6.5 Data Referencing in Invokes: DataTable, List & Dictionary Arguments

6.5 Data Referencing in Invokes: DataTable, List & Dictionary Arguments

Core Concept: The exam description lists "Data referencing in invokes: Dt, List, Dictionary" as its own task. It tests one idea that surprises many developers: DataTable, List, and Dictionary are reference types. Passing one to Invoke Workflow File, Invoke Code, or Invoke Method passes a reference to the same object. So an argument marked In can still have its contents changed by the invoked code. What an In argument does prevent is replacing the caller's variable with a new object.

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Value Types Versus Reference Types

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Three Scenarios to Know Cold

1. Mutating the object through an In argument

' Caller: dt_Invoices has 10 rows; invoke Enrich.xaml with in_dtInvoices (direction In)
' Inside Enrich.xaml:
in_dtInvoices.Rows.Add("INV-11", 250.0)
in_dtInvoices.Rows(0)("Status") = "Checked"
' Back in the caller: dt_Invoices now has 11 rows and row 0 shows Status = "Checked"

The same applies to List.Add, List.Remove, Dictionary(key) = value, and Dictionary.Remove. REFramework's in_Config dictionary is passed In to many workflows, so any workflow that writes to it changes the configuration for the rest of the job. That is why the template treats Config as read-only after Initialization.

2. Replacing the object through an In argument

' Inside the invoked workflow (in_dtInvoices is In):
in_dtInvoices = dt_Filtered   ' points the local argument at a different table
' Back in the caller: dt_Invoices is unchanged

Assigning a new object only redirects the argument inside the invoked workflow. To hand a new table back, use an Out argument, such as out_dtFiltered, or In/Out (io_dtInvoices).

3. Protecting the caller's data

When the invoked workflow must experiment on a table without touching the original, pass or create a copy:

Dim dt_Work As DataTable = in_dtInvoices.Copy()   ' schema + rows, independent object
Dim lst_Work As New List(Of String)(in_lstIds)    ' new list with the same items
Dim dict_Work As New Dictionary(Of String, Object)(in_Config) ' shallow copy of the dictionary

A dictionary copy made this way is shallow: if a value is itself a reference type, such as a DataTable stored in the dictionary, both dictionaries still point to that same table.

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Invoke Code and Invoke Method Follow the Same Rules

• Invoke Code: an argument with direction In of type DataTable can be modified in place with Rows.Add or by editing cells, and the caller sees the change. Declare an Out or In/Out argument only when the code creates a new object that must be returned.
• Invoke Method: calling an instance method on a TargetObject, such as lst_Items.Sort() or dt_Report.Clear(), changes that object in place. There is no copy.

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The Isolated Exception

Invoke Workflow File has an Isolated option that runs the invoked workflow in a separate robot process, for example to contain a crash-prone application. Across process boundaries, arguments are serialized and copied rather than shared:

Isolated invocations also require argument types that can be serialized. A live object such as a UI element handle or an open database connection cannot cross into the separate process.

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Parallel Branches and Shared References

Because every branch of a Parallel or Parallel For Each activity sees the same objects, two branches that add rows to the same DataTable are writing to one object. A DataTable is safe for concurrent reads only, so give each branch its own table and merge the results afterwards.

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Checklist for Reviews

1. For every DataTable, List, or Dictionary argument, ask whether the invoked workflow is allowed to change it. If not, document it as read-only or pass a copy.
2. Use Out or In/Out whenever the invoked workflow assigns a new object that the caller needs.
3. Do not rely on in-place changes when the invocation is Isolated.
4. Name arguments with their direction prefix (in_, out_, io_) so readers can see the contract. UiPath's Workflow Analyzer includes naming rules for this.
5. Remember that strings never change in place; in_Text.Trim() returns a new string.