Added AsReadOnly extension methods for IDictionary<T, U> and IList<T>

[Mrxx99]

Resolves #33033

I have made the code changes, but I have some problems:
- Using the ReadOnlyDictionary<T, U> in the CollectionExtensions class in System.Collections.csproj is not possible. It can't find the reference and I don't know what to do here. Adding the System.ObjectModel.csproj to the System.Collections.sln and adding a project reference from that project to System.Collections.csproj did not work.
- Adding the ref project for System.ObjectModel to the ref project for System.Collections complains about a cycle because System.ObjectModel already references System.Collections. With that is it even possible to add the extension method for ReadOnlyDictionary to CollectionExtensions in System.Collections? (ReadOnlyCollection is no problem because it is inside the System.Runtime.dll)
- Where should XML-Doc comments be added? The other methods in CollectionExtensions have none but they show up in intellisense.

Can someone please help me here?

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

---

Resolves #33033

I have made the code changes, but I have some problems:

• Using the ReadOnlyDictionary<T, U> in the CollectionExtensions class in System.Collections.csproj is not possible. It can't find the reference and I don't know what to do here. Adding the System.ObjectModel.csproj to the System.Collections.sln and adding a project reference from that project to System.Collections.csproj did not work.
• Adding the ref project for System.ObjectModel to the ref project for System.Collections complains about a cycle because System.ObjectModel already references System.Collections. With that is it even possible to add the extension method for ReadOnlyDictionary to CollectionExtensions in System.Collections? (ReadOnlyCollection is no problem because it is inside the System.Runtime.dll)
• Where should XML-Doc comments be added? The other methods in CollectionExtensions have none but they show up in intellisense.

Can someone please help me here?

Author:	Mrxx99
Assignees:	-
Labels:	area-System.Collections, new-api-needs-documentation, community-contribution
Milestone:	-

[eiriktsarpalis]

Where should XML-Doc comments be added? The other methods in CollectionExtensions have none but they show up in intellisense.

@Mrxx99 please go ahead and add triple-slash documentation directly on the implementation method. We will sync this up the dotnet-api-docs repo at a later stage.

[Mrxx99]

@eiriktsarpalis I added the documentation comments. I looked at the documentation comments for the List.AsReadOnly() method and the type and constructor documentation for ReadOnlyCollection and ReadOnlyDictionary<TKey, TValue> and combined the information from them. I hope they are okay.

List.AsReadOnly()

//
// Summary:
//     Returns a read-only System.Collections.ObjectModel.ReadOnlyCollection\`1 wrapper
//     for the current collection.
//
// Returns:
//     An object that acts as a read-only wrapper around the current System.Collections.Generic.List`1.

ReadOnlyCollection

//
// Summary:
//     Provides the base class for a generic read-only collection.
//
// Type parameters:
//   T:
//     The type of elements in the collection.

//
// Summary:
//     Initializes a new instance of the System.Collections.ObjectModel.ReadOnlyCollection`1
//     class that is a read-only wrapper around the specified list.
//
// Parameters:
//   list:
//     The list to wrap.
//
// Exceptions:
//   T:System.ArgumentNullException:
//     list is null.

ReadOnlyDictionary

//
// Summary:
//     Represents a read-only, generic collection of key/value pairs.
//
// Type parameters:
//   TKey:
//     The type of keys in the dictionary.
//
//   TValue:
//     The type of values in the dictionary.

//
// Summary:
//     Initializes a new instance of the System.Collections.ObjectModel.ReadOnlyDictionary`2
//     class that is a wrapper around the specified dictionary.
//
// Parameters:
//   dictionary:
//     The dictionary to wrap.
//
// Exceptions:
//   T:System.ArgumentNullException:
//     dictionary is null.