问为什么blobServiceClient.GetUserDelegationKeyAsync失败了，但blobClient.DownloadToAsync成功了？

EN

 public class AppBlobService
{
    private readonly string _connectionString;
    public AppBlobService(string connectionString)
    {
        _connectionString = connectionString;
    }
    public async Task<Uri?> CreateSasTokenUri(string containerName, string blobName)
    {
        var blobServiceClient = new BlobServiceClient(_connectionString);
        var containerClient = blobServiceClient.GetBlobContainerClient(containerName);
        var blobClient = containerClient.GetBlobClient(blobName);
        Uri ret = null;
        try
        {
            if (await blobClient.ExistsAsync())
            {
                ret = await GetUserDelegationSasBlob(blobClient);
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e);
        }
        return ret;

    }
    private async Task<Uri> GetUserDelegationSasBlob(BlobClient blobClient)
    {
        BlobServiceClient blobServiceClient =
            blobClient.GetParentBlobContainerClient().GetParentBlobServiceClient();

        Azure.Storage.Blobs.Models.UserDelegationKey userDelegationKey =
            await blobServiceClient.GetUserDelegationKeyAsync(DateTimeOffset.UtcNow,
                DateTimeOffset.UtcNow.AddDays(7));

        BlobSasBuilder sasBuilder = new BlobSasBuilder()
        {
            BlobContainerName = blobClient.BlobContainerName,
            BlobName = blobClient.Name,
            Resource = "b",
            StartsOn = DateTimeOffset.UtcNow,
            ExpiresOn = DateTimeOffset.UtcNow.AddDays(7)
        };

        sasBuilder.SetPermissions(BlobSasPermissions.Read );

        BlobUriBuilder blobUriBuilder = new BlobUriBuilder(blobClient.Uri)
        {
            Sas = sasBuilder.ToSasQueryParameters(userDelegationKey,
                blobServiceClient.AccountName)
        };

        return blobUriBuilder.ToUri();
    }
    public async Task<BlobEntity> GetAsync(string containerName, string blobName)
    {
        var ret = new BlobEntity(containerName, blobName);
        var blobServiceClient = new BlobServiceClient(_connectionString);
        var containerClient = blobServiceClient.GetBlobContainerClient(containerName);
        var blobClient = containerClient.GetBlobClient(blobName);
        if (await blobClient.ExistsAsync())
        {
            var ms = new MemoryStream();
            await blobClient.DownloadToAsync(ms);
            ret.DataStream = ms;
        }

        return ret;
    }
}