Make some edge-case GetCustomAttribute() behavior match NETFx behavior. (#5368)

This was discovered while reviewing incoming CoreCLR changes
that enable the use of generic custom attributes. With these
changes (plus a separate change that comments out an assert in NUTC)
the tests introduced for that feature pass in Project N.


Object[] ICustomAttributeProvider.GetCustomAttributes()

 If the passed in attribute type is an open type or a value type,
 the return array is of type Object[] rather than the exact
 array type (which would be impossible in the case of an open type.)
 Chances are the array length would be zero though I don't
 hardcode that assumption.


IEnumerable<Attribute> CustomAttributeExtensions.GetCustomAttributes()

 If the passed in attribute type is an open type, the code internally
 generates a Object[] array which then decays to null due to a
 "as Attribute[]" conversion.

 If the passed in attribute type is a value type, it gets thrown
 out by the "must derive from System.Attribute" check so that
 case doesn't apply to this api.


Attribute.GetCustomAttributes()

 Everything that applies to CustomAttributeExtensions applies
 to this.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Helpers.cs b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Helpers.cs
index 90943008c..5d851fb09 100644
--- a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Helpers.cs
+++ b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/Helpers.cs
@@ -189,8 +189,14 @@ namespace System.Reflection.Runtime.General
                 object instantiatedAttribute = cad.Instantiate();
                 attributes.Add(instantiatedAttribute);
             }
+
+            // This is here for desktop compatibility. ICustomAttribute.GetCustomAttributes() normally returns an array of the 
+            // exact attribute type requested except in two cases: when the passed in type is an open type and when 
+            // it is a value type. In these two cases, it returns an array of type Object[].
+            bool useObjectArray = actualElementType.ContainsGenericParameters || actualElementType.IsValueType;
             int count = attributes.Count;
-            object[] result = (object[])Array.CreateInstance(actualElementType, count);
+            object[] result = useObjectArray ? new object[count] : (object[])Array.CreateInstance(actualElementType, count);
+
             attributes.CopyTo(result, 0);
             return result;
         }