Port #3841 to 6.1: Introduce app context switch for setting MSF=true

src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/ConnectionString/DbConnectionStringDefaults.cs

@@ -39,7 +39,7 @@ internal static class DbConnectionStringDefaults
         internal const int MaxPoolSize = 100;
         internal const int MinPoolSize = 0;
         internal const bool MultipleActiveResultSets = false;
-        internal const bool MultiSubnetFailover = false;
+        internal static bool MultiSubnetFailover => LocalAppContextSwitches.EnableMultiSubnetFailoverByDefault;
         internal const int PacketSize = 8000;
         internal const string Password = "";
         internal const bool PersistSecurityInfo = false;

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs

@@ -22,6 +22,7 @@ private enum Tristate : byte
         internal const string LegacyVarTimeZeroScaleBehaviourString = @"Switch.Microsoft.Data.SqlClient.LegacyVarTimeZeroScaleBehaviour";
         internal const string UseConnectionPoolV2String = @"Switch.Microsoft.Data.SqlClient.UseConnectionPoolV2";
         private const string IgnoreServerProvidedFailoverPartnerString = @"Switch.Microsoft.Data.SqlClient.IgnoreServerProvidedFailoverPartner";
+        private const string EnableMultiSubnetFailoverByDefaultString = @"Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault";
 
         // this field is accessed through reflection in tests and should not be renamed or have the type changed without refactoring NullRow related tests
         private static Tristate s_legacyRowVersionNullBehavior;
@@ -32,6 +33,7 @@ private enum Tristate : byte
         private static Tristate s_legacyVarTimeZeroScaleBehaviour;
         private static Tristate s_useConnectionPoolV2;
         private static Tristate s_ignoreServerProvidedFailoverPartner;
+        private static Tristate s_multiSubnetFailoverByDefault;
 
 
 #if NET
@@ -263,5 +265,30 @@ public static bool IgnoreServerProvidedFailoverPartner
                 return s_ignoreServerProvidedFailoverPartner == Tristate.True;
             }
         }
+
+        /// <summary>
+        /// When set to true, the default value for MultiSubnetFailover connection string property
+        /// will be true instead of false. This enables parallel IP connection attempts for 
+        /// improved connection times in multi-subnet environments.
+        /// This app context switch defaults to 'false'.
+        /// </summary>
+        public static bool EnableMultiSubnetFailoverByDefault
+        {
+            get
+            {
+                if (s_multiSubnetFailoverByDefault == Tristate.NotInitialized)
+                {
+                    if (AppContext.TryGetSwitch(EnableMultiSubnetFailoverByDefaultString, out bool returnedValue) && returnedValue)
+                    {
+                        s_multiSubnetFailoverByDefault = Tristate.True;
+                    }
+                    else
+                    {
+                        s_multiSubnetFailoverByDefault = Tristate.False;
+                    }
+                }
+                return s_multiSubnetFailoverByDefault == Tristate.True;
+            }
+        }
     }
 }

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlConnectionString.cs

@@ -45,7 +45,7 @@ internal static class DEFAULT
             internal const bool MARS = DbConnectionStringDefaults.MultipleActiveResultSets;
             internal const int Max_Pool_Size = DbConnectionStringDefaults.MaxPoolSize;
             internal const int Min_Pool_Size = DbConnectionStringDefaults.MinPoolSize;
-            internal const bool MultiSubnetFailover = DbConnectionStringDefaults.MultiSubnetFailover;
+            internal static bool MultiSubnetFailover => DbConnectionStringDefaults.MultiSubnetFailover;
             internal const int Packet_Size = DbConnectionStringDefaults.PacketSize;
             internal const string Password = DbConnectionStringDefaults.Password;
             internal const bool Persist_Security_Info = DbConnectionStringDefaults.PersistSecurityInfo;

src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs

@@ -32,6 +32,7 @@ public sealed class LocalAppContextSwitchesHelper : IDisposable
     #if NETFRAMEWORK
     private readonly PropertyInfo _disableTnirByDefaultProperty;
     #endif
+    private readonly PropertyInfo _enableMultiSubnetFailoverByDefaultProperty;
 
     // These fields are used to capture the original switch values.
     private readonly FieldInfo _legacyRowVersionNullBehaviorField;
@@ -48,10 +49,13 @@ public sealed class LocalAppContextSwitchesHelper : IDisposable
     private readonly Tristate _useConnectionPoolV2Original;
     private readonly FieldInfo _ignoreServerProvidedFailoverPartnerField;
     private readonly Tristate _ignoreServerProvidedFailoverPartnerOriginal;
-#if NETFRAMEWORK
+    #if NETFRAMEWORK
     private readonly FieldInfo _disableTnirByDefaultField;
     private readonly Tristate _disableTnirByDefaultOriginal;
-#endif
+    #endif
+    private readonly FieldInfo _multiSubnetFailoverByDefaultField;
+    private readonly Tristate _multiSubnetFailoverByDefaultOriginal;
+
 
     #endregion
 
@@ -139,6 +143,10 @@ void InitProperty(string name, out PropertyInfo property)
             out _disableTnirByDefaultProperty);
         #endif
 
