[6.0] Port #3841 Introduce app context switch for setting MSF=true by default

src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/DbConnectionStringCommon.cs

@@ -973,7 +973,7 @@ internal static class DbConnectionStringDefaults
         internal const bool IntegratedSecurity = false;
         internal const int LoadBalanceTimeout = 0; // default of 0 means don't use
         internal const bool MultipleActiveResultSets = false;
-        internal const bool MultiSubnetFailover = false;
+        internal static readonly bool MultiSubnetFailover = LocalAppContextSwitches.EnableMultiSubnetFailoverByDefault;
         internal const int MaxPoolSize = 100;
         internal const int MinPoolSize = 0;
         internal const int PacketSize = 8000;

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

@@ -21,6 +21,7 @@ private enum Tristate : byte
         internal const string UseMinimumLoginTimeoutString = @"Switch.Microsoft.Data.SqlClient.UseOneSecFloorInTimeoutCalculationDuringLogin";
         internal const string LegacyVarTimeZeroScaleBehaviourString = @"Switch.Microsoft.Data.SqlClient.LegacyVarTimeZeroScaleBehaviour";
         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;
@@ -30,6 +31,7 @@ private enum Tristate : byte
         // this field is accessed through reflection in Microsoft.Data.SqlClient.Tests.SqlParameterTests and should not be renamed or have the type changed without refactoring related tests
         private static Tristate s_legacyVarTimeZeroScaleBehaviour;
         private static Tristate s_ignoreServerProvidedFailoverPartner;
+        private static Tristate s_multiSubnetFailoverByDefault;
 
 #if NET
         static LocalAppContextSwitches()
@@ -236,5 +238,30 @@ 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

@@ -43,7 +43,7 @@ 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 readonly 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/FunctionalTests/LocalAppContextSwitchesTests.cs

@@ -14,6 +14,7 @@ public class LocalAppContextSwitchesTests
         [InlineData("LegacyRowVersionNullBehavior", false)]
         [InlineData("MakeReadAsyncBlocking", false)]
         [InlineData("UseMinimumLoginTimeout", true)]
+        [InlineData("EnableMultiSubnetFailoverByDefault", false)]
         public void DefaultSwitchValue(string property, bool expectedDefaultValue)
         {
             var switchesType = typeof(SqlCommand).Assembly.GetType("Microsoft.Data.SqlClient.LocalAppContextSwitches");