PowerShell Parse JSON with header and nested items into lines

• Indeed, the nested loop must refer to the outer loop's iteration variable rather than to the input object graph as a whole, as Daniel points out.

• If you mistakenly access the whole object graph in the nested loop with $array.Data.Items, you perform member-access enumeration on the $array.Data array, meaning that the .Item property values across all array elements are returned.

To put it all together (variables renamed for clarity); note that member-access enumeration is still selectively used, namely in $product.Items.Amount in order to obtain the amounts across all .Item elements:

# Note: -Raw makes Get-Content much faster by reading the entire file
#       content into as single, multiline string.
$objectGraphfromJson = Get-Content -Raw response.json | ConvertFrom-Json

# Loop over all elements of the array in the `.Data` property.
foreach ($product in $objectGraphfromJson.Data) {
  # Create and output the header object.
  [pscustomobject] @{
    row = 'Header'
    ID = $product.ID
    Code = $product.Code
    # Sum up the amounts across all child items.
    Cost = ($product.Items.Amount | Measure-Object -Sum).Sum
  }
  # For each `.Data` element, loop over its items and 
  # create and output an object for each.
  foreach ($item in $product.Items) {
    [pscustomobject] @{
      row = 'Item ' + $item.LineNo
      ID = $product.ID
      Code = $product.Code
      Cost = $item.Amount
    }  
  }
}

Output (the equivalent of what you requested, based on PowerShell's automatic for-display formatting of the objects that were output):

row               ID Code       Cost
---               -- ----       ----
Header 9663696221792 08099991 21.130
Item 1 9663696221792 08099991  5.050
Item 2 9663696221792 08099991 16.080
Header 9663696221793 08028001 26.130
Item 1 9663696221793 08028001 26.130

First loop

foreach ($root in $array.data)

Inside loop (inside the root loop)

foreach ($item in $root.items)