Add USP0022, UnityObjectIfNullCoalescingSuppressor

doc/USP0022.md

@@ -0,0 +1,32 @@
+# USP0022 Prefer reference equality
+
+We have a dedicated diagnostic `UNT0029` to prevent is-null check with `UnityEngine.Object`. But `IDE0270` will suggest to use is-null check over reference equality method.
+
+## Suppressed Diagnostic ID
+
+IDE0270 - Null check can be simplified (if null check)
+
+## Examples of code that produces a suppressed diagnostic
+```csharp
+using UnityEngine;
+
+class Camera : MonoBehaviour
+{
+	public void M()
+	{
+		Camera item = FindItem() as Camera;
+		if (item == null)
+			throw new System.InvalidOperationException();
+	}
+
+	MonoBehaviour FindItem() => null;
+}
+```
+
+## Why is the diagnostic reported?
+
+Under normal circumstances, `if (item == null) throw new System.InvalidOperationException()` can be simplified to `Camera item = FindItem() as Camera ?? throw new System.InvalidOperationException();`.
+
+## Why do we suppress this diagnostic?
+
+Unity has overridden the `==` operator for `UnityEngine.Object`. If you use the `==` operator to compare a `UnityEngine.Object` to null, it will return true if the `UnityEngine.Object` is destroyed, even if the object itself isn't actually null. The `??` operator cannot be overridden in this way, and therefore behaves inconsistently with the `==` operator, because it checks for null in a different way.

doc/index.md

@@ -65,3 +65,4 @@ ID | Suppressed ID | Justification
 [USP0019](USP0012.md) | IDE0051 | Don't flag private methods with PreserveAttribute or UsedImplicitlyAttribute as unused
 [USP0020](USP0020.md) | IDE0052 | The Unity runtime invokes Unity messages
 [USP0021](USP0021.md) | IDE0041 | Prefer reference equality
+[USP0022](USP0022.md) | IDE0270 | Unity objects should not use if null coalescing

src/Microsoft.Unity.Analyzers.Tests/UnityObjectNullHandlingSuppressorTests.cs

@@ -219,6 +219,7 @@ public void Update()
 		await VerifyCSharpDiagnosticAsync(test, suppressor);
 	}
 
+
 	[Fact]
 	public async Task UseIsNullFromObjectSuppressed()
 	{
@@ -241,6 +242,30 @@ public void Update()
 		await VerifyCSharpDiagnosticAsync(test, suppressor);
 	}
 
+	[Fact]
+	public async Task IfNullCoalescingSuppressed()
+	{
+		const string test = @"
+using UnityEngine;
+
+class Camera : MonoBehaviour
+{
+	public void M()
+	{
+		Camera item = FindItem() as Camera;
+		if (item == null)
+			throw new System.InvalidOperationException();
+	}
+
+	MonoBehaviour FindItem() => null;
+}";
+
+		var suppressor = ExpectSuppressor(UnityObjectNullHandlingSuppressor.IfNullCoalescingRule)
+			.WithLocation(9, 3);
+
+		await VerifyCSharpDiagnosticAsync(test, suppressor);
+	}
+
 	[Fact]
 	public async Task LegitUseIsNullNotSuppressed()
 	{

src/Microsoft.Unity.Analyzers/Resources/Strings.resx

@@ -232,6 +232,9 @@
   <data name="UnityObjectNullCoalescingSuppressorJustification" xml:space="preserve">
     <value>Unity objects should not use null coalescing.</value>
   </data>
+  <data name="UnityObjectIfNullCoalescingSuppressorJustification" xml:space="preserve">
+    <value>Unity objects should not use if null coalescing.</value>
+  </data>
   <data name="UnityObjectNullPropagationSuppressorJustification" xml:space="preserve">
     <value>Unity objects should not use null propagation.</value>
   </data>
@@ -611,4 +614,4 @@
   <data name="GetLocalPositionAndRotationDiagnosticTitle" xml:space="preserve">
     <value>Inefficient localPosition/localRotation read</value>
   </data>
-</root>
+</root>

src/Microsoft.Unity.Analyzers/UnityObjectNullHandling.cs

@@ -289,7 +289,12 @@ public class UnityObjectNullHandlingSuppressor : DiagnosticSuppressor
 		suppressedDiagnosticId: "IDE0074",
 		justification: Strings.UnityObjectCoalescingAssignmentSuppressorJustification);
 
-	public override ImmutableArray<SuppressionDescriptor> SupportedSuppressions => ImmutableArray.Create(NullCoalescingRule, NullPropagationRule, CoalescingAssignmentRule, UseIsNullRule);
+	internal static readonly SuppressionDescriptor IfNullCoalescingRule = new(
+		id: "USP0022",
+		suppressedDiagnosticId: "IDE0270",
+		justification: Strings.UnityObjectIfNullCoalescingSuppressorJustification);
+
+	public override ImmutableArray<SuppressionDescriptor> SupportedSuppressions => ImmutableArray.Create(NullCoalescingRule, NullPropagationRule, CoalescingAssignmentRule, UseIsNullRule, IfNullCoalescingRule);
 
 	public override void ReportSuppressions(SuppressionAnalysisContext context)
 	{