When-Process-block-is-called

When process block is called

PowerShell functions and script blocks consist of blocks. If none of them is explicitly specified then the end block is assumed for functions and script blocks and the process block for filters, the special form of functions.

One of the blocks is process. It is supposed to be called with each item from the data piped to a command. There are three edge cases. One of them may be not intuitively expected.

When a null is piped to a command then its process block is called once with the null item. It looks reasonable, one item is piped and it is null.

$null | Invoke-MyCommand

When an empty array or empty result from a command is piped to a command then its process block is not called. It looks reasonable, nothing is piped.

@() | Invoke-MyCommand
Get-Nothing | Invoke-MyCommand

It may be not obvious that the process is called with a null item when a command is invoked without a pipeline. Thus, it works as if null is piped. One may expect it to work as if nothing is piped.

Invoke-MyCommand

The following scripts illustrate the last two facts.

• Test-1.Simple.function.ps1
• Test-2.Advanced.function.ps1

The script Test-3.Pipe.nothing.ps1 shows what happens if nothing is piped to a command, either directly or via a result variable. Depending on PowerShell versions these scenarios may be different.