is there any way of specifying the type of a generic class during runtime instead of at compile time?

my problem is that i have file several different versions of a file with each version having different properties and methods and it is not unitl compile time that i can findout what version of the file it is. Below is examples of the classes that i have

public class version1_0
{
   //example member
   public int total;
}

public class version2_2
{
   //example member
   public string extendedDesc;
}

public Gen<T>
{
   public T file;

   //other methods are here
}

public enum Versions
{
   v1, //represents the version1_0 class
   v2 //represents the version2_2 class
}

i want to add the generic class "Gen" to the class below using the _file variable and then set the type of the generic class Gen by the version that was passed into the constructor so i can specify the type of the Gen variable at compile time only at run time.
public class myFile
{
   private Gen??? _file;

   public myFile(Versions)
   {
      //set type of _file here by using Versions enum.
   }
}

is there any other way that i could go about solving this problem?

many thanks in advance.

Glenn