fix(csharp): disable feature flag cache by default (#358)

## 🥞 Stacked PR
Use this
[link](https://github.com/adbc-drivers/databricks/pull/358/files) to
review incremental changes.
-
[**stack/disable-config-cache**](#358)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/358/files)]

---------
## What's Changed



Please fill in a description of the changes here.

**This contains breaking changes.** <!-- Remove this line if there are
no breaking changes. -->

Closes #NNN.

---------

Co-authored-by: Jade Wang <jade.wang+data@databricks.com>

Author: jadewang-db

csharp/src/DatabricksParameters.cs

@@ -360,7 +360,7 @@ public class DatabricksParameters : SparkParameters
         /// <summary>
         /// Whether to enable the feature flag cache for fetching remote configuration from the server.
         /// When enabled, the driver fetches feature flags from the Databricks server and merges them with local properties.
-        /// Default value is true if not specified.
+        /// Default value is false if not specified.
         /// </summary>
         public const string FeatureFlagCacheEnabled = "adbc.databricks.feature_flag_cache_enabled";
 

csharp/src/FeatureFlagCache.cs

@@ -307,9 +307,9 @@ public async Task<IReadOnlyDictionary<string, string>> MergePropertiesWithFeatur
 
             try
             {
-                // Check if feature flag cache is enabled (default: true)
-                if (localProperties.TryGetValue(DatabricksParameters.FeatureFlagCacheEnabled, out string? enabledStr) &&
-                    bool.TryParse(enabledStr, out bool enabled) &&
+                // Check if feature flag cache is enabled (default: false)
+                if (!localProperties.TryGetValue(DatabricksParameters.FeatureFlagCacheEnabled, out string? enabledStr) ||
+                    !bool.TryParse(enabledStr, out bool enabled) ||
                     !enabled)
                 {
                     activity?.AddEvent(new ActivityEvent("feature_flags.skipped",

csharp/src/Telemetry/TelemetryConfiguration.cs

@@ -81,9 +81,9 @@ public sealed class TelemetryConfiguration
 
         /// <summary>
         /// Gets or sets whether telemetry is enabled.
-        /// Default is true.
+        /// Default is false.
         /// </summary>
-        public bool Enabled { get; set; } = true;
+        public bool Enabled { get; set; } = false;
 
         /// <summary>
         /// Gets or sets the batch size for telemetry metrics.

csharp/test/E2E/FeatureFlagCacheE2ETest.cs

@@ -17,6 +17,7 @@
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using Apache.Arrow.Adbc;
 using Apache.Arrow.Adbc.Tests;
 using Xunit;
 using Xunit.Abstractions;
@@ -49,7 +50,7 @@ public async Task TestFeatureFlagCacheInitialization()
             Skip.If(string.IsNullOrEmpty(hostName), "Cannot determine host name from test configuration");
 
             // Act - Create a connection which initializes the feature flag cache
-            using var connection = NewConnection(TestConfiguration);
+            using var connection = NewConnectionWithFeatureFlagCache();
 
             // Assert - The connection should be created successfully
             Assert.NotNull(connection);
@@ -97,8 +98,8 @@ public async Task TestFeatureFlagCacheSharedAcrossConnections()
             var cache = FeatureFlagCache.GetInstance();
 
             // Act - Create two connections to the same host
-            using var connection1 = NewConnection(TestConfiguration);
-            using var connection2 = NewConnection(TestConfiguration);
+            using var connection1 = NewConnectionWithFeatureFlagCache();
+            using var connection2 = NewConnectionWithFeatureFlagCache();
 
             // Assert - Both connections should work properly
             Assert.NotNull(connection1);
@@ -134,7 +135,7 @@ public async Task TestFeatureFlagCachePersistsAfterConnectionClose()
             OutputHelper?.WriteLine($"[FeatureFlagCacheE2ETest] Initial cache count: {cache.CachedHostCount}");
 
             // Act - Create and close a connection
-            using (var connection = NewConnection(TestConfiguration))
+            using (var connection = NewConnectionWithFeatureFlagCache())
             {
                 // Connection is active, cache should have a context for this host
                 Assert.NotNull(connection);
@@ -176,7 +177,7 @@ public async Task TestFeatureFlagCachePersistsAfterConnectionClose()
         public async Task TestConnectionWithFeatureFlagsExecutesQueries()
         {
             // Arrange
-            using var connection = NewConnection(TestConfiguration);
+            using var connection = NewConnectionWithFeatureFlagCache();
 
             // Act - Execute multiple queries to ensure feature flags don't interfere
             var queries = new[]
@@ -202,6 +203,18 @@ public async Task TestConnectionWithFeatureFlagsExecutesQueries()
             }
         }
 
