Support nullability in SetMeasurementEventCallback (fixes #68025) (#68071)

Author: soroshsabz

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/MeterListener.cs

@@ -117,33 +117,37 @@ public void EnableMeasurementEvents(Instrument instrument, object? state = null)
         /// <param name="measurementCallback">The callback which can be used to get measurement recording of numeric type T.</param>
         public void SetMeasurementEventCallback<T>(MeasurementCallback<T>? measurementCallback) where T : struct
         {
-            if (measurementCallback is MeasurementCallback<byte> byteCallback)
+            if (measurementCallback is null)
             {
-                _byteMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : byteCallback;
+                measurementCallback = (instrument, measurement, tags, state) => { /* no-op */};
             }
-            else if (measurementCallback is MeasurementCallback<int> intCallback)
+            if (typeof(T) == typeof(byte))
+            {
+                _byteMeasurementCallback = (MeasurementCallback<byte>)(object)measurementCallback;
+            }
+            else if (typeof(T) == typeof(int))
             {
-                _intMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : intCallback;
+                _intMeasurementCallback = (MeasurementCallback<int>)(object)measurementCallback;
             }
-            else if (measurementCallback is MeasurementCallback<float> floatCallback)
+            else if (typeof(T) == typeof(float))
             {
-                _floatMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : floatCallback;
+                _floatMeasurementCallback = (MeasurementCallback<float>)(object)measurementCallback;
             }
-            else if (measurementCallback is MeasurementCallback<double> doubleCallback)
+            else if (typeof(T) == typeof(double))
             {
-                _doubleMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : doubleCallback;
+                _doubleMeasurementCallback = (MeasurementCallback<double>)(object)measurementCallback;
             }
-            else if (measurementCallback is MeasurementCallback<decimal> decimalCallback)
+            else if (typeof(T) == typeof(decimal))
             {
-                _decimalMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : decimalCallback;
+                _decimalMeasurementCallback = (MeasurementCallback<decimal>)(object)measurementCallback;
             }
-            else if (measurementCallback is MeasurementCallback<short> shortCallback)
+            else if (typeof(T) == typeof(short))
             {
-                _shortMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : shortCallback;
+                _shortMeasurementCallback = (MeasurementCallback<short>)(object)measurementCallback;
             }
-            else if (measurementCallback is MeasurementCallback<long> longCallback)
+            else if (typeof(T) == typeof(long))
             {
-                _longMeasurementCallback = (measurementCallback is null) ? ((instrument, measurement, tags, state) => { /* no-op */}) : longCallback;
+                _longMeasurementCallback = (MeasurementCallback<long>)(object)measurementCallback;
             }
             else
             {

src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs

@@ -894,6 +894,36 @@ public void MultipleListenersTest()
             }).Dispose();
         }
 
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void NullMeasurementEventCallbackTest()
+        {
+             RemoteExecutor.Invoke(() => {
+                Meter meter = new Meter("NullMeasurementEventCallbackTest");
+
+                Counter<int> counter = meter.CreateCounter<int>("Counter");
+
+                MeterListener listener = new MeterListener();
+
+                listener.InstrumentPublished = (theInstrument, theListener) => theListener.EnableMeasurementEvents(theInstrument, theInstrument);
+
+                int count = 0;
+                listener.SetMeasurementEventCallback<int>((inst, measurement, tags, state) => count++);
+
+                listener.Start();
+
+                Assert.Equal(0, count);
+
+                counter.Add(1);
+                Assert.Equal(1, count);
+
+                listener.SetMeasurementEventCallback<int>(null);
+                counter.Add(1);
+                Assert.Equal(1, count);
+
+                Assert.Throws<InvalidOperationException>(() => listener.SetMeasurementEventCallback<ulong>(null));
+            }).Dispose();
+        }
+
         [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
         public void EnableListeneingMultipleTimesWithDifferentState()
         {