Create the FormattableString class and perhaps another related types.

Reference: https://docs.microsoft.com/en-us/dotnet/api/system.formattablestring?view=netframework-4.7.2

Create BlockingCollection<T> class and related concurrent classes.

Implement LINQ expression without runtime compilation.

Create SemaphoreSlim class.

Create the BufferedStream class.

On full Framework every await without ConfigureAwait(false) continues on current synchronization context. Must ensure that the behavior of Windows CE implementation is the same.

Create the SortedDictionary class.

Implement asynchronous methods of StreamReader.

Create ImmutableArray<T> from Immutable package.

Reference: https://blogs.msdn.microsoft.com/dotnet/2013/06/24/please-welcome-immutablearrayt/

Create missing methods of Interlocked class.

• Read = Interlocked.CompareExchange(ref value, 0, 0).
• MemoryBarrier = Thread.MemoryBarrier().

The current implementation of AsyncTaskMethodBuilder does not check whether the Task is being canceled when the Task is not initialized.

Support the extension methods SkipLast and TakeLast for IEnumerable<T>.

The method Wait of the class Task is throwing an exception when called.

   at System.Threading.Monitor.Exit(Object obj)
   at System.Threading.Monitor2.ReleaseAll(Object obj)
   at System.Threading.Condition.Wait(Int32 millisecondsTimeout)
   at System.Threading.Monitor2.Wait(Object obj, Int32 millisecondsTimeout)
   at System.Threading.ManualResetEventSlim.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at System.Threading.Tasks.Task.<>c__DisplayClass119_0`1.<Run>b__0()
   at System.Threading.Tasks.Task`1.ExecuteTaskAction()
   at System.Threading.Tasks.Task.TaskStartAction(Object stateObject)
   at System.Threading.ThreadPool.WorkItem.doWork(Object o)
   at System.Threading.Timer.ring()

Add the field Empty to the System.String2 class.

Create the implementation of InvalidEnumArgumentException exception type.

Implement an extension method for Thread indicating whether the thread is running or not.

Suggestion: use Join(0) and check return flag. True - terminated, False - still running (https://stackoverflow.com/a/8034178/7710026).

Implement System.Index and System.Range to allow usage of index and range features of C# 8.0.

Support the ConvertAll method from Array.

Extend System.Collections.Generic.Comparer<T> to match new API.

Support asynchronous BeginRead and BeginWrite for the following internal streams:

• System.Net.HttpReadStream (Read/Write)
• System.Net.HttpWriteStream (Write)
• System.IO.Ports.SerialStream (Read/Write)

Support TaskContinuationOptions flags for continuation tasks.

Support conversion to IReadOnlyCollection and IReadOnlyList from Array, List, ICollection and others.

// The following code should work
IReadOnlyCollection<byte> value1 = (new byte[] { 15, 7, 4 }).AsReadOnly();
IReadOnlyList<byte> value2 = (new byte[] { 15, 7, 4 }).AsReadOnly();
IReadOnlyList<byte> value3 = new List<byte>(new byte[] { 15, 7, 4 }).AsReadOnlyList();

Support string formatting of TimeSpan instances.

The Stream.CopyTo extension method should use AsyncStream.CopyTo method when applicable.

Support types from System.Diagnostics.Contracts namespace.

The asynchronous method of WebRequest class it not affected by the Timeout property, but the TPL extensions should.

References:

• HttpWebRequest.Timeout Property
• HttpWebRequest.BeginGetResponse Method

Create the System.Net.Mail.MailAddress class.

Create GetValue(object) overload for PropertyInfo class.

Extend existing API of System.Reflection to support new features.

Create code to allow using async streams.

Ref: Tutorial: Generate and consume async streams using C# 8.0 and .NET Core 3.0

Implement the interfaces IStructuralComparable and IStructuralEquatable for Array.

Support the Parse method from Guid.

The class AsyncTaskMethodBuilder<TResult> should be a struct.

Tried to partial implementation of Tasks, in particular waited for task to complete within timeout. Had graceful completion but default task result.

STR:
Run sample on Windows CE

ER:
To get the graceful result(123) and no timeouts.

AR:
No timeouts, but default result

See sample below.

using System;
using System.Threading;
using System.Threading.Tasks;
using Task = System.Threading.Tasks.Task;

namespace TestApp
{
    class Program
    {
        static void Main(string[] args)
        {
            var runningTask = CalculateSomething();
            var result = Execute(runningTask, TimeSpan.FromSeconds(10));//succeeds but result is 0, not 123 in Execute and Execute1 methods

            Console.WriteLine(result);
            Console.ReadLine();
        }

        private static async Task<int> CalculateSomething()
        {
            Console.WriteLine("Calculating....");
            await Task.Delay(TimeSpan.FromSeconds(5));
            Console.WriteLine("Done");

            return 123;
        }

        private static T Execute<T>(Task<T> task, TimeSpan timeout)
        {
            if (task.Wait(timeout))
            {
                Console.WriteLine("Got graceful result");
                return task.Result;
            }

            Console.WriteLine("Timeout");
            return default(T);
        }

        private static T Execute1<T>(Task<T> task, TimeSpan timeout)
        {
            var tcs = new CancellationTokenSource(timeout);

            var runningTask = Task.Run(() => task, tcs.Token);

            var result = runningTask.GetAwaiter().GetResult();

            return result;
        }
    }
}

Create ContinueWith methods overloads for Task<TResult>.

Implement the method ConvertAll of Array class.

my mean is OS.
Because Microsoft gave up Windows CE system update and support. The .net is crumbling in handheld devices. Android began to enter in large numbers, and java and kotlin began to eat market share. After all this is completely switched, what else can we do with .net?

Use the variant alternatives of Action and Func delegates.

Create the LocalizableAttribute attribute from System.ComponentModel namespace.

Implement System.HashCode to combine hash codes into a single hash code.

Implement missing Run method overloads of Task class.

Change the static method Empty<T>() of the class Array2 to define a public modifier.

Extend Trace class from System.Diagnostics to improve compatibility.

References:

• https://www.devmedia.com.br/system-diagnostics-estendendo-o-namespace-do-net-framework/2320

Support the property IsConstructedGenericType from Type class.

public override bool IsConstructedGenericType 
{
    get { return IsGenericType && !IsGenericTypeDefinition; } 
} 

NETStandard.WindowsCE 1.7.1 works, but 1.8.0 can't work.

The awaiter for ValueTask does not implement INotifyCompletion interface.

error CS4027: 'ConfiguredValueTaskAwaitable<bool>.ConfiguredValueTaskAwaiter' does not implement 'INotifyCompletion'

Add support to System.Span<T> from System.Memory.

Extend Activator2 class to allow access to private constructor.

Support the IValueTaskSource interface at System.Threading.Tasks.Sources namespace.

Implement a wrapper to control thread state for a thread.