Question

I have been assigned to Convert a VB.NET project to C# and I got stuck. I am using a class called RsiOPCAuto, but I don’t think that I’ll have to go into to much detail into explaining how it works. Let’s just get on with my issue.

So basicly what i do is grabbing an object from my class using this code:

public partial class FrmPartialMain : Form
{
    RsiOPCAuto.OPCServer oOpcServer;
    public FrmPartialMain()
    {
        InitializeComponent();
        object RsiOPCAuto;
        object oOPCList;

        oOpcServer = new RsiOPCAuto.OPCServer();
        oOPCList = oOpcServer.GetOPCServers();

So far, so good. By adding a watch I can see that oOPCList now have the value {string[1..4]}.

Now I want to put these four strings into a combo box. I do this with a simple for loop:

for (int i = 0; i <= oOPCList.Length; i++)
{
    cboServer.Items.Add(oOPCList[i]);
}

Edit: Scratch that, changed this to a much nicer foreach loop.

Even though this object now functions as a string array both the oOPCList.Length and (oOPCList[i]) get errors:

.Length:
Error 1 ‘object’ does not contain a definition for ‘Length’ and no extension method ‘Length’ accepting a first argument of type ‘object’ could be found (are you missing a using directive or an assembly reference?)

oOPCList[i]:
Error 2 Cannot apply indexing with [] to an expression of type ‘object’

I bet it’s the simplest thing but I just can’t see it, help is very much appreciated and if there’s anything else you need to know be sure to ask 🙂

PS. It might be worth mentioning that I have tried some different ways to convert the object to a string array but I continuously get an error telling me that I can not convert system.string[*] to system.string[].

This is the VB.NET code that I am converting:

Friend Class frmPartialMain
    Inherits System.Windows.Forms.Form
        Dim oOpcServer As RsiOPCAuto.OPCServer
        Private Sub frmPartialMain_Load(ByVal eventSender As System.Object, ByVal eventArgs As  System.EventArgs) Handles MyBase.Load
            Dim RsiOPCAuto As Object
            Dim oOPCList() As Object
            Dim i As Integer

            oOpcServer = New RsiOPCAuto.OPCServer
            oOPCList = oOpcServer.GetOPCServers
            For i = LBound(oOPCList) To UBound(oOPCList)
                cboServer.Items.Add(oOPCList(i))
            Next i

Answer 1

You need to cast the return value of GetOPCServers to an object first, then to an Array, because this method returns a dynamic type. You can’t directly cast to a string[] because strong-typed arrays that are not 0-based are not supported by C#. After the cast, you need to call Cast<string> to get a strong typed enumerable, over which you can iterate:

IEnumerable<string> oOPCList;

oOpcServer = new RsiOPCAuto.OPCServer();
oOPCList = ((Array)(object)oOpcServer.GetOPCServers()).Cast<string>();

Furthermore, you would be better off using a foreach loop, because it is a lot more readable:

foreach(var item in oOPCList)
    cboServer.Items.Add(item);

---

The strange cast first to object, then to Array, and then to IEnumerable<string> via Cast<string> is needed because of the following:

GetOPCServers returns a dynamic type. Trying to access that dynamic instance in any way – even via a call to GetType triggers an InvalidCastException. Therefore, it first needs to be cast to object so it no longer is a dynamic type. After that, we can cast it to an Array, the only supported way in C# to work with non-zero-based arrays. But Array is not strong typed, so we append the call to Cast<string> to get a strong typed enumerable.