Annotate config.GetValue() with [NotNullIfNotNull]

src/libraries/Directory.Build.targets

@@ -170,6 +170,7 @@
   <!-- Adds Nullable annotation attributes to C# non .NETCoreApp builds. -->
   <ItemGroup Condition="'$(Nullable)' != '' and
                         '$(Nullable)' != 'disable' and
+                        '$(SkipIncludeNullableAttributes)' != 'true' and
                         '$(MSBuildProjectExtension)' == '.csproj' and
                         '$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
     <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\NullableAttributes.cs" Link="System\Diagnostics\CodeAnalysis\NullableAttributes.cs" />

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/ConfigurationBindingGenerator.Emitter.cs

@@ -15,6 +15,7 @@ private sealed partial class Emitter
             private readonly TypeIndex _typeIndex;
             private readonly bool _emitEnumParseMethod;
             private readonly bool _emitGenericParseEnum;
+            private readonly bool _emitNotNullIfNotNull;
             private readonly bool _emitThrowIfNullMethod;
 
             private readonly SourceWriter _writer = new();
@@ -26,6 +27,7 @@ public Emitter(SourceGenerationSpec sourceGenSpec)
                 _typeIndex = new TypeIndex(sourceGenSpec.ConfigTypes);
                 _emitEnumParseMethod = sourceGenSpec.EmitEnumParseMethod;
                 _emitGenericParseEnum = sourceGenSpec.EmitGenericParseEnum;
+                _emitNotNullIfNotNull = sourceGenSpec.EmitNotNullIfNotNull;
                 _emitThrowIfNullMethod = sourceGenSpec.EmitThrowIfNullMethod;
             }
 

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/ConfigurationBindingGenerator.Parser.cs

@@ -57,6 +57,7 @@ internal sealed partial class Parser(CompilationData compilationData)
                     ConfigTypes = _createdTypeSpecs.Values.OrderBy(s => s.TypeRef.FullyQualifiedName).ToImmutableEquatableArray(),
                     EmitEnumParseMethod = _emitEnumParseMethod,
                     EmitGenericParseEnum = _emitGenericParseEnum,
+                    EmitNotNullIfNotNull = _typeSymbols.NotNullIfNotNullAttribute is not null,
                     EmitThrowIfNullMethod = IsThrowIfNullMethodToBeEmitted()
                 };
             }

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

@@ -73,6 +73,7 @@ private void EmitGetValueMethods()
                 if (ShouldEmitMethods(MethodsToGen.ConfigBinder_GetValue_T_key_defaultValue))
                 {
                     EmitStartDefinition_Get_Or_GetValue_Overload(MethodsToGen.ConfigBinder_GetValue_T_key_defaultValue, documentation);
+                    EmitNotNullIfNotNull(Identifier.defaultValue);
                     _writer.WriteLine($"public static T? {Identifier.GetValue}<T>(this {Identifier.IConfiguration} {Identifier.configuration}, string {Identifier.key}, T {Identifier.defaultValue}) => " +
                         $"(T?)({expressionForGetValueCore}({Identifier.configuration}, typeof(T), {Identifier.key}) ?? {Identifier.defaultValue});");
                 }
@@ -87,11 +88,20 @@ private void EmitGetValueMethods()
                 if (ShouldEmitMethods(MethodsToGen.ConfigBinder_GetValue_TypeOf_key_defaultValue))
                 {
                     EmitStartDefinition_Get_Or_GetValue_Overload(MethodsToGen.ConfigBinder_GetValue_TypeOf_key_defaultValue, documentation);
+                    EmitNotNullIfNotNull(Identifier.defaultValue);
                     _writer.WriteLine($"public static object? {Identifier.GetValue}(this {Identifier.IConfiguration} {Identifier.configuration}, Type {Identifier.type}, string {Identifier.key}, object? {Identifier.defaultValue}) => " +
                         $"{expressionForGetValueCore}({Identifier.configuration}, {Identifier.type}, {Identifier.key}) ?? {Identifier.defaultValue};");
                 }
             }
 
+            private void EmitNotNullIfNotNull(string parameterName)
+            {
+                if (_emitNotNullIfNotNull)
+                {
+                    _writer.WriteLine($"[return: global::System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof({parameterName}))]");
+                }
+            }
+
             private void EmitBindMethods_ConfigurationBinder()
             {
                 if (!ShouldEmitMethods(MethodsToGen.ConfigBinder_Bind))

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Parser/KnownTypeSymbols.cs

@@ -64,6 +64,7 @@ internal sealed class KnownTypeSymbols
         public INamedTypeSymbol? MemberInfo  { get; }
         public INamedTypeSymbol? ParameterInfo { get; }
         public INamedTypeSymbol? Delegate   { get; }
+        public INamedTypeSymbol? NotNullIfNotNullAttribute { get; }
 
         public KnownTypeSymbols(CSharpCompilation compilation)
         {
@@ -132,6 +133,9 @@ public KnownTypeSymbols(CSharpCompilation compilation)
             IntPtr = Compilation.GetSpecialType(SpecialType.System_IntPtr);
             UIntPtr = Compilation.GetSpecialType(SpecialType.System_UIntPtr);
             Delegate = Compilation.GetSpecialType(SpecialType.System_Delegate);
+
+            // Only generate nullable attributes if available
+            NotNullIfNotNullAttribute = compilation.GetBestTypeByMetadataName("System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute");
         }
     }
 }

src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Specs/SourceGenerationSpec.cs

