From 13a5b4f6531159c8eaf2c54121545b2a29e4e20c Mon Sep 17 00:00:00 2001
From: HeyItsGilbert <me@gilbertsanchez.com>
Date: Wed, 29 Jul 2026 04:12:29 +0000
Subject: [PATCH] fix: clear error when rule Condition is null (#83)

Test-FeatureFlag/Test-Condition threw an opaque 'You cannot call a
method on a null-valued expression' error when a rule's Condition was
$null (omitted or misspelled key). ConditionTransformAttribute called
.GetType() on the null input before any of its type checks ran.

ConditionTransformAttribute.Transform() now null-guards up front and
throws a clear ArgumentNullException naming the problem: the rule must
define a Condition. FeatureFlagTransformAttribute gained the same
null guard for consistency (same GetType()-on-null pattern for a null
-FeatureFlag).

Added coverage: Test-Condition -Condition $null, and a Test-FeatureFlag
rule with no Condition, both assert the new clear error message.
---
 CHANGELOG.md                       |  6 ++++++
 Gatekeeper/Classes/FeatureFlag.ps1 |  8 ++++++++
 tests/Classes.tests.ps1            |  6 ++++++
 tests/Test-FeatureFlag.tests.ps1   | 12 ++++++++++++
 4 files changed, 32 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6c862b0..2c4be8c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   bind and always threw). `-PropertySet` is now a mandatory parameter on
   `Test-FeatureFlag`, matching `Test-Condition`, so the missing value is
   reported with a clear mandatory-parameter error instead. (#84)
+- `Test-Condition`/`Test-FeatureFlag` no longer throw an opaque
+  "You cannot call a method on a null-valued expression" error when a rule's
+  `Condition` is `$null` (e.g. omitted or misspelled). `ConditionTransformAttribute`
+  now null-guards and raises a clear `ArgumentNullException` explaining that the
+  rule must define a `Condition`. `FeatureFlagTransformAttribute` gained the same
+  guard for a null `FeatureFlag`. (#83)
 
 ## [1.0.0] 2026-06-05
 
diff --git a/Gatekeeper/Classes/FeatureFlag.ps1 b/Gatekeeper/Classes/FeatureFlag.ps1
index 7bc05de..c1e718d 100644
--- a/Gatekeeper/Classes/FeatureFlag.ps1
+++ b/Gatekeeper/Classes/FeatureFlag.ps1
@@ -236,6 +236,10 @@ class GatekeeperPath {
 class FeatureFlagTransformAttribute : System.Management.Automation.ArgumentTransformationAttribute {
 
     [object] Transform([System.Management.Automation.EngineIntrinsics]$engineIntrinsics, [object] $inputData) {
+        if ($null -eq $inputData) {
+            throw [System.ArgumentNullException]::new('FeatureFlag',
+                "FeatureFlag cannot be null.")
+        }
         if ($inputData -is [FeatureFlag]) {
             return $inputData
         }
@@ -258,6 +262,10 @@ class FeatureFlagTransformAttribute : System.Management.Automation.ArgumentTrans
 class ConditionTransformAttribute : System.Management.Automation.ArgumentTransformationAttribute {
 
     [object] Transform([System.Management.Automation.EngineIntrinsics]$engineIntrinsics, [object] $inputData) {
+        if ($null -eq $inputData) {
+            throw [System.ArgumentNullException]::new('Condition',
+                "Rule has no Condition. Each rule must define a 'Condition'.")
+        }
         if ($inputData -is [Condition]) {
             return $inputData
         }
diff --git a/tests/Classes.tests.ps1 b/tests/Classes.tests.ps1
index 0b4d4b8..6c3356d 100644
--- a/tests/Classes.tests.ps1
+++ b/tests/Classes.tests.ps1
@@ -216,6 +216,12 @@ Describe 'ConditionTransformAttribute' {
             Test-Condition -Condition 42 -PropertySet $script:propertySet -Context $script:context
         } | Should -Throw -ExpectedMessage '*Cannot convert type*'
     }
+
+    It 'Throws a clear, actionable error when Condition is null' {
+        {
+            Test-Condition -Condition $null -PropertySet $script:propertySet -Context $script:context
+        } | Should -Throw -ExpectedMessage "*Rule has no Condition*"
+    }
 }
 
 Describe 'PropertySetTransformAttribute' {
diff --git a/tests/Test-FeatureFlag.tests.ps1 b/tests/Test-FeatureFlag.tests.ps1
index 6696da1..12696bc 100644
--- a/tests/Test-FeatureFlag.tests.ps1
+++ b/tests/Test-FeatureFlag.tests.ps1
@@ -85,6 +85,18 @@ Describe 'Test-FeatureFlag' {
         }
     }
 
+    Context 'Malformed rules' {
+        It 'Rule with a null Condition throws a clear, actionable error' {
+            $flag = [FeatureFlag]::new(@{
+                Name = 'MissingConditionFlag'; DefaultEffect = 'Deny'; Version = '1.0.0'; Author = 'Test'
+                Rules = @(@{ Name = 'Allow group'; Effect = 'Allow' })
+            })
+            {
+                Test-FeatureFlag -FeatureFlag $flag -PropertySet $script:propertySet -Context $script:context
+            } | Should -Throw -ExpectedMessage '*Rule has no Condition*'
+        }
+    }
+
     Context 'Ordering — first-match-wins' {
         It 'Deny before matching Allow returns $false (Deny wins)' {
             $flag = [FeatureFlag]::new(@{