Document Previewer

The document previewer renders a file inline in the browser — typically inside a modal panel — without triggering a download. Your handler generates or loads the file bytes and passes them directly in the /runevent response.

When your handler calls PreviewDocument(), the command is returned in the /runevent response. The SDK receives it and opens the viewer.

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Method signature

cmd.PreviewDocument(string fileId, byte[]? data, string fieldName = "attached_files", object? context = null)

Parameter	Description
fileId	Filename shown to the user in the viewer (e.g. "contract.pdf")
data	File content as byte[]. The bytes are Base64-encoded and embedded in the response. Pass null to have the SDK stream the file by path instead.
fieldName	Optional. The widget field this file is associated with. Defaults to "attached_files".
context	Optional. Record context object (pluginCode, formCode, guid, pGuid) — used when streaming large files.

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Generating and previewing a PDF

The most common pattern is to generate PDF bytes on demand and pass them directly:

public override async Task Form_onPrint()
{
    byte[] pdfBytes = await _pdfService.RenderAsync("ContractTemplate", record.GetData());
    cmd.PreviewDocument("contract.pdf", pdfBytes);
}

The bytes are Base64-encoded and embedded in the /runevent response — no separate file storage step is required.

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Previewing a stored file

If the file is already in storage, load the bytes and pass them the same way:

public async Task Form_onClick(string widgetName)
{
    if (widgetName == "previewattachmentbtn")
    {
        string? filePath = record.GetField("attachment")?.ToString();

        if (string.IsNullOrEmpty(filePath))
        {
            ShowMessage("warning", "No attachment found.");
            return;
        }

        byte[] fileBytes = await _fileStorage.GetFileBytesAsync(filePath);
        string fileName  = Path.GetFileName(filePath);

        cmd.PreviewDocument(fileName, fileBytes);
    }
}

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Large files

For files larger than ~10 MB, pass null for data and provide a context object so the SDK can stream the file by path instead of embedding the bytes in the response:

object previewContext = new
{
    pluginCode = Context.PluginCode,
    formCode   = Context.FormCode,
    guid       = Context.Guid,
    pGuid      = Context.PGuid
};

cmd.PreviewDocument(filePath, fileBytes?.Length < 10 * 1024 * 1024 ? fileBytes : null, "attached_files", previewContext);

When data is null, the SDK uses fileId as a path and streams the content via /getlinkedfilebychunks.

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Supported file types

The built-in viewer supports PDF files, images (PNG, JPG, GIF, etc.), and Excel spreadsheets.