[Hangfire] Fix NullReferenceException for unresolvable jobs

src/OpenTelemetry.Instrumentation.Hangfire/CHANGELOG.md

@@ -5,6 +5,10 @@
 * Assemblies are now digitally signed using cosign.
   ([#4637](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/4637))
 
+* Fixed `NullReferenceException` in the metrics filter when a job's definition
+  cannot be resolved.
+  ([#4731](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/4731))
+
 ## 1.16.0-beta.1
 
 Released 2026-Jun-24

src/OpenTelemetry.Instrumentation.Hangfire/Implementation/JobNameFormatter.cs

@@ -9,8 +9,13 @@ namespace OpenTelemetry.Instrumentation.Hangfire.Implementation;
 
 internal static class JobNameFormatter
 {
-    internal static string FormatJobName(this Job job)
+    internal static string FormatJobName(this Job? job)
     {
+        if (job is null)
+        {
+            return "UNKNOWN";
+        }
+
         var sb = new StringBuilder()
             .Append(job.Type.ToGenericTypeString())
             .Append('.')

test/OpenTelemetry.Instrumentation.Hangfire.Tests/JobNameFormatterTests.cs

@@ -0,0 +1,33 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+using Hangfire;
+using Hangfire.Common;
+using OpenTelemetry.Instrumentation.Hangfire.Implementation;
+
+namespace OpenTelemetry.Instrumentation.Hangfire.Tests;
+
+public class JobNameFormatterTests
+{
+    [Fact]
+    public void FormatJobName_Returns_TypeAndMethod_When_Job_Is_Resolved()
+    {
+        var job = Job.FromExpression<TestJob>(x => x.Execute());
+        var backgroundJob = new BackgroundJob("job-id", job, DateTime.UtcNow);
+
+        var name = backgroundJob.FormatJobName();
+
+        Assert.Equal("TestJob.Execute", name);
+    }
+
+    [Fact]
+    public void FormatJobName_Returns_Unknown_When_Job_Is_Unresolvable()
+    {
+        // null! mimics Hangfire's runtime state for a job whose definition failed to deserialize.
+        var backgroundJob = new BackgroundJob("job-id", job: null!, DateTime.UtcNow);
+
+        var name = backgroundJob.FormatJobName();
+
+        Assert.Equal("UNKNOWN", name);
+    }
+}