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Check the HTTP response before giving its body to a PDF converter. Keep the HttpResponseMessage returned by GetAsync or SendAsync, test IsSuccessStatusCode, and only then read PDF bytes or a stream. On a non-2xx response, read a bounded error body, record the status, and return or throw a meaningful application error. This prevents an HTML error page, JSON problem document, CAPTCHA, or empty response from reaching PDF code.
The failure boundary: HTTP first, PDF second
HTTP status and PDF validity answer different questions. IsSuccessStatusCode is true for every status from 200 through 299, not just 200. A 201, 202, 204, or 205 can therefore be successful. However, a 204 normally has no document body, and a 2xx response can still contain malformed or incorrectly labelled content.
PDF processing should begin only after transport handling succeeds. In the success branch, check useful format signals such as the Content-Type value application/pdf and, when appropriate, the file signature. RFC 8118 specifies that PDF files start with the characters %PDF- followed by a version number. Neither a media type nor a filename is proof by itself: servers can mislabel content, so the parser remains the final format check.
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This pattern preserves the response, distinguishes status codes, allows inspection of an error payload, and leaves room for retry or custom result handling.
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using System.Net;
using System.Net.Http;
public static async Task<Stream> DownloadPdfAsync(
HttpClient httpClient,
Uri uri,
CancellationToken cancellationToken = default)
{
using HttpResponseMessage response = await httpClient.GetAsync(
uri,
HttpCompletionOption.ResponseHeadersRead,
cancellationToken);
if (!response.IsSuccessStatusCode)
{
string errorBody = await ReadBoundedTextAsync(
response.Content,
maxCharacