How to Dynamically Build Calculated Properties with Select-Object in PowerShell

Learn how to build dynamic calculated properties in PowerShell using `Select-Object`. Discover effective solutions through closures and scriptblocks for streamlining your scripts.
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How to Dynamically Build Calculated Properties with Select-Object in PowerShell

When working with data returned from REST APIs in PowerShell, you may often find yourself needing to extract specific properties from complex JSON structures. This is especially true when the properties are nested within subproperties with repetitive names. In situations like these, using Select-Object with calculated properties can simplify your code significantly. However, constructing these calculated properties dynamically can present challenges, especially when trying to reference dynamic field names within expressions. In this post, we’ll explore how to effectively build calculated properties using closures or dynamic scriptblocks to avoid redundant code.

Understanding the Problem

Imagine you’re querying a REST API that returns JSON data structured like this:

[[See Video to Reveal this Text or Code Snippet]]

To extract the values of these properties with PowerShell, a typical approach would be to use Select-Object and specify the properties in a hashtable. However, if you need to handle multiple fields and want to avoid rewriting the hashtable for every property, things can get cumbersome. In such cases, it is necessary to create a dynamic way of referencing these properties.

Suppose you created a filter function like this:

[[See Video to Reveal this Text or Code Snippet]]

You might expect to invoke it like this:

[[See Video to Reveal this Text or Code Snippet]]

However, you might notice that the $field variable does not resolve as expected in the expression block. Instead of yielding $_."year".result, it stays as $_."$field".result. So, how do we make this work? Let’s explore two effective solutions.

Solution 1: Create a Closure

The first solution involves using PowerShell’s closure capabilities. A closure is a way to remember the environment around it, thus allowing us to dynamically bind variable references. Here’s how to implement it:

Step-by-Step Implementation

Create Testing Data: First, create some dummy objects for testing purposes:

[[See Video to Reveal this Text or Code Snippet]]

Define the Filter Function: Create the filter function that takes advantage of closures:

[[See Video to Reveal this Text or Code Snippet]]

Utilize the Function: Now, you can call this filter with Select-Object:

[[See Video to Reveal this Text or Code Snippet]]

This will now work as you expect, resolving the dynamic names correctly.

Solution 2: Construct Scriptblock from Source Code

Another approach is to dynamically generate a scriptblock by constructing its source code and then executing it. Here’s the implementation:

Step-by-Step Implementation

Reuse the Testing Data: Use the same dummy objects from above.

Define the Filter Function with Scriptblocks:

[[See Video to Reveal this Text or Code Snippet]]

Call the Function: Just like the previous solution, you can call it as follows:

[[See Video to Reveal this Text or Code Snippet]]

By using this method, you can also dynamically create calculated properties without rewriting a lot of code, and it will properly evaluate your field names.

Conclusion

In conclusion, dynamically building calculated properties in PowerShell with Select-Object can save you time and make your scripts much cleaner and more manageable. By leveraging closures or dynamically constructing scriptblocks, you can avoid repetitive code and easily manipulate your data. Whether you choose to use closures for in-place binding of field variables or scriptblocks for generating expressions, you will find that both methods are efficient and effective ways to work with JSON data in PowerShell.

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