Implement the C# experiments API

This closely follows the Kotlin implementation.

glean-core/csharp/Glean/Glean.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using Mozilla.Glean.FFI;
@@ -324,6 +325,87 @@ namespace Mozilla.Glean
             }
         }
 
+        /// <summary>
+        /// Indicate that an experiment is running. Glean will then add an
+        /// experiment annotation to the environment which is sent with pings. This
+        /// information is not persisted between runs.
+        /// </summary>
+        /// <param name="experimentId">The id of the active experiment (maximum 100 bytes)</param>
+        /// <param name="branch">The experiment branch (maximum 100 bytes)</param>
+        /// <param name="extra">Optional metadata to output with the ping</param>
+        public void SetExperimentActive(string experimentId, string branch, Dictionary<string, string> extra = null)
+        {
+            // The Map is sent over FFI as a pair of arrays, one containing the
+            // keys, and the other containing the values.
+            string[] keys = null;
+            string[] values = null;
+
+            Int32 numKeys = 0;
+            if (extra != null)
+            {
+                // While the `ToArray` functions below could throw `ArgumentNullException`, this would
+                // only happen if `extra` (and `extra.Keys|Values`) is null. Which is not the case, since
+                // we're specifically checking this.
+                // Note that the order of `extra.Keys` and `extra.Values` is unspecified, but guaranteed
+                // to be the same. See
+                // https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.values?view=netstandard-2.0#remarks
+                keys = extra.Keys.ToArray();
+                values = extra.Values.ToArray();
+                numKeys = extra.Count();
+            }
+
+            // We dispatch this asynchronously so that, if called before the Glean SDK is
+            // initialized, it doesn't get ignored and will be replayed after init.
+            Dispatchers.LaunchAPI(() => {
+                LibGleanFFI.glean_set_experiment_active(
+                    experimentId,
+                    branch,
+                    keys,
+                    values,
+                    numKeys
+                );
+            });
+        }
+
+        /// <summary>
+        /// Indicate that an experiment is no longer running.
+        /// </summary>
+        /// <param name="experimentId">The id of the experiment to deactivate.</param>
+        public void SetExperimentInactive(string experimentId)
+        {
+            // We dispatch this asynchronously so that, if called before the Glean SDK is
+            // initialized, it doesn't get ignored and will be replayed after init.
+            Dispatchers.LaunchAPI(() => {
+                LibGleanFFI.glean_set_experiment_inactive(experimentId);
+            });
+        }
+
+        /// <summary>
+        /// Tests whether an experiment is active, for testing purposes only.
+        /// </summary>
+        /// <param name="experimentId">The id of the experiment to look for.</param>
+        /// <returns>true if the experiment is active and reported in pings, otherwise false</returns>
+        public bool TestIsExperimentActive(string experimentId)
+        {
+            Dispatchers.AssertInTestingMode();
+
+            return LibGleanFFI.glean_experiment_test_is_active(experimentId) != 0;
+        }
+
+        /// <summary>
+        /// Returns the stored data for the requested active experiment, for testing purposes only.
+        /// </summary>
+        /// <param name="experimentId">The id of the experiment to look for.</param>
+        /// <exception cref="System.NullReferenceException">Thrown when there is no data for the experiment.</exception>
+        /// <returns>The `RecordedExperimentData` for the experiment</returns>
+        public RecordedExperimentData TestGetExperimentData(string experimentId)
+        {
+            Dispatchers.AssertInTestingMode();
+
+            string rawData = LibGleanFFI.glean_experiment_test_get_data(experimentId).AsString();
+            return RecordedExperimentData.FromJsonString(rawData);
+        }
+
         /// <summary>
         /// TEST ONLY FUNCTION.
         /// Resets the Glean state and triggers init again.