+        InitProperty(
+            "EnableMultiSubnetFailoverByDefault",
+            out _enableMultiSubnetFailoverByDefaultProperty);
+
         // A local helper to capture the original value of a switch.
         void InitField(string name, out FieldInfo field, out Tristate value)
         {
@@ -195,6 +203,11 @@ void InitField(string name, out FieldInfo field, out Tristate value)
             out _disableTnirByDefaultField,
             out _disableTnirByDefaultOriginal);
         #endif
+
+        InitField(
+            "s_multiSubnetFailoverByDefault",
+            out _multiSubnetFailoverByDefaultField,
+            out _multiSubnetFailoverByDefaultOriginal);
     }
 
     /// <summary>
@@ -255,6 +268,10 @@ void RestoreField(FieldInfo field, Tristate value)
             _disableTnirByDefaultOriginal);
         #endif
 
+        RestoreField(
+            _multiSubnetFailoverByDefaultField,
+            _multiSubnetFailoverByDefaultOriginal);
+
         if (failedFields.Count > 0)
         {
             throw new Exception(
@@ -327,6 +344,11 @@ public bool DisableTnirByDefault
     }
     #endif
 
+    public bool EnableMultiSubnetFailoverByDefault
+    {
+        get => (bool)_enableMultiSubnetFailoverByDefaultProperty.GetValue(null);
+    }
+
     // These properties get or set the like-named underlying switch field value.
     //
     // They all fail the test if the value cannot be retrieved or set.
@@ -408,6 +430,12 @@ public Tristate DisableTnirByDefaultField
     }
     #endif
 
+    public Tristate EnableMultiSubnetFailoverByDefaultField
+    {
+        get => GetValue(_multiSubnetFailoverByDefaultField);
+        set => SetValue(_multiSubnetFailoverByDefaultField, value);
+    }
+
 #endregion
 
     #region Private Helpers

src/Microsoft.Data.SqlClient/tests/FunctionalTests/LocalAppContextSwitchesTests.cs

@@ -20,6 +20,7 @@ public class LocalAppContextSwitchesTests
         #if NETFRAMEWORK
         [InlineData("DisableTnirByDefault", false)]
         #endif
+        [InlineData("EnableMultiSubnetFailoverByDefault", false)]
         public void DefaultSwitchValue(string property, bool expectedDefaultValue)
         {
             var switchesType = typeof(SqlCommand).Assembly.GetType("Microsoft.Data.SqlClient.LocalAppContextSwitches");

src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionStringTest.cs

@@ -17,7 +17,7 @@ public SqlConnectionStringTest()
             _appContextSwitchHelper = new LocalAppContextSwitchesHelper();
         }
 
-#if NETFRAMEWORK
+        #if NETFRAMEWORK
         [Theory]
         [InlineData("test.database.windows.net", true, Tristate.True, true)]
         [InlineData("test.database.windows.net", false, Tristate.True, false)]
@@ -60,7 +60,50 @@ public void TestDefaultTnir(string dataSource, bool? tnirEnabledInConnString, Tr
             // Assert
             Assert.Equal(expectedValue, connectionString.TransparentNetworkIPResolution);
         }
-#endif
+        #endif
+
+        /// <summary>
+        /// Test MSF values when set through connection string and through app context switch.
+        /// </summary>
+        [Theory]
+        [InlineData(true, Tristate.True, true)]
+        [InlineData(false, Tristate.True, false)]
+        [InlineData(null, Tristate.True, true)]
+        [InlineData(true, Tristate.False, true)]
+        [InlineData(false, Tristate.False, false)]
+        [InlineData(null, Tristate.False, false)]
+        [InlineData(null, Tristate.NotInitialized, false)]
+        public void TestDefaultMultiSubnetFailover(bool? msfInConnString, Tristate msfEnabledAppContext, bool expectedValue)
+        {
+            _appContextSwitchHelper.EnableMultiSubnetFailoverByDefaultField = msfEnabledAppContext;
+
+            SqlConnectionStringBuilder builder = new();
+            if (msfInConnString.HasValue)
+            {
+                builder.MultiSubnetFailover = msfInConnString.Value;
+            }
+            SqlConnectionString connectionString = new(builder.ConnectionString);
+
+            Assert.Equal(expectedValue, connectionString.MultiSubnetFailover);
+        }
+
+        /// <summary>
+        /// Tests that MultiSubnetFailover=true cannot be used with FailoverPartner.
+        /// </summary>
+        [Fact]
+        public void TestMultiSubnetFailoverWithFailoverPartnerThrows()
+        {
+            _appContextSwitchHelper.EnableMultiSubnetFailoverByDefaultField = Tristate.True;
+
+            SqlConnectionStringBuilder builder = new()
+            {
+                DataSource = "server",
+                FailoverPartner = "partner",
+                InitialCatalog = "database"
+            };
+
+            Assert.Throws<ArgumentException>(() => new SqlConnectionString(builder.ConnectionString));
+        }
 
         public void Dispose()
         {