Introduce app context switch for setting MSF=true by default

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

@@ -27,6 +27,7 @@ private enum Tristate : byte
         private const string TruncateScaledDecimalString = @"Switch.Microsoft.Data.SqlClient.TruncateScaledDecimal";
         private const string IgnoreServerProvidedFailoverPartnerString = @"Switch.Microsoft.Data.SqlClient.IgnoreServerProvidedFailoverPartner";
         private const string EnableUserAgentString = @"Switch.Microsoft.Data.SqlClient.EnableUserAgent";
+        private const string EnableMultiSubnetFailoverByDefaultString = @"Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault";
 
         #if NET
         private const string GlobalizationInvariantModeString = @"System.Globalization.Invariant";
@@ -52,6 +53,7 @@ private enum Tristate : byte
         private static Tristate s_truncateScaledDecimal;
         private static Tristate s_ignoreServerProvidedFailoverPartner;
         private static Tristate s_enableUserAgent;
+        private static Tristate s_multiSubnetFailoverByDefault;
 
         #if NET
         private static Tristate s_globalizationInvariantMode;
@@ -511,6 +513,31 @@ public static bool DisableTnirByDefault
                 return s_disableTnirByDefault == Tristate.True;
             }
         }
-        #endif
+#endif
+
+        /// <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/tests/Common/LocalAppContextSwitchesHelper.cs

@@ -33,8 +33,8 @@ public sealed class LocalAppContextSwitchesHelper : IDisposable
     private readonly PropertyInfo _truncateScaledDecimalProperty;
     private readonly PropertyInfo _ignoreServerProvidedFailoverPartner;
     private readonly PropertyInfo _enableUserAgent;
-
-    #if NET
+    private readonly PropertyInfo _enableMultiSubnetFailoverByDefaultProperty;
+#if NET
     private readonly PropertyInfo _globalizationInvariantModeProperty;
     #endif
 
@@ -69,8 +69,9 @@ public sealed class LocalAppContextSwitchesHelper : IDisposable
     private readonly Tristate _ignoreServerProvidedFailoverPartnerOriginal;
     private readonly FieldInfo _enableUserAgentField;
     private readonly Tristate _enableUserAgentOriginal;
-
-    #if NET
+    private readonly FieldInfo _multiSubnetFailoverByDefaultField;
+    private readonly Tristate _multiSubnetFailoverByDefaultOriginal;
+#if NET
     private readonly FieldInfo _globalizationInvariantModeField;
     private readonly Tristate _globalizationInvariantModeOriginal;
     #endif
@@ -181,7 +182,11 @@ void InitProperty(string name, out PropertyInfo property)
             "EnableUserAgent",
             out _enableUserAgent);
 
-        #if NET
+        InitProperty(
+            "EnableMultiSubnetFailoverByDefault",
+            out _enableMultiSubnetFailoverByDefaultProperty);
+
+#if NET
         InitProperty(
             "GlobalizationInvariantMode",
             out _globalizationInvariantModeProperty);
@@ -269,7 +274,12 @@ void InitField(string name, out FieldInfo field, out Tristate value)
             out _enableUserAgentField,
             out _enableUserAgentOriginal);
 
-        #if NET
+        InitField(
+            "s_multiSubnetFailoverByDefault",
+            out _multiSubnetFailoverByDefaultField,
+            out _multiSubnetFailoverByDefaultOriginal);
+
+#if NET
         InitField(
             "s_globalizationInvariantMode",
             out _globalizationInvariantModeField,
@@ -281,8 +291,8 @@ void InitField(string name, out FieldInfo field, out Tristate value)
             "s_useManagedNetworking",
             out _useManagedNetworkingField,
             out _useManagedNetworkingOriginal);
-        #endif
-        
+#endif
+
         #if NETFRAMEWORK
         InitField(
             "s_disableTnirByDefault",
@@ -359,6 +369,10 @@ void RestoreField(FieldInfo field, Tristate value)
             _enableUserAgentField,
             _enableUserAgentOriginal);
 
+        RestoreField(
+            _multiSubnetFailoverByDefaultField,
+            _multiSubnetFailoverByDefaultOriginal);
+
         #if NET
         RestoreField(
             _globalizationInvariantModeField,
@@ -474,6 +488,11 @@ public bool EnableUserAgent
         get => (bool)_enableUserAgent.GetValue(null);
     }
 
+    public bool EnableMultiSubnetFailoverByDefault
+    {
+        get => (bool)_enableMultiSubnetFailoverByDefaultProperty.GetValue(null);
+    }
+
     #if NET
     /// <summary>
     /// Access the LocalAppContextSwitches.GlobalizationInvariantMode property.
@@ -608,7 +627,13 @@ public Tristate EnableUserAgentField
         set => SetValue(_enableUserAgentField, value);
     }
 
-    #if NET
+    public Tristate EnableMultiSubnetFailoverByDefaultField
+    {
+        get => GetValue(_multiSubnetFailoverByDefaultField);
+        set => SetValue(_multiSubnetFailoverByDefaultField, value);
+    }
+
+#if NET
     /// <summary>
     /// Get or set the LocalAppContextSwitches.GlobalizationInvariantMode switch value.
     /// </summary>

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

@@ -27,6 +27,7 @@ public void TestDefaultAppContextSwitchValues()
         Assert.True(LocalAppContextSwitches.UseCompatibilityAsyncBehaviour);
         Assert.False(LocalAppContextSwitches.UseConnectionPoolV2);
         Assert.False(LocalAppContextSwitches.TruncateScaledDecimal);
+        Assert.False(LocalAppContextSwitches.EnableMultiSubnetFailoverByDefault);
 #if NETFRAMEWORK
         Assert.False(LocalAppContextSwitches.DisableTnirByDefault);
         Assert.False(LocalAppContextSwitches.UseManagedNetworking);

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

@@ -58,6 +58,48 @@ public void TestDefaultTnir(string dataSource, bool? tnirEnabledInConnString, Tr
             Assert.Equal(expectedValue, connectionString.TransparentNetworkIPResolution);
         }
 #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()
         {