Build DataTable

Build DataTable

Type: build-datatable  ·  Package: DataTable Activities v1.0.5  ·  Output: DataTable

Creates a DataTable from scratch from its rows. Use it when you need to build a table yourself — to accumulate results, prepare data to write to Excel, or set up a structure you will add rows to — instead of reading it from a source.

How it works

In the Designer, data is edited with a table editor: you define the columns and type in the rows. Each row is a { column: value } object and each cell accepts a literal value or an expression. You can also pass an expression that returns a list of row objects (for example, a list of dictionaries).

• Columns: come from the row keys, in the order they first appear. If a row lacks a key that other rows have, that cell is left empty.
• Column type: inferred from the first non-null value in the column: text → string, integer → int/long, decimal → double/decimal, boolean → bool, date → DateTime. If every value is null or of another type, the column accepts any value (object).
• Empty cells: a null value or an empty string is stored as a cell with no value (DBNull).
• Incompatible values: if a non-empty value cannot be converted to its column type (for example, text in a column inferred as numeric), the activity fails naming the row and column; it never stores a wrong value silently.
• If data is already a DataTable (because the expression returns one), it is passed through as is.
• List items that are not row objects are ignored; if data is not a list, the result is an empty table with no columns.

Required parameters

Parameter	Type	Default	Description
data	List	—	Table rows. Each row is a { column: value } object; cells accept literals or expressions. Also accepts an expression that returns a list of dictionaries or a DataTable.

Optional parameters

None.

Output

Returns a DataTable with one column per distinct row key and one row per row object, in the same order. Since columns come from rows, an empty data gives a table with no columns and no rows.

Common errors

Message	Cause and fix
The value in row …, column '…' is not compatible with the column type '…'.	A value does not fit the type inferred from the first non-null value of that column (rows are counted from 1). Use the same type for the whole column: for example, all numbers or all text.

Example

Create an invoice table with two rows and write it to Excel:

Build DataTable
  data:
     { Customer: "Northwind Trading", Total: 1500000 }
     { Customer: "Southern Supplies", Total: 820000 }
  → output: summary

Log                 message = "Rows created: " + rowCount(summary)
Write Excel Sheet   path = "C:/out/summary.xlsx"   data = summary

An empty table with columns, ready to fill

Columns are defined by the rows, so to start with an empty table that has a structure, create a sample row and empty it with Clear DataTable: it keeps columns and types. Then, inside a For Each, add rows with Add Data Row. To dump a DataTable, Write Excel Sheet and Write CSV take it directly.

Related activities

• Add Data Row — add rows to the created table.
• Clear DataTable — remove the rows and keep the structure.
• Add Column — add a calculated column to an existing table.
• Read Excel Sheet — get a DataTable from a file.