CoreActivity, make From/Recipient/Conversation optional (nullable fields)

[rido-min]

This pull request introduces comprehensive changes to support nullability for key properties in activity-related classes and their usage throughout the codebase. The main goal is to improve robustness when handling activities that may lack sender, recipient, or conversation information, and to ensure consistent null handling in both implementation and tests.

Key changes include:

• Making From, Recipient, and Conversation properties nullable in activity and related classes.
• Updating constructors, methods, and serialization logic to handle possible null values.
• Adjusting tests to account for and verify nullability.

The most important changes are:

Core Schema and Model Updates:

• Made the From, Recipient, and Conversation properties nullable in CoreActivity, TeamsActivity, and related classes, and updated constructors to handle null inputs gracefully. This ensures activities can be created and processed even when some participants or conversation info are missing. [1] [2] [3] [4]
• Changed entity and attachment collections (EntityList, TeamsAttachment) to return null instead of empty collections when input JSON arrays are null, standardizing the nullability contract. [1] [2]

Method and API Usage Adjustments:

• Updated various method signatures and internal usages to accept nullable types and to check for nulls before accessing properties, including in WithFrom, WithRecipient, and WithConversation builder methods, and in AgenticIdentity extraction logic. [1] [2] [3] [4] [5] [6] [7]

Test Suite Updates:

• Updated unit tests to expect nulls for properties and collections where appropriate, and to use null-safe assertions and property access throughout. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Constructor and Null Handling Improvements:

• Constructors for TeamsConversationAccount and TeamsConversation now handle null arguments without throwing exceptions, returning early if input is null. [1] [2]

General Codebase Consistency:

• Ensured all usages of From, Recipient, Conversation, Entities, and Attachments are null-safe across the codebase, including in activity updates, deletions, and proactive messaging scenarios. [1] [2]

These changes collectively make the bot framework more resilient to incomplete or partially populated activities and improve code safety and clarity when handling optional data.

[Copilot]

Pull request overview

This PR updates the bot core/app activity schema to support nullable From, Recipient, and Conversation (plus nullable Entities/Attachments collections), and adjusts key call sites and unit tests to be null-safe when activities are partially populated.

Changes:

• Made CoreActivity.From, .Recipient, and .Conversation nullable and updated related callers (e.g., ConversationClient, compat layer) to use null-safe identity extraction.
• Standardized EntityList / TeamsAttachment parsing helpers to return null (instead of empty collections) when the source JSON arrays are null.
• Updated multiple unit tests (core/apps/compat) to align with the new nullability expectations.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
core/src/Microsoft.Teams.Bot.Core/Schema/CoreActivity.cs	Makes From/Recipient/Conversation nullable in the core schema.
core/src/Microsoft.Teams.Bot.Core/Schema/CoreActivityBuilder.cs	Adjusts builder API to accept nullable From.
core/src/Microsoft.Teams.Bot.Core/ConversationClient.cs	Uses null-safe From?.GetAgenticIdentity() for request options and delete.
core/src/Microsoft.Teams.Bot.Compat/CompatTeamsInfo.cs	Null-safe identity extraction from From?.Properties.
core/src/Microsoft.Teams.Bot.Compat/CompatConversations.cs	Null-safe identity extraction from Activity?.From?.Properties.
core/src/Microsoft.Teams.Bot.Apps/Schema/TeamsConversationAccount.cs	Allows null input in conversion ctor.
core/src/Microsoft.Teams.Bot.Apps/Schema/TeamsConversation.cs	Allows null input in conversion ctor.
core/src/Microsoft.Teams.Bot.Apps/Schema/TeamsAttachment.cs	Returns null when source JSON array is null.
core/src/Microsoft.Teams.Bot.Apps/Schema/TeamsActivity.cs	Null-safe conversion from CoreActivity and nullable Teams shadow properties.
core/src/Microsoft.Teams.Bot.Apps/Schema/Entities/Entity.cs	EntityList.FromJsonArray returns null when source JSON array is null.
core/samples/CompatBot/EchoBot.cs	Updates sample to use ca.From?.Properties safely.
core/test/Microsoft.Teams.Bot.Core.UnitTests/Schema/CoreActivityTests.cs	Updates reply assertions to be null-safe.
core/test/Microsoft.Teams.Bot.Core.UnitTests/MiddlewareTests.cs	Avoids dereferencing possibly-null Recipient in setup.
core/test/Microsoft.Teams.Bot.Core.UnitTests/CoreActivityBuilderTests.cs	Updates builder tests for nullable participants.
core/test/Microsoft.Teams.Bot.Core.UnitTests/ConversationClientTests.cs	Updates expected exception type for null conversation.
core/test/Microsoft.Teams.Bot.Core.UnitTests/BotApplicationTests.cs	Avoids dereferencing possibly-null Recipient in setup.
core/test/Microsoft.Teams.Bot.Compat.UnitTests/CompatActivityTests.cs	Updates compat assertions for nullable collections/participants.
core/test/Microsoft.Teams.Bot.Apps.UnitTests/TeamsActivityTests.cs	Adds/relocates tests for derived type mapping and minimal JSON output.
core/test/Microsoft.Teams.Bot.Apps.UnitTests/TeamsActivityBuilderTests.cs	Updates Teams builder tests for nullable participants and nullable collections.

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[Copilot]

The XML doc for TeamsConversationAccount(ConversationAccount? conversationAccount) still states the parameter "Cannot be null", but the signature and implementation now explicitly allow null (early return). Please update the doc comment to match the new nullability contract.

[Copilot]

Unused using directive Microsoft.AspNetCore.Mvc.ApplicationParts appears to be unused in this test file. Since some projects in this repo treat warnings as errors, it’s safer to remove it to avoid CS8019 build failures in stricter configurations.

Suggested change

using Microsoft.AspNetCore.Mvc.ApplicationParts;