在C＃中基于变量创建变量类型

Question

I am parsing DataField 's out of a larger Message . The Message is in the format RecordHeader|DataFieldKey|DataField|DataFieldKey|DataField|DataFieldKey|DataField|DataFieldKey|DataField (There are no delimiters, just there for effect.) The DataFieldKey block defines the length and type of the DataField that follows. I have a DataField class which is used to inherit base functionality for various DataField derived types

After I parse my DataFieldKey I know the type I need to create and the chunk of the byte[] to copy out of the message into the DataField.

switch(fieldId)
{
    case (0):
        GPSData dataField = new GPSData(dataFieldContent);
        break;
    case (1):
        DebugEvent dataField = new DebugEvent(dataFieldContent);
        break;
    case (2):
        DigitalData dataField = new DigitalData(dataFieldContent);
        break;
    case (3):
        Driver dataField = new Driver(dataFieldContent);
        break;
    //case etc
    default:
        Debug.WriteLine("Message {0}. Location {1} Unable to process unknown data field type ({2})", recordHeader.SequenceNumber, dfk.FieldPosition, fieldId);
        break;
}

This leads to errors like the following Cannot implicitly convert type 'Namespace.TripType' to 'Namespace.GPSData'

I can see that the name is defined in the first case and all the following fail. I'm not sure what to do here. I want to be able to create the correct type for the DataField, move on to call a method or two within it, and then rinse repeat for the next field. Is there some inheritance trick I could be using? Do I need to name each var dataField0 to dataFieldX?

switch(fieldType) is lazy hardcode for checking DataField.FieldId

Have tried the following, but it just has more errors. I have renamed DataField to DataFieldBase as I also have two Interfaces IDataFieldBase and IDataFieldImpl to make sure I implement things everywhere.

DataFieldBase dataField = null;

switch(fieldId)
{
    case (0):
        dataField = new GPSData();
        dataField.Content = dataFieldContent;
        (dataField as GPSData).TestArray = dataFieldContent;
        break;
//etc

• Cannot use local variable 'dataField' before it is declared
  • Occured everywhere dataField ws used (3 times)
  • I thought DataFieldBase dataField = null; declared it
• 'Ctrack.DMT.Message.GPSData' does not contain a constructor that takes 0 arguments
  • I don't actually have a no argument constructor
  • After I made one this error went away

Answer 1

To the best of my knowledge this won't work in the way that you have set it up, because even though the objects in second, third, etc conditions of the switch statement only get created at runtime, the compiler still sees them as the type that the first condition creates them as at compile time.

If all the dataField types inherit from a common ancestor, use that as your dataField type for every switch condition, or even declare it as the ancestor type with value null before the switch and then just assign the various values during the switch conditions.

If they don't inherit from a common ancestor, then see if you can't design in that way with common data and functionality in the ancestor class, or even use an interface.

Edit:

Or declare the dataField as type object with a null value before the switch?

Code to try:

class CommonData
{
    public string CommonStuff { get; set; }
}

class GPSData : CommonData
{
    public string GPSStuff { get; set; }
}

class DigitalData : CommonData
{
    public string DigitalStuff { get; set; }
}

class DriverData : CommonData
{
    public string DriverStuff { get; set; }
}

public static class FieldStuff
{
    public static void TryStuff(int aFieldId)
    {
        CommonData dataField = null;

        switch (aFieldId)
        {
            case (0):
                dataField = new GPSData();
                dataField.CommonStuff = "abcd";
                (dataField as GPSData).GPSStuff = "bcde";
                break;
            case (1):
                dataField = new DigitalData();
                dataField.CommonStuff = "abcd";
                (dataField as DigitalData).DigitalStuff = "cdef";
                break;
            case (2):
                dataField = new DriverData();
                dataField.CommonStuff = "abcd";
                (dataField as DriverData).DriverStuff = "defg";
                break;
            default:
                break;
        }
    }
}