Fix MissingMethodException in GitHub Copilot attachment handling

[Copilot]

Motivation and Context

Binary incompatibility with GitHub.Copilot.SDK v0.1.18+ caused MissingMethodException when sending messages with DataContent attachments. The SDK changed from a base class with settable Type property to specific typed attachment classes.

Description

Updated GitHub.Copilot.SDK to v0.1.23

• Changed from v0.1.18 in Directory.Packages.props

Fixed attachment API usage in GitHubCopilotAgent.cs

- attachments.Add(new UserMessageDataAttachmentsItem
+ attachments.Add(new UserMessageDataAttachmentsItemFile
  {
-   Type = UserMessageDataAttachmentsItemType.File,
    Path = tempFilePath,
    DisplayName = Path.GetFileName(tempFilePath)
  });

The new SDK provides three typed classes for attachments:

• UserMessageDataAttachmentsItemFile - for file attachments
• UserMessageDataAttachmentsItemDirectory - for directory attachments
• UserMessageDataAttachmentsItemSelection - for text selections

Each has a read-only Type property with the appropriate constant value.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• www.nuget.org
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET: [Bug]: MissingMethodException in ProcessDataContentAttachmentsAsync due to binary incompatibility with GitHub.Copilot.SDK</issue_title>
<issue_description>### Description

When using the NuGet package Microsoft.Agents.AI.GitHub.Copilot (version 1.0.0-preview.260209.1), a MissingMethodException occurs when sending messages with attachments.

Code Sample

using GitHub.Copilot.SDK;
using Microsoft.Agents.AI;
using Microsoft.Agents.AI.GitHub.Copilot;
using Microsoft.Extensions.AI;

using var copilotClient = new CopilotClient();
var config = new SessionConfig { Model = "claude-3.5-sonnet" };
var agent = new GitHubCopilotAgent(copilotClient, config);

await agent.CreateSessionAsync();

string filePath = "Program.cs"; 
byte[] fileBytes = await File.ReadAllBytesAsync(filePath);
string mimeType = "text/plain"; 

var message = new ChatMessage(ChatRole.User, "What's the best practice to test private methods?");

message.Contents.Add(new DataContent(fileBytes, mimeType));

var messages = new List<ChatMessage> { message };

try
{
    Console.WriteLine($"Sending message with attachment: {filePath}...");
    var response = await agent.RunAsync(messages);
    Console.WriteLine("--- Agent Response ---");
    Console.WriteLine(response);
}
catch (Exception ex)
{
    Console.WriteLine($"Error: {ex.Message}");
    if (ex.InnerException != null)
        Console.WriteLine($"Inner: {ex.InnerException.Message}");
}

Error Messages / Stack Traces

Method not found: 'Void GitHub.Copilot.SDK.UserMessageDataAttachmentsItem.set_Type(GitHub.Copilot.SDK.UserMessageDataAttachmentsItemType)'.
at Microsoft.Agents.AI.GitHub.Copilot.GitHubCopilotAgent.<ProcessDataContentAttachmentsAsync>d__33.MoveNext()

Package Versions

Microsoft.Agents.AI.GitHub.Copilot: 1.0.0-preview.260209.1

.NET Version

.NET 9.0

Additional Context

Investigation
The Cause: The property UserMessageDataAttachmentsItem.Type in the latest GitHub.Copilot.SDK appears to have changed its setter (e.g., from set to init), breaking binary compatibility with the pre-compiled NuGet package.

Evidence: - Using the NuGet package results in the error above.

However, cloning this repository and referencing the source code directly works perfectly, because the code is re-compiled against the latest SDK during the local build.</issue_description>

<agent_instructions>Fix the bug reported in this issue. As part of the fix update to the latest version of the GitHub.Copilot.SDK and fix any other breaking changes.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET: [Bug]: MissingMethodException in ProcessDataContentAttachmentsAsync due to binary incompatibility with GitHub.Copilot.SDK #3869

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