即使函数失败，也会创建输出 blob

Question

I have an Azure Function that is triggered by some blob creation, is doing some transforms (OCR mostly) and writes the result again to Blob:

[FunctionName("Ocr")]
public static async Task Run([BlobTrigger("input/{name}")]CloudBlockBlob myBlob,[Blob("output/{name}_{DateTime}.txt", FileAccess.Write)] TextWriter resultTextFile, string name,ILogger log)
{
    try
    {
        var ocrResult = await DoOcr(name);
        // This only happens if no exception was thrown
        await resultTextFile.WriteAsync(ocrResult.Text);
    }
    catch (Exception e)
    {
        log.LogError(e, $"Exception during processing. Cannot process document {name}");
    }
}

This works all fine and well. However, if something goes wrong inside the "DoOcr()" function, an exception is thrown - and caught it inside my catch block - the Function ends and an empty new blob was created at "output/{name}_{DateTime}.txt".

"WriteAsync()" gets never called but why is the file still created? It does not happen if I use "CloudBlockBlob" instead of "TextWriter" in the binding.

Answer 1

As Kyle said, TextWriter writer = new StreamWriter(stream); . So when you use TextWriter as blob output, it will initializes a new instance of the StreamWriter class for the specified file on the specified path. If the file exists, it can be either overwritten or appended to. If the file does not exist, this constructor creates a new file.

While you use CloudBlockBlob as blob output, it will initializes a new instance of the CloudBlockBlob class using an absolute URI to the blob which read as an in-memory stream and then get dumped to the file. So, if the stream is interrupt, it will not create a output new file.