From 65d060c90f25536bfbaf7f8a760eed247a497f2d Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Wed, 2 Sep 2026 11:43:55 +0200
Subject: [PATCH] fix(simmy): return null from FaultGenerator when no fault is
 generated

FaultGenerator's implicit conversion dereferenced the nullable Outcome
returned by the underlying generator via `!.Value`, throwing
InvalidOperationException ("Nullable object must have a value.") when the
generator produced no outcome - an empty generator, or registered weights
summing to zero. ChaosFaultStrategy treats a null fault as "do not inject"
and only catches OperationCanceledException, so the exception escaped and
failed the execution the strategy was meant to leave untouched.

Propagate the nullable outcome with `?.Exception`, mirroring
OutcomeGenerator<TResult>, so no fault is injected when none is generated.
---
 src/Polly.Core/Simmy/Fault/FaultGenerator.cs   |  2 +-
 .../Simmy/Fault/FaultGeneratorTests.cs         | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/Polly.Core/Simmy/Fault/FaultGenerator.cs b/src/Polly.Core/Simmy/Fault/FaultGenerator.cs
index c5931c0b161..cfb943f327c 100644
--- a/src/Polly.Core/Simmy/Fault/FaultGenerator.cs
+++ b/src/Polly.Core/Simmy/Fault/FaultGenerator.cs
@@ -78,6 +78,6 @@ public FaultGenerator AddException<TException>(int weight = DefaultWeight)
 
         var generatorDelegate = generator._helper.CreateGenerator();
 
-        return args => new ValueTask<Exception?>(generatorDelegate(args.Context)!.Value.Exception);
+        return args => new ValueTask<Exception?>(generatorDelegate(args.Context)?.Exception);
     }
 }
diff --git a/test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs b/test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs
index 1b7e2cc99a0..fd77255d42f 100644
--- a/test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs
+++ b/test/Polly.Core.Tests/Simmy/Fault/FaultGeneratorTests.cs
@@ -39,6 +39,24 @@ public void AddException_FactoryWithResilienceContext_Ok()
         Generate(generator).ShouldBeOfType<InvalidOperationException>();
     }
 
+    [Fact]
+    public void NoExceptionRegistered_ShouldReturnNull()
+    {
+        var generator = new FaultGenerator();
+
+        Generate(generator).ShouldBeNull();
+    }
+
+    [Fact]
+    public void AllWeightsZero_ShouldReturnNull()
+    {
+        var generator = new FaultGenerator();
+
+        generator.AddException<InvalidOperationException>(weight: 0);
+
+        Generate(generator).ShouldBeNull();
+    }
+
     private static Exception? Generate(FaultGenerator generator)
     {
         Func<FaultGeneratorArguments, ValueTask<Exception?>> func = generator;