[OTLP] Fix NullReferenceException when no bucket boundaries configured for a view (#6773)

Author: martincostello

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -23,6 +23,9 @@ Notes](../../RELEASENOTES.md).
 * Added support for `Meter.TelemetrySchemaUrl` property.
   ([#6731](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6731))
 
+* Fix `NullReferenceException` when no bucket boundaries configured for a view.
+  ([#6773](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6773))
+
 ## 1.14.0
 
 Released 2025-Nov-12

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufOtlpMetricSerializer.cs

@@ -409,7 +409,7 @@ private static int WriteNumberDataPoint(byte[] buffer, int writePosition, int fi
                     writePosition,
                     in exemplar,
                     ProtobufOtlpMetricFieldNumberConstants.NumberDataPoint_Exemplars,
-                    static (byte[] buffer, int writePosition, in Exemplar exemplar) => ProtobufSerializer.WriteFixed64WithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.Exemplar_Value_As_Int, (ulong)exemplar.LongValue));
+                    static (buffer, writePosition, in exemplar) => ProtobufSerializer.WriteFixed64WithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.Exemplar_Value_As_Int, (ulong)exemplar.LongValue));
             }
         }
 
@@ -472,7 +472,7 @@ private static int WriteDoubleExemplars(byte[] buffer, int writePosition, int fi
                     writePosition,
                     in exemplar,
                     fieldNumber,
-                    static (byte[] buffer, int writePosition, in Exemplar exemplar) => ProtobufSerializer.WriteDoubleWithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.Exemplar_Value_As_Double, exemplar.DoubleValue));
+                    static (buffer, writePosition, in exemplar) => ProtobufSerializer.WriteDoubleWithTag(buffer, writePosition, ProtobufOtlpMetricFieldNumberConstants.Exemplar_Value_As_Double, exemplar.DoubleValue));
             }
         }
 
@@ -516,7 +516,10 @@ private static int WriteHistogramBuckets(byte[] buffer, int writePosition, Histo
     {
         writePosition = WriteBucketCounts(buffer, writePosition, buckets.BucketCounts);
 
-        writePosition = WriteExplicitBounds(buffer, writePosition, buckets.ExplicitBounds!);
+        if (buckets.ExplicitBounds is { } explicitBounds)
+        {
+            writePosition = WriteExplicitBounds(buffer, writePosition, explicitBounds);
+        }
 
         return writePosition;
 

src/OpenTelemetry/Metrics/View/ExplicitBucketHistogramConfiguration.cs

@@ -38,7 +38,7 @@ public double[]? Boundaries
                     throw new ArgumentException($"Histogram boundaries are invalid. Histogram boundaries must be in ascending order with distinct values.", nameof(value));
                 }
 
-                this.CopiedBoundaries = value.ToArray();
+                this.CopiedBoundaries = [.. value];
             }
             else
             {

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/Implementation/Serializer/ProtobufOtlpMetricSerializerTests.cs

@@ -13,12 +13,51 @@ namespace OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests.Implementation.Seri
 
 public static class ProtobufOtlpMetricSerializerTests
 {
+    private const string HistogramName = "histogram";
+
     [Fact]
     public static async Task WriteMetricsData_Serializes_Metrics_Correctly()
     {
         // Arrange
         var metrics = GenerateMetrics();
 
+        // Act and Assert
+        await WriteMetricsAndAssertSnapshot(metrics);
+    }
+
+    [Fact]
+    public static async Task WriteMetricsData_Serializes_Metrics_With_Explicit_Boundaries()
+    {
+        // Arrange
+        var metrics = GenerateMetrics((builder) =>
+        {
+            builder.AddView(
+                instrumentName: HistogramName,
+                new ExplicitBucketHistogramConfiguration { Boundaries = [1, 2, 4, 8, 16] });
+        });
+
+        // Act and Assert
+        await WriteMetricsAndAssertSnapshot(metrics);
+    }
+
+    [Fact]
+    public static async Task WriteMetricsData_Serializes_Metrics_With_No_Boundaries()
+    {
+        // Arrange
+        var metrics = GenerateMetrics((builder) =>
+        {
+            builder.AddView(
+                instrumentName: HistogramName,
+                new ExplicitBucketHistogramConfiguration { Boundaries = [] });
+        });
+
+        // Act and Assert
+        await WriteMetricsAndAssertSnapshot(metrics);
+    }
+
+    private static async Task WriteMetricsAndAssertSnapshot(Batch<Metric> metrics)
+    {
+        // Arrange
         var attributes = new Dictionary<string, object>
         {
             ["service.name"] = "OpenTelemetry-DotNet",
@@ -54,7 +93,7 @@ await Verify(stream, "bin")
             .UseDirectory("snapshots");
     }
 
-    private static Batch<Metric> GenerateMetrics()
+    private static Batch<Metric> GenerateMetrics(Action<MeterProviderBuilder>? configure = null)
     {
         // Arrange
         Batch<Metric> metrics = default;
@@ -94,6 +133,8 @@ private static Batch<Metric> GenerateMetrics()
                     experimentalOptions,
                     configureExporterInstance: (_) => exporter));
 
+            configure?.Invoke(builder);
+
             using var meterProvider = builder.Build();
             using var meter = new Meter(meterName);
 
@@ -103,7 +144,7 @@ private static Batch<Metric> GenerateMetrics()
             var gauge = meter.CreateGauge<int>("gauge");
             gauge.Record(42);
 
-            var histogram = meter.CreateHistogram<int>("histogram");
+            var histogram = meter.CreateHistogram<int>(HistogramName);
             histogram.Record(100);
 
             Assert.True(meterProvider.ForceFlush());

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/Implementation/Serializer/snapshots/ProtobufOtlpMetricSerializerTests.WriteMetricsData_Serializes_Metrics_With_Explicit_Boundaries.verified.bin

@@ -102,7 +102,7 @@ private static void VerifyEventMessage(MethodInfo eventMethod, EventWrittenEvent
         string expectedMessage = eventArguments.Length == 0
             ? GetEventAttribute(eventMethod).Message!
             : string.Format(CultureInfo.InvariantCulture, GetEventAttribute(eventMethod).Message!, eventArguments);
-        string actualMessage = string.Format(CultureInfo.InvariantCulture, actualEvent.Message!, actualEvent.Payload!.ToArray());
+        string actualMessage = string.Format(CultureInfo.InvariantCulture, actualEvent.Message!, [.. actualEvent.Payload!]);
         AssertEqual(nameof(VerifyEventMessage), expectedMessage, actualMessage);
     }