Slackblocks 2.4.0

slackblocks for .NET

Validated, immutable Slack Block Kit construction for .NET 8 and newer.

The NuGet package uses the same version as the Python, TypeScript, Go, and Java packages and is released only as part of the coordinated slackblocks release train.

Installation

dotnet add package Slackblocks

Build a block

Every value is an immutable class. Pass named arguments to its constructor, which validates them against Slack's documented rules:

using Slackblocks.Blocks;
using Slackblocks.Elements;

var block = new SectionBlock(
    text: "A deployment is ready for review.",
    accessory: new ButtonElement("Review", "review_deployment", value: "deploy-482"));

Required arguments come first, so the common ones can be passed positionally. Strings convert to Slack text objects: a field such as a section's text sends mrkdwn, and a field Slack requires as plain text, such as a button label, sends plain_text. Pass new PlainText(...) or new MarkdownText(...) when you need options such as emoji or verbatim.

Known Slack limits, required fields, mutually exclusive fields, and composition restrictions are checked in the constructor. A failure throws ValidationException with a stable ErrorCategory and field path:

using Slackblocks;
using Slackblocks.Blocks;

try
{
    _ = new HeaderBlock(new string('x', 151));
}
catch (ValidationException error)
{
    Console.WriteLine($"{error.Category}: {error.Path}"); // LengthExceeded: HeaderBlock.text.text
}

Read values

Properties return what you passed. Optional values are nullable, and collections are read-only lists that are empty when unset:

using Slackblocks.Elements;

var review = (ButtonElement)block.Accessory!;
Console.WriteLine(review.ActionId); // review_deployment
Console.WriteLine(block.Fields.Count); // 0

JSON

ToJson() returns compact Slack JSON, and ToJsonNode() returns a mutable copy. Values also serialize directly with System.Text.Json, including through role interfaces such as IBlock:

using System.Text.Json;
using Slackblocks.Blocks;

IBlock[] blocks = [new HeaderBlock("Deploy 482"), block];
string json = JsonSerializer.Serialize(new { channel = "C0123456", text = "Deploy 482", blocks });

Use MessagePayload, WebhookMessage, MessageResponse, ModalView, and HomeTabView when you need a complete, validated payload.

Send a message

slackblocks builds payloads and leaves delivery to you. Post a WebhookMessage to an incoming webhook with HttpClient:

using System.Text;
using Slackblocks.Payloads;

using var http = new HttpClient();
var message = new WebhookMessage(text: "A deployment is ready", blocks: [block]);
using var content = new StringContent(message.ToJson(), Encoding.UTF8, "application/json");
using var response = await http.PostAsync(Environment.GetEnvironmentVariable("SLACK_WEBHOOK_URL"), content);
response.EnsureSuccessStatusCode();

For the Web API, send a MessagePayload to chat.postMessage with a bot token. When you already use a client library such as SlackNet, pass the JSON from ToJson() to its raw request support. Several names, such as SectionBlock, exist in both libraries, so alias one namespace if a file imports both.

Fields Slack adds later

Every constructor accepts additionalFields for Slack fields that have no named parameter yet. The values are validated with the rest of the object, and a field cannot be set both there and through its named parameter:

using Slackblocks.Blocks;

var divider = new DividerBlock(additionalFields: new Dictionary<string, object?> { ["future_flag"] = true });

Higher-level components

Paginator, Accordion, and AccordionSection return ordinary blocks, so their results spread into any block collection:

using Slackblocks.Blocks;
using Slackblocks.Components;
using Slackblocks.Payloads;

IBlock[] results = [new SectionBlock(text: "one"), new SectionBlock(text: "two"), new SectionBlock(text: "three")];
var page = new MessagePayload(
    "C0123456",
    blocks: [new HeaderBlock("Results"), .. Paginator.Create("results", results, pageSize: 2)]);

Compatibility and conformance

• .NET 8 and newer, with no dependencies beyond the framework.
• Versioned in lockstep with every supported slackblocks language.
• All shared valid fixtures and invalid-case categories are mandatory; the C# skip list is empty.
• Value types are generated from java/generator/model.json, the model the Java package is generated from, so both expose the same fields, limits, and documentation.

Development

cd csharp
dotnet test
dotnet format --verify-no-changes

The build treats warnings as errors with the .NET analyzers enabled. The tests run every shared conformance fixture and every C# documentation snippet.

The Block Kit model is generated. Edit java/generator/model.json, regenerate the Java package as its README describes, then run:

python3 csharp/generator/generate_models.py

See the C# documentation, API reference, and repository-level contributing guide.