best use collection general classes

consider following code:

abstract category someclassx<t>
{
  // blah
}

class someclassy: someclassx<int>
{
  // blah
}

class someclassz: someclassx<long>
{
  // blah
}

i wish collection someclassx<t>'s, however, isn't illusory given someclassx<int> != someclassx<long> list<someclassx<>> isn't allowed.

so fortitude have someclassx<t> exercise an interface interpretation collection as, where isomeclassx interface:

class collectionofsomeclassx: list<isomeclassx>
{
  // blah
}

is best proceed this, there improved way?