+        /// <summary>
+        /// Creates a connection with feature flag cache explicitly enabled.
+        /// </summary>
+        private AdbcConnection NewConnectionWithFeatureFlagCache()
+        {
+            var parameters = GetDriverParameters(TestConfiguration);
+            parameters[DatabricksParameters.FeatureFlagCacheEnabled] = "true";
+            var driver = new DatabricksDriver();
+            var database = driver.Open(parameters);
+            return database.Connect(new Dictionary<string, string>());
+        }
+
         /// <summary>
         /// Gets the normalized host name from test configuration.
         /// Strips protocol prefix if present (e.g., "https://host" -> "host").

csharp/test/Unit/FeatureFlagCacheTests.cs

@@ -567,6 +567,64 @@ public async Task FeatureFlagContext_BackgroundRefreshError_ContinuesRefreshLoop
 
         #endregion
 
+        #region MergePropertiesWithFeatureFlagsAsync Default Behavior Tests
+
+        [Fact]
+        public async Task MergePropertiesWithFeatureFlagsAsync_PropertyNotSet_ReturnsLocalProperties()
+        {
+            // Arrange - No FeatureFlagCacheEnabled property set (default: false)
+            var localProperties = new Dictionary<string, string>
+            {
+                ["host"] = TestHost,
+                ["some_property"] = "some_value"
+            };
+            var cache = FeatureFlagCache.GetInstance();
+
+            // Act
+            var result = await cache.MergePropertiesWithFeatureFlagsAsync(localProperties, DriverVersion);
+
+            // Assert - Should return local properties unchanged (feature flags skipped)
+            Assert.Same(localProperties, result);
+        }
+
+        [Fact]
+        public async Task MergePropertiesWithFeatureFlagsAsync_PropertySetToFalse_ReturnsLocalProperties()
+        {
+            // Arrange - FeatureFlagCacheEnabled explicitly set to false
+            var localProperties = new Dictionary<string, string>
+            {
+                ["host"] = TestHost,
+                [DatabricksParameters.FeatureFlagCacheEnabled] = "false"
+            };
+            var cache = FeatureFlagCache.GetInstance();
+
+            // Act
+            var result = await cache.MergePropertiesWithFeatureFlagsAsync(localProperties, DriverVersion);
+
+            // Assert - Should return local properties unchanged (feature flags skipped)
+            Assert.Same(localProperties, result);
+        }
+
+        [Fact]
+        public async Task MergePropertiesWithFeatureFlagsAsync_PropertySetToInvalidValue_ReturnsLocalProperties()
+        {
+            // Arrange - FeatureFlagCacheEnabled set to a non-boolean value
+            var localProperties = new Dictionary<string, string>
+            {
+                ["host"] = TestHost,
+                [DatabricksParameters.FeatureFlagCacheEnabled] = "notabool"
+            };
+            var cache = FeatureFlagCache.GetInstance();
+
+            // Act
+            var result = await cache.MergePropertiesWithFeatureFlagsAsync(localProperties, DriverVersion);
+
+            // Assert - Should return local properties unchanged (can't parse as bool)
+            Assert.Same(localProperties, result);
+        }
+
+        #endregion
+
         #region Helper Methods
 
         /// <summary>

csharp/test/Unit/Telemetry/TelemetryConfigurationTests.cs

@@ -50,7 +50,7 @@ public void TelemetryConfiguration_DefaultValues_AreCorrect()
             var config = new TelemetryConfiguration();
 
             // Assert
-            Assert.True(config.Enabled);
+            Assert.False(config.Enabled);
             Assert.Equal(100, config.BatchSize);
             Assert.Equal(5000, config.FlushIntervalMs);
             Assert.Equal(3, config.MaxRetries);
@@ -106,7 +106,7 @@ public void TelemetryConfiguration_InvalidProperty_UsesDefault()
 
             // Assert - should use defaults for invalid values
             Assert.Equal(100, config.BatchSize); // default
-            Assert.True(config.Enabled); // default
+            Assert.False(config.Enabled); // default
             Assert.Equal(5000, config.FlushIntervalMs); // default
         }
 
@@ -117,7 +117,7 @@ public void TelemetryConfiguration_NullProperties_UsesDefaults()
             var config = TelemetryConfiguration.FromProperties(null);
 
             // Assert
-            Assert.True(config.Enabled);
+            Assert.False(config.Enabled);
             Assert.Equal(100, config.BatchSize);
             Assert.Equal(5000, config.FlushIntervalMs);
         }
@@ -132,7 +132,7 @@ public void TelemetryConfiguration_EmptyProperties_UsesDefaults()
             var config = TelemetryConfiguration.FromProperties(properties);
 
             // Assert
-            Assert.True(config.Enabled);
+            Assert.False(config.Enabled);
             Assert.Equal(100, config.BatchSize);
             Assert.Equal(5000, config.FlushIntervalMs);
         }
@@ -187,7 +187,7 @@ public void TelemetryConfiguration_InvalidEnvironmentVariable_UsesDefault()
 
             // Assert - should use defaults for invalid environment variables
             Assert.Equal(100, config.BatchSize);
-            Assert.True(config.Enabled);
+            Assert.False(config.Enabled);
         }
 
         [Fact]