@@ -12,6 +12,7 @@ public sealed record SourceGenerationSpec
         public required ImmutableEquatableArray<TypeSpec> ConfigTypes { get; init; }
         public required bool EmitEnumParseMethod { get; set; }
         public required bool EmitGenericParseEnum { get; set; }
+        public required bool EmitNotNullIfNotNull { get; set; }
         public required bool EmitThrowIfNullMethod { get; set; }
     }
 }

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

@@ -32,10 +32,12 @@ public static partial class ConfigurationBinder
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("In case the type is non-primitive, the trimmer cannot statically analyze the object's type so its members may be trimmed.")]
         public static object? GetValue(this Microsoft.Extensions.Configuration.IConfiguration configuration, [System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute(System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.All)] System.Type type, string key) { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("In case the type is non-primitive, the trimmer cannot statically analyze the object's type so its members may be trimmed.")]
+        [return: System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static object? GetValue(this Microsoft.Extensions.Configuration.IConfiguration configuration, [System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute(System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.All)] System.Type type, string key, object? defaultValue) { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("In case the type is non-primitive, the trimmer cannot statically analyze the object's type so its members may be trimmed.")]
         public static T? GetValue<[System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute(System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.All)] T>(this Microsoft.Extensions.Configuration.IConfiguration configuration, string key) { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("In case the type is non-primitive, the trimmer cannot statically analyze the object's type so its members may be trimmed.")]
+        [return: System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static T? GetValue<[System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute(System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes.All)] T>(this Microsoft.Extensions.Configuration.IConfiguration configuration, string key, T defaultValue) { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresDynamicCodeAttribute("Binding strongly typed objects to configuration values requires generating dynamic code at runtime, for example instantiating generic types.")]
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("In case the type is non-primitive, the trimmer cannot statically analyze the object's type so its members may be trimmed.")]

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

@@ -167,6 +167,7 @@ public static void Bind(this IConfiguration configuration, object? instance, Act
         /// <param name="defaultValue">The default value to use if no value is found.</param>
         /// <returns>The converted value.</returns>
         [RequiresUnreferencedCode(TrimmingWarningMessage)]
+        [return: NotNullIfNotNull(nameof(defaultValue))]
         public static T? GetValue<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] T>(this IConfiguration configuration, string key, T defaultValue)
         {
             return (T?)GetValue(configuration, typeof(T), key, defaultValue);
@@ -198,6 +199,7 @@ public static void Bind(this IConfiguration configuration, object? instance, Act
         /// <param name="defaultValue">The default value to use if no value is found.</param>
         /// <returns>The converted value.</returns>
         [RequiresUnreferencedCode(TrimmingWarningMessage)]
+        [return: NotNullIfNotNull(nameof(defaultValue))]
         public static object? GetValue(
             this IConfiguration configuration,
             [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)]

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

@@ -326,11 +326,15 @@ public void CanBindToObjectProperty()
         [Fact]
         public void GetNullValue()
         {
-            var dic = new Dictionary<string, string>
+            #nullable enable
+            #pragma warning disable IDE0004 // Cast is redundant
+
+            var dic = new Dictionary<string, string?>
             {
                 {"Integer", null},
                 {"Boolean", null},
                 {"Nested:Integer", null},
+                {"String", null},
                 {"Object", null }
             };
             var configurationBuilder = new ConfigurationBuilder();
@@ -341,13 +345,16 @@ public void GetNullValue()
             Assert.False(config.GetValue<bool>("Boolean"));
             Assert.Equal(0, config.GetValue<int>("Integer"));
             Assert.Equal(0, config.GetValue<int>("Nested:Integer"));
+            Assert.Null(config.GetValue<string>("String"));
             Assert.Null(config.GetValue<ComplexOptions>("Object"));
 
             // Generic overloads with default value.
-            Assert.True(config.GetValue("Boolean", true));
-            Assert.Equal(1, config.GetValue("Integer", 1));
-            Assert.Equal(1, config.GetValue("Nested:Integer", 1));
-            Assert.Equal(new NestedConfig(""), config.GetValue("Object", new NestedConfig("")));
+            Assert.True((bool)config.GetValue("Boolean", true));
+            Assert.Equal(1, (int)config.GetValue("Integer", 1));
+            Assert.Equal(1, (int)config.GetValue("Nested:Integer", 1));
+            // [NotNullIfNotNull] avoids CS8600: Converting possible null value to non-nullable type.
+            Assert.Equal("s", (string)config.GetValue("String", "s"));
+            Assert.Equal(new NestedConfig(""), (NestedConfig)config.GetValue("Object", new NestedConfig("")));
 
