Ensure Bind can handle null from GetSection

src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs

@@ -298,6 +298,11 @@ private static void BindInstance(
                 return;
             }
 
+            if (config is null)
+            {
+                return;
+            }
+
             var section = config as IConfigurationSection;
             string? configValue = section?.Value;
             if (configValue != null && TryConvertValue(type, configValue, section?.Path, out object? convertedValue, out Exception? error))

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

@@ -888,5 +888,11 @@ public int MyIntProperty
             }
         }
 
+        public class SimplePoco
+        {
+            public string A { get; set; }
+            public string B { get; set; }
+        }
+
     }
 }

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

@@ -12,6 +12,7 @@
 using Microsoft.Extensions.Configuration;
 #endif
 using Microsoft.Extensions.Configuration.Test;
+using Moq;
 using Xunit;
 
 namespace Microsoft.Extensions
@@ -1767,7 +1768,7 @@ public void EnsureCallingThePropertySetter()
             Assert.Equal(0, options.OtherCodeNullable);
             Assert.Equal("default", options.OtherCodeString);
             Assert.Null(options.OtherCodeNull);
-            Assert.Null(options.OtherCodeUri);            
+            Assert.Null(options.OtherCodeUri);
         }
 
         [Fact]
@@ -2238,7 +2239,7 @@ void TestUntypedOverloads(IConfiguration? configuration, string? key)
                 Assert.Throws<ArgumentNullException>(() => configuration.GetValue(typeof(GeolocationClass), key, new GeolocationClass()));
                 Assert.Throws<ArgumentNullException>(() => configuration.GetValue(typeof(Geolocation), key));
                 Assert.Throws<ArgumentNullException>(() => configuration.GetValue(typeof(Geolocation), key, defaultValue: null));
-                Assert.Throws<ArgumentNullException>(() => configuration.GetValue(typeof(Geolocation), key, default(Geolocation)));    
+                Assert.Throws<ArgumentNullException>(() => configuration.GetValue(typeof(Geolocation), key, default(Geolocation)));
             }
         }
 
@@ -2404,5 +2405,29 @@ public void SharedChildInstance()
             config.GetSection("A").Bind(instance);
             Assert.Equal("localhost", instance.ConnectionString);
         }
+
+        // Moq heavily utilizes RefEmit, which does not work on most aot workloads
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsReflectionEmitSupported))]
+        public void CanBindToMockConfiugrationSection()
+        {
+            const string expectedA = "hello";
+
+            var mockConfSection = new Mock<IConfigurationSection>();
+            var aSection = new Mock<IConfigurationSection>();
+            aSection.Setup(m => m.Value).Returns(expectedA);
+
+            // only mock one of the two properties, the other will return a null section.
+            // runtime binder uses GetSection
+            mockConfSection.Setup(config => config.GetSection(nameof(SimplePoco.A))).Returns(aSection.Object);
+            // source gen uses indexer
+            mockConfSection.Setup(config => config[nameof(SimplePoco.A)]).Returns(aSection.Object?.Value);
+            mockConfSection.Setup(config => config.GetChildren()).Returns(new[] { aSection.Object });
+
+            SimplePoco result = new();
+            mockConfSection.Object.Bind(result);
+
+            Assert.Equal(expectedA, result.A);
+            Assert.Equal(default(string), result.B);
+        }
     }
 }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Microsoft.Extensions.Configuration.Binder.SourceGeneration.Tests.csproj

@@ -43,6 +43,7 @@
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.Options.ConfigurationExtensions\src\Microsoft.Extensions.Options.ConfigurationExtensions.csproj" SkipUseReferenceAssembly="true" />
     <ProjectReference Include="..\..\src\Microsoft.Extensions.Configuration.Binder.csproj" SkipUseReferenceAssembly="true" />
     <ProjectReference Include="..\..\gen\Microsoft.Extensions.Configuration.Binder.SourceGeneration.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="true" />
+    <PackageReference Include="Moq" Version="$(MoqVersion)" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/UnitTests/Microsoft.Extensions.Configuration.Binder.Tests.csproj

@@ -25,6 +25,7 @@
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.Configuration.Json\src\Microsoft.Extensions.Configuration.Json.csproj" SkipUseReferenceAssembly="true" />
     <ProjectReference Include="$(LibrariesProjectRoot)Microsoft.Extensions.FileProviders.Abstractions\src\Microsoft.Extensions.FileProviders.Abstractions.csproj" SkipUseReferenceAssembly="true" />
     <ProjectReference Include="..\..\src\Microsoft.Extensions.Configuration.Binder.csproj" SkipUseReferenceAssembly="true" />
+    <PackageReference Include="Moq" Version="$(MoqVersion)" />
   </ItemGroup>
 
 </Project>