Disable packet multiplexing design by default

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

@@ -69,16 +69,19 @@ static LocalAppContextSwitches()
         /// <summary>
         /// In TdsParser the ProcessSni function changed significantly when the packet
         /// multiplexing code needed for high speed multi-packet column values was added.
-        /// In case of compatibility problems this switch will change TdsParser to use
-        /// the previous version of the function.
+        /// The switch is enabled by default and retains the old ProcessSni design.
+        /// Use this switch to enable experimental design that uses the new ProcessSni
+        /// behavior using the packet multiplexer.
         /// </summary>
         public static bool UseCompatibilityProcessSni
         {
             get
             {
                 if (s_useCompatibilityProcessSni == Tristate.NotInitialized)
                 {
-                    if (AppContext.TryGetSwitch(UseCompatibilityProcessSniString, out bool returnedValue) && returnedValue)
+                    // Check if the switch has been set by the AppContext switch directly
+                    // If it has not been set, we default to true.
+                    if (!AppContext.TryGetSwitch(UseCompatibilityProcessSniString, out bool returnedValue) || returnedValue)
                     {
                         s_useCompatibilityProcessSni = Tristate.True;
                     }
@@ -95,7 +98,7 @@ public static bool UseCompatibilityProcessSni
         /// In TdsParser the async multi-packet column value fetch behaviour is capable of
         /// using a continue snapshot state in addition to the original replay from start
         /// logic.
-        /// This switch disables use of the continue snapshot state. This switch will always
+        /// This switch disables use of the continue snapshot state and is enabled by default. This switch will always
         /// return true if <see cref="UseCompatibilityProcessSni"/> is enabled because the 
         /// continue state is not stable without the multiplexer.
         /// </summary>
@@ -114,7 +117,7 @@ public static bool UseCompatibilityAsyncBehaviour
 
                 if (s_useCompatibilityAsyncBehaviour == Tristate.NotInitialized)
                 {
-                    if (AppContext.TryGetSwitch(UseCompatibilityAsyncBehaviourString, out bool returnedValue) && returnedValue)
+                    if (!AppContext.TryGetSwitch(UseCompatibilityAsyncBehaviourString, out bool returnedValue) || returnedValue)
                     {
                         s_useCompatibilityAsyncBehaviour = Tristate.True;
                     }

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

@@ -25,7 +25,7 @@ public static bool IsUsingCompatibilityProcessSni
                 {
                     return foundValue;
                 }
-                return false;
+                return true; // Default to true if the switch is not set
             }
         }
 

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

@@ -23,8 +23,8 @@ public void TestDefaultAppContextSwitchValues()
         Assert.False(LocalAppContextSwitches.MakeReadAsyncBlocking);
         Assert.True(LocalAppContextSwitches.UseMinimumLoginTimeout);
         Assert.True(LocalAppContextSwitches.LegacyVarTimeZeroScaleBehaviour);
-        Assert.False(LocalAppContextSwitches.UseCompatibilityProcessSni);
-        Assert.False(LocalAppContextSwitches.UseCompatibilityAsyncBehaviour);
+        Assert.True(LocalAppContextSwitches.UseCompatibilityProcessSni);
+        Assert.True(LocalAppContextSwitches.UseCompatibilityAsyncBehaviour);
         Assert.False(LocalAppContextSwitches.UseConnectionPoolV2);
         Assert.False(LocalAppContextSwitches.TruncateScaledDecimal);
 #if NETFRAMEWORK