             // Type overloads.
             Assert.Null(config.GetValue(typeof(bool), "Boolean"));
@@ -356,16 +363,22 @@ public void GetNullValue()
             Assert.Null(config.GetValue(typeof(ComplexOptions), "Object"));
 
             // Type overloads with default value.
+            // [NotNullIfNotNull] avoids CS8605: Unboxing a possibly null value.
             Assert.True((bool)config.GetValue(typeof(bool), "Boolean", true));
             Assert.Equal(1, (int)config.GetValue(typeof(int), "Integer", 1));
             Assert.Equal(1, (int)config.GetValue(typeof(int), "Nested:Integer", 1));
-            Assert.Equal(new NestedConfig(""), config.GetValue("Object", new NestedConfig("")));
+            // [NotNullIfNotNull] avoids CS8600: Converting possible null value to non-nullable type.
+            Assert.Equal("s", (string)config.GetValue(typeof(string), "String", "s"));
+            Assert.Equal(new NestedConfig(""), (NestedConfig)config.GetValue("Object", new NestedConfig("")));
 
             // GetSection tests.
             Assert.False(config.GetSection("Boolean").Get<bool>());
             Assert.Equal(0, config.GetSection("Integer").Get<int>());
             Assert.Equal(0, config.GetSection("Nested:Integer").Get<int>());
             Assert.Null(config.GetSection("Object").Get<ComplexOptions>());
+
+            #pragma warning restore IDE0004
+            #nullable restore
         }
 
         [Fact]

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Baselines/netcoreapp/ConfigurationBinder/GetValue.generated.txt

@@ -39,6 +39,7 @@ namespace Microsoft.Extensions.Configuration.Binder.SourceGeneration
 
         /// <summary>Extracts the value with the specified key and converts it to the specified type.</summary>
         [InterceptsLocation(@"src-0.cs", 16, 24)]
+        [return: global::System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static T? GetValue<T>(this IConfiguration configuration, string key, T defaultValue) => (T?)(BindingExtensions.GetValueCore(configuration, typeof(T), key) ?? defaultValue);
 
         /// <summary>Extracts the value with the specified key and converts it to the specified type.</summary>
@@ -47,6 +48,7 @@ namespace Microsoft.Extensions.Configuration.Binder.SourceGeneration
 
         /// <summary>Extracts the value with the specified key and converts it to the specified type.</summary>
         [InterceptsLocation(@"src-0.cs", 17, 24)]
+        [return: global::System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static object? GetValue(this IConfiguration configuration, Type type, string key, object? defaultValue) => BindingExtensions.GetValueCore(configuration, type, key) ?? defaultValue;
         #endregion IConfiguration extensions.
 

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Baselines/netcoreapp/ConfigurationBinder/GetValue_T_Key_DefaultValue.generated.txt

@@ -35,6 +35,7 @@ namespace Microsoft.Extensions.Configuration.Binder.SourceGeneration
         #region IConfiguration extensions.
         /// <summary>Extracts the value with the specified key and converts it to the specified type.</summary>
         [InterceptsLocation(@"src-0.cs", 12, 20)]
+        [return: global::System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static T? GetValue<T>(this IConfiguration configuration, string key, T defaultValue) => (T?)(BindingExtensions.GetValueCore(configuration, typeof(T), key) ?? defaultValue);
         #endregion IConfiguration extensions.
 

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Baselines/netcoreapp/ConfigurationBinder/GetValue_TypeOf_Key_DefaultValue.generated.txt

@@ -35,6 +35,7 @@ namespace Microsoft.Extensions.Configuration.Binder.SourceGeneration
         #region IConfiguration extensions.
         /// <summary>Extracts the value with the specified key and converts it to the specified type.</summary>
         [InterceptsLocation(@"src-0.cs", 11, 20)]
+        [return: global::System.Diagnostics.CodeAnalysis.NotNullIfNotNull(nameof(defaultValue))]
         public static object? GetValue(this IConfiguration configuration, Type type, string key, object? defaultValue) => BindingExtensions.GetValueCore(configuration, type, key) ?? defaultValue;
         #endregion IConfiguration extensions.
 

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

@@ -8,6 +8,9 @@
     <NoWarn>$(NoWarn);SYSLIB1103,SYSLIB1104</NoWarn>
     <InterceptorsPreviewNamespaces>$(InterceptorsPreviewNamespaces);Microsoft.Extensions.Configuration.Binder.SourceGeneration</InterceptorsPreviewNamespaces>
     <EnableConfigurationBindingGenerator>true</EnableConfigurationBindingGenerator>
+
+    <!-- Avoid conflict with Microsoft.Extensions.Configuration.Binder (via InternalsVisibleTo) -->
+    <SkipIncludeNullableAttributes>true</SkipIncludeNullableAttributes>
   </PropertyGroup>
 
   <PropertyGroup>