Fix using right configuration provider order when getting configuration value (#117474)

Author: tarekgh

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs

@@ -3007,5 +3007,36 @@ public void BindArraysWithNullAndOtherValues()
             Assert.Equal([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], instance.ByteArray3);
 #endif
         }
+
+        [Fact]
+        public void TestProvidersOrder()
+        {
+            string jsonConfig1 = @"
+            {
+                ""SimplePoco"": {
+                    ""A"": ""Provider1A"",
+                    ""B"": ""Provider1B"",
+                },
+            }";
+
+            // Missing B in the second provider should not override the value from the first provider.
+            string jsonConfig2 = @"
+            {
+                ""SimplePoco"": {
+                    ""A"": ""Provider2A"",
+                },
+            }";
+
+            var configuration = new ConfigurationBuilder()
+                        .AddJsonStream(new MemoryStream(Encoding.UTF8.GetBytes(jsonConfig1)))
+                        .AddJsonStream(new MemoryStream(Encoding.UTF8.GetBytes(jsonConfig2)))
+                        .Build().GetSection("SimplePoco");
+
+            SimplePoco? result = configuration.Get<SimplePoco>();
+
+            Assert.NotNull(result);
+            Assert.Equal("Provider2A", result.A); // Value should come from the last provider
+            Assert.Equal("Provider1B", result.B); // B should not be overridden by the second provider
+        }
     }
 }

src/libraries/Microsoft.Extensions.Configuration/src/InternalConfigurationRootExtensions.cs

@@ -42,8 +42,15 @@ internal static IEnumerable<IConfigurationSection> GetChildrenImplementation(thi
 
         internal static bool TryGetConfiguration(this IConfigurationRoot root, string key, out string? value)
         {
-            foreach (IConfigurationProvider provider in root.Providers)
+            // common cases Providers is IList<IConfigurationProvider> in ConfigurationRoot
+            IList<IConfigurationProvider> providers = root.Providers is IList<IConfigurationProvider> list
+                ? list
+                : root.Providers.ToList();
+
+            // ensure looping in the reverse order
+            for (int i = providers.Count - 1; i >= 0; i--)
             {
+                IConfigurationProvider provider = providers[i];
                 if (provider.TryGet(key, out value))
                 {
                     return true;