Modbus TCP: Configuring Runtime Subscription List Access

Runtime access to the Modbus TCP subscription list requires changing two access modifiers in ModbusBase.vb. Making only SubscriptionList public fails because its element type, SubscriptionInfo, remains private.

Root Cause of the Accessibility Error

SubscriptionList is declared as List(Of SubscriptionInfo). A public member cannot expose a private nested type, which causes the compiler message: SubscriptionList cannot expose type ModbusBase.SubscriptionInfo in namespace AdvancedHMIDrivers through class ModbusBase.

Expose the Subscription List

1. Open ModbusBase.vb and locate the subscription-list declaration, reported at approximately line 290.
2. Change Private SubscriptionList As New List(Of SubscriptionInfo) to a public declaration.
3. Locate Private Structure SubscriptionInfo a few lines below it and change that declaration to public as well.

Public SubscriptionList As New List(Of SubscriptionInfo)

Public Structure SubscriptionInfo
    ' Existing structure members remain unchanged.
End Structure

Read Subscribed Addresses at Runtime

Access the list through the Modbus TCP communication instance. The following loop reads each subscription and writes its address to the console:

For i = 0 To ModbusTCPCom1.SubscriptionList.Count - 1
    Console.WriteLine(ModbusTCPCom1.SubscriptionList(i).Address.Address)
Next

The same iteration can supply a runtime ListBox click handler; replace the console output with the existing ListBox insertion method. The evidence does not define a specific ListBox control name or insertion call.

Verification and Scope

Verify that the accessibility error is cleared and that the loop returns the addresses currently held by ModbusTCPCom1.SubscriptionList. This change exposes the existing collection directly; the evidence does not establish whether modifying that collection externally is supported, so use it for inspection unless the driver implementation confirms otherwise.

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