Provide System.Reflection.Context package readme (#106398)

Author: eNeRGy164

src/libraries/System.Reflection.Context/src/PACKAGE.md

@@ -0,0 +1,133 @@
+## About
+
+Provides the `CustomReflectionContext` class to enable adding or removing custom attributes from reflection objects, or adding dummy properties to those objects, without re-implementing the complete reflection model.
+
+## Key Features
+
+<!-- The key features of this package -->
+
+* Create a custom reflection context to control how types and members are presented during reflection.
+* Easily extend or customize the reflection model to fit specialized application needs.
+
+## How to Use
+
+<!-- A compelling example on how to use this package with code, as well as any specific guidelines for when to use the package -->
+
+Defining a custom `Attribute` and implement a `CustomReflectionContext` to add the attribute to specic methods.
+
+```csharp
+using System.Reflection;
+using System.Reflection.Context;
+
+[AttributeUsage(AttributeTargets.Method)]
+class CustomAttribute : Attribute
+{
+}
+
+class CustomContext : CustomReflectionContext
+{
+    // Called whenever the reflection context checks for custom attributes.
+    protected override IEnumerable<object> GetCustomAttributes(MemberInfo member, IEnumerable<object> declaredAttributes)
+    {
+        // Add custom attribute to "ToString" members
+        if (member.Name == "ToString")
+        {
+            yield return new CustomAttribute();
+        }
+
+        // Keep existing attributes as well
+        foreach (var attr in declaredAttributes)
+        {
+            yield return attr;
+        }
+    }
+}
+```
+
+Inspecting the `string` type with both the default and custom reflection context.
+
+```csharp
+Type type = typeof(string);
+
+// Representation of the type in the default reflection context
+TypeInfo typeInfo = type.GetTypeInfo();
+
+Console.WriteLine("\"To*\" members and their attributes in the default reflection context");
+foreach (MemberInfo memberInfo in typeInfo.DeclaredMembers)
+{
+    if (memberInfo.Name.StartsWith("To"))
+    {
+        Console.WriteLine(memberInfo.Name);
+        foreach (Attribute attribute in memberInfo.GetCustomAttributes())
+        {
+            Console.WriteLine($" - {attribute.GetType()}");
+        }
+    }
+}
+Console.WriteLine();
+
+// Output:
+// "To*" members and their attributes in the default reflection context
+// ToCharArray
+// ToCharArray
+// ToString
+// ToString
+// ToLower
+// ToLower
+// ToLowerInvariant
+// ToUpper
+// ToUpper
+// ToUpperInvariant
+
+// Representation of the type in the customized reflection context
+CustomContext customContext = new();
+TypeInfo customTypeInfo = customContext.MapType(typeInfo);
+
+Console.WriteLine("\"To*\" members and their attributes in the customized reflection context");
+foreach (MemberInfo memberInfo in customTypeInfo.DeclaredMembers)
+{
+    if (memberInfo.Name.StartsWith("To"))
+    {
+        Console.WriteLine(memberInfo.Name);
+        foreach (Attribute attribute in memberInfo.GetCustomAttributes())
+        {
+            Console.WriteLine($" - {attribute.GetType()}");
+        }
+    }
+}
+
+// Output:
+// "To*" members and their attributes in the customized reflection context
+// ToCharArray
+// ToCharArray
+// ToString
+//  - CustomAttribute
+// ToString
+//  - CustomAttribute
+// ToLower
+// ToLower
+// ToLowerInvariant
+// ToUpper
+// ToUpper
+// ToUpperInvariant
+```
+## Main Types
+
+<!-- The main types provided in this library -->
+
+The main type provided by this library is:
+
+* `System.Reflection.Context.CustomReflectionContext`
+
+## Additional Documentation
+
+<!-- Links to further documentation. Remove conceptual documentation if not available for the library. -->
+
+* [API documentation](https://learn.microsoft.com/dotnet/api/system.reflection.context)
+
+## Feedback & Contributing
+
+<!-- How to provide feedback on this package and contribute to it -->
+
+System.Reflection.Context is released as open source under the [MIT license](https://licenses.nuget.org/MIT).
+Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).

src/libraries/System.Reflection.Context/src/System.Reflection.Context.csproj

@@ -7,12 +7,7 @@
     <IsPackable>true</IsPackable>
     <AddNETFrameworkPlaceholderFileToPackage>true</AddNETFrameworkPlaceholderFileToPackage>
     <AddNETFrameworkAssemblyReferenceToPackage>true</AddNETFrameworkAssemblyReferenceToPackage>
-    <PackageDescription>Provides classes that enable customized reflection contexts.
-
-Commonly Used Types:
-System.Reflection.Context.CustomReflectionContext</PackageDescription>
-    <!-- TODO: Add package README file: https://github.com/dotnet/runtime/issues/99358 -->
-    <EnableDefaultPackageReadmeFile>false</EnableDefaultPackageReadmeFile>
+    <PackageDescription>Provides CustomReflectionContext to enable customized reflection contexts.</PackageDescription>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->