined) public class IntersectionClosedPath : Path, IEnumerable where T : IntSegment { public IntersectionClosedPath(List inEdges) : base() { Segments = inEdges; } } I can't understand why this assignment is not allowed. (I don't need to make a deep copy of the incoming List). C# generics generic-list link|improve this question edited Mar 16 at 17:17phoog11k921 asked Mar 16 at 15:32gwizardry835 85% accept rate.
Change List inEdges to List inEdges and it will work. The problem is that Segments is known as a List, where T : IntSegment, while inEdges is a List. (For reasons I'll not go into here unless you ask, such an assignment is not allowed.
Look up variance/covariance/contravariance if you're interested. ).
1 that's a good alternative solution – Andras Zoltan Mar 16 at 15:43 Thanks this works...i'll bear the reading material in mind for later. I guess the only risk is that I have a collection which is supposed to contain the derived members but doesn't - if I accidently send in a List of the base. I'm not worried about this now, so no problem.
– gwizardry Mar 16 at 15:50 Very new to generics....but enjoying the possibilities achieveable with them! – gwizardry Mar 16 at 15:52 1 With the generics restrictions that exist, you won't be able to compile something that accidentally sends in a List of the base. What you send in will always have to be a List, and T will always have to match exactly.
E.g. This (assume class ExtendedIntSegment : IntSegment exists) won't even compile: new IntersectionClosedPath(new List()); Because ExtendedIntSegment and IntSegment aren't exactly the same. If you make those two types the same, either of those, it'll compile and work just fine, with great type safety.
– Tim S. Mar 16 at 16:01.
A classic problem. A List cannot be implicitly converted to a List. You can cast the items in a List and create a List like this: listOfBase = listOfDerived.Cast().ToList().
1 I'm being a pedant, but this won't quite work - Cast() returns an IEnumerable, not List – Andras Zoltan Mar 16 at 15:40 Grr! Forgot the ToList. Fixed now.
– Kendall Frey Mar 16 at 15:42 I don't quite understand this as i'm not trying to convert from base to derived but the other way round. – gwizardry Mar 16 at 15:49 D'oh! I must be still sleeping.
– Kendall Frey Mar 16 at 15:50.
A List is not equivalent to a List where T : TBase. Why? Because List doesn't inherit from List, only the generic type parameters are related.
You can instead do this in the constructor: Segments = inEdges.Cast().ToList() I would also change the constructor parameter to IEnumerable Equally, it might be that @Tim S. Has the best solution based on what you want to achieve. Personally I believe he's probably nailed it.
– gwizardry Mar 16 at 16:04 @gwizardry - In IntersectionClosedPath yes. – Andras Zoltan Mar 16 at 16:39.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.
Related Questions
Why can I access a derived private member function via a base class pointer to a derived object?
Casting List of derived class to List of base class still returning objects of derived class?
In a templated derived class, why do I need to qualify base class member names with “this->” inside a member function?
When a derived class object is deleted which gets deleted first - the base class object or the derived class object?
C++ setting attribute of a derived class to an unknown class object, derived from same base class?
Collection of derived classes that have generic base class?