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Powershell as a T4 Text Template alternative
T4 Text templates is a tool for code (or any text really) generation that is built into Visual Studio. A couple of weeks ago I started working on a project that needed a lot of code generation but I could not allow a dependency to Visual Studio so I started writing Powershell scripts instead. I was surprised how nice it turned out to be. Three features of Powershell turns out to be particularly useful:
- Variable expansions in strings; i.e. “The variable ‘var’ has value: $var”
- Here strings (that may contain any characters): @”…”@
- Multiple returns: { “row1”; “row2”; “row3” } yields an array of the three rows.
(At first when I discovered the multiple returns feature in Powershell I though it was weird and I wondered how it could actually have made it into the language. Now I know better…)
Now, as an example, lets say that we want to generate a html table from a collection of objects and their properties. Using the features above the task can be expressed as:
function ConvertTo-HtmlTable($header, $objects, $properties)
{
@"
<html>
<body>
<h1>$header</h1>
<table border=`"1`">
"@
if (!$properties) {
$properties = Get-Member -InputObject $o -MemberType Properties | select -expandproperty name
}
" <tr>"
foreach($p in $properties) { " <th>$p</th>" }
" </tr>"
foreach($o in $objects)
{
" <tr>"
foreach($p in $properties)
{
$value = Invoke-Expression ("`$o." + $p)
" <td>$value</td>"
}
" </tr>"
}
@"
</table>
</body>
</html>
"@
}
$objects = Get-Process
ConvertTo-HtmlTable `
"Process information" $objects Id, Name, PrivateMemorySize | Out-File "page.html"
Sometimes I think generating code written in Powershell even outshines a corresponding T4 templates implementation. Nice!
Code Contemplation 