[Mono-bugs] [Bug 78297][Nor] New - interface constraints to generics are considered value types

bugzilla-daemon at bugzilla.ximian.com bugzilla-daemon at bugzilla.ximian.com
Thu May 4 22:07:43 EDT 2006

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Changed by gregdavisfromnj at hotmail.com.


--- shadow/78297	2006-05-04 22:07:43.000000000 -0400
+++ shadow/78297.tmp.2298	2006-05-04 22:07:43.000000000 -0400
@@ -0,0 +1,54 @@
+Bug#: 78297
+Product: Mono: Compilers
+Version: 1.1
+OS: other
+OS Details: 
+Status: NEW   
+Priority: Normal
+Component: C#
+AssignedTo: rharinath at novell.com                            
+ReportedBy: gregdavisfromnj at hotmail.com               
+QAContact: mono-bugs at ximian.com
+TargetMilestone: ---
+Summary: interface constraints to generics are considered value types
+Description of Problem:  
+Create a generic container class of any type T, and assign null to a T.   
+That is bad because T might be a value type.  Constrain T to an arbitrary  
+interface such as IAmNotAValueType, and that should guarantee that Ts are  
+not value types.  The compiler doesn't agree.  Implementing that 
+interface in AmNotAValueType, which is abstract, and only specifies 
+abstract methods for IAmNotAValueType methods convinces the compiler to 
+allow a null assignment to T.  See the attached file. 
+Steps to reproduce the problem:  
+1.  Make a generic class of type T whose only constraint on T is an 
+2.  Assign null to a T someplace in the class; gmcs dissaproves. 
+3.  Make an abstract class whose only members are abstract, and which 
+implements the aforementioned interface. 
+4.  Constrain T with the abstract class; gmcs is happy. 
+Actual Results:  
+Foobar.cs(25,5): error CS0403: Cannot convert null to the type parameter 
+`T' becaues it could be a value type. Consider using `default (T)' 
+Foobar.cs(25,5): error CS0037: Cannot convert null to `T' because it is a 
+value type 
+Compilation failed: 2 error(s), 0 warnings 
+Expected Results:  
+a Foobar.dll 
+How often does this happen?   
+every time 
+Additional Information:  
+compile the attached file with "gmcs -target:library Foobar.cs"

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