C# - How to detect data type combined with nullable property

I am using reflection to retrieve generic class object property, by detecting their data type (e.g System.String, System.DateTime, etc) and convert value based on the data type, e.g:

                switch (prop.PropertyType.FullName)
                {
                    case "System.String":
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            string.Empty : Convert.ToString(_propertyDataValue));
                        break;
                    case "System.Int32":
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            -1 : Convert.ToInt32(_propertyDataValue));
                        break;
                    case "System.DateTime":
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            DateTime.MinValue : Convert.ToDateTime(_propertyDataValue));
                        break;
                    case "System.Double":
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            0 : Convert.ToDouble(_propertyDataValue));
                        break;
                    case "System.Boolean":
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            false : Convert.ToBoolean(_propertyDataValue));
                        break;
                    default:
                        prop.SetValue(resultObject, _propertyDataValue == DBNull.Value ?
                            string.Empty : Convert.ToString(_propertyDataValue));
                        break;
                }

However, when I met the property defined as int?, double? or DateTime? which would be a Nullable type, I cannot figure out which exact data type for the property, the reflection only gives me the type as "System.Nullable", is there anyway to detect the combined data type behind?