Fix SetProvider to return immediately if user-defined provider found

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlAuthenticationProviderManager.cs

@@ -186,7 +186,7 @@ public bool SetProvider(SqlAuthenticationMethod authenticationMethod, SqlAuthent
                     if (candidateMethod == authenticationMethod)
                     {
                         _sqlAuthLogger.LogError(nameof(SqlAuthenticationProviderManager), methodName, $"Failed to add provider {GetProviderType(provider)} because a user-defined provider with type {GetProviderType(_providers[authenticationMethod])} already existed for authentication {authenticationMethod}.");
-                        break;
+                        return false; // return here to avoid replacing user-defined provider
                     }
                 }
             }

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

@@ -5,7 +5,7 @@
 using System;
 using System.Security;
 using System.Threading.Tasks;
-using Microsoft.Identity.Client;
+using Microsoft.Data.SqlClient.FunctionalTests.DataCommon;
 using Xunit;
 
 namespace Microsoft.Data.SqlClient.Tests
@@ -50,6 +50,18 @@ private void InvalidCombinationCheck(SqlCredential credential)
             }
         }
 
+        [Fact]
+        public async Task IsDummySqlAuthenticationProviderSetByDefault()
+        {
+            var provider = SqlAuthenticationProvider.GetProvider(SqlAuthenticationMethod.ActiveDirectoryInteractive);
+
+            Assert.NotNull(provider);
+            Assert.Equal(typeof(DummySqlAuthenticationProvider), provider.GetType());
+
+            var token = await provider.AcquireTokenAsync(null);
+            Assert.Equal(token.AccessToken, DummySqlAuthenticationProvider.DUMMY_TOKEN_STR);
+        }
+
         [Fact]
         public void CustomActiveDirectoryProviderTest()
         {

src/Microsoft.Data.SqlClient/tests/FunctionalTests/App.config

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<configuration>
+  <configSections>
+    <section name="SqlClientAuthenticationProviders" type="Microsoft.Data.SqlClient.SqlClientAuthenticationProviderConfigurationSection,Microsoft.Data.SqlClient" />
+  </configSections>
+  <SqlClientAuthenticationProviders>
+    <providers>
+      <add name="active directory interactive" type="Microsoft.Data.SqlClient.FunctionalTests.DataCommon.DummySqlAuthenticationProvider, FunctionalTests" />
+    </providers>
+  </SqlClientAuthenticationProviders>
+</configuration>

src/Microsoft.Data.SqlClient/tests/FunctionalTests/DataCommon/DummySqlAuthenticationProvider.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading.Tasks;
+
+namespace Microsoft.Data.SqlClient.FunctionalTests.DataCommon
+{
+    /// <summary>
+    /// Dummy class to override default Sql Authentication provider in functional tests.
+    /// This type returns a dummy access token and is only used for registration test from app.config file.
+    /// Since no actual connections are intended to be made in Functional tests, 
+    /// this type is added by default to validate config file registration scenario.
+    /// </summary>
+    public class DummySqlAuthenticationProvider : SqlAuthenticationProvider
+    {
+        public static string DUMMY_TOKEN_STR = "dummy_access_token";
+
+        public override Task<SqlAuthenticationToken> AcquireTokenAsync(SqlAuthenticationParameters parameters)
+        => Task.FromResult(new SqlAuthenticationToken(DUMMY_TOKEN_STR, new DateTimeOffset(DateTime.Now.AddHours(2))));
+
+        // Supported authentication modes don't matter for dummy test, but added to demonstrate config file usage.
+        public override bool IsSupported(SqlAuthenticationMethod authenticationMethod)
+        => authenticationMethod == SqlAuthenticationMethod.ActiveDirectoryInteractive;
+    }
+}

src/Microsoft.Data.SqlClient/tests/FunctionalTests/Microsoft.Data.SqlClient.FunctionalTests.csproj

@@ -24,6 +24,7 @@
     <Compile Include="AlwaysEncryptedTests\Utility.cs" />
     <Compile Include="AssertExtensions.cs" />
     <Compile Include="DataCommon\AssemblyResourceManager.cs" />
+    <Compile Include="DataCommon\DummySqlAuthenticationProvider.cs" />
     <Compile Include="DataCommon\SystemDataResourceManager.cs" />
     <Compile Include="DataCommon\TestUtility.cs" />
     <Compile Include="LocalizationTest.cs" />