Pass group of values as user function parameters

[Fatima]

Hi
I need your help: I have  report. I must write a user function to handle my values in separate groups.
So I need to pass every values that exist in each groups by expression such as :
= MyFunction(Fields.doneValue)

and in code behind (c#):
public static double? MyFunction(List<double?> valueList)
{
    double? result;
    foreach(var itm in valueList)
     {
          if(item.HasValue)
          result += item.value;
     }
    return result;
}

but this code has error due to its parameter type, List of values can't pass to user function.
what do I do??