glean-core/csharp/Glean/Metrics/RecordedExperimentData.cs

@@ -0,0 +1,57 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+using System;
+using System.Collections.Generic;
+using System.Text.Json;
+
+namespace Mozilla.Glean.Private
+{
+    /// <summary>
+    /// Deserialized experiment data.
+    /// </summary>
+    public sealed class RecordedExperimentData
+    {
+        /// <summary>
+        /// The experiment's branch as set through `SetExperimentActive`.
+        /// </summary>
+        public readonly string Branch;
+
+        /// <summary>
+        /// Any extra data associated with this experiment through `SetExperimentActive`.
+        /// </summary>
+        public readonly Dictionary<string, string> Extra;
+
+        // This constructor is only useful for tests.
+        internal RecordedExperimentData() { }
+
+        RecordedExperimentData(string branch, Dictionary<string, string> extra)
+        {
+            Branch = branch;
+            Extra = extra;
+        }
+
+        public static RecordedExperimentData FromJsonString(string json)
+        {
+            try
+            {
+                JsonDocument data = JsonDocument.Parse(json);
+                JsonElement root = data.RootElement;
+
+                string branch = root.GetProperty("branch").GetString();
+                Dictionary<string, string> processedExtra = new Dictionary<string, string>();
+                JsonElement rawExtraMap = root.GetProperty("extra");
+                foreach (var entry in rawExtraMap.EnumerateObject())
+                {
+                    processedExtra.Add(entry.Name, entry.Value.GetString());
+                }
+                return new RecordedExperimentData(branch, processedExtra);
+            }
+            catch (Exception)
+            {
+                return null;
+            }
+        }
+    }
+}

glean-core/csharp/GleanTests/GleanTests.cs

@@ -2,6 +2,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+using System.Collections.Generic;
 using System.IO;
 using Xunit;
 using static Mozilla.Glean.Glean;
@@ -34,6 +35,61 @@ namespace Mozilla.Glean.Tests
             GleanInstance.HandleBackgroundEvent();
         }
 
+        [Fact]
+        public void TestExperimentsRecording()
+        {
+            GleanInstance.SetExperimentActive(
+                "experiment_test", "branch_a"
+            );
+            GleanInstance.SetExperimentActive(
+            "experiment_api", "branch_b",
+            new Dictionary<string, string>() { { "test_key", "value" } }
+            );
+            Assert.True(GleanInstance.TestIsExperimentActive("experiment_api"));
+            Assert.True(GleanInstance.TestIsExperimentActive("experiment_test"));
+
+            GleanInstance.SetExperimentInactive("experiment_test");
+
+            Assert.True(GleanInstance.TestIsExperimentActive("experiment_api"));
+            Assert.False(GleanInstance.TestIsExperimentActive("experiment_test"));
+
+            var storedData = GleanInstance.TestGetExperimentData("experiment_api");
+            Assert.Equal("branch_b", storedData.Branch);
+            Assert.Single(storedData.Extra);
+            Assert.Equal("value", storedData.Extra["test_key"]);
+        }
+
+        [Fact]
+        public void TestExperimentsRecordingBeforeGleanInits()
+        {
+            // This test relies on Glean not being initialized and task queuing to be on.
+            GleanInstance.TestDestroyGleanHandle();
+            Dispatchers.QueueInitialTasks = true;
+
+            GleanInstance.SetExperimentActive(
+                "experiment_set_preinit", "branch_a"
+            );
+
+            GleanInstance.SetExperimentActive(
+                "experiment_preinit_disabled", "branch_a"
+            );
+
+            GleanInstance.SetExperimentInactive("experiment_preinit_disabled");
+
+            // This will init glean and flush the dispatcher's queue.
+            string tempDataDir = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName());
+            GleanInstance.Reset(
+                applicationId: "org.mozilla.csharp.tests",
+                applicationVersion: "1.0-test",
+                uploadEnabled: true,
+                configuration: new Configuration(),
+                dataDir: tempDataDir
+                );
+
+            Assert.True(GleanInstance.TestIsExperimentActive("experiment_set_preinit"));
+            Assert.False(GleanInstance.TestIsExperimentActive("experiment_preinit_disabled"));
+        }
+
         [Fact]
         public void SettingMaxEventsDoesNotCrash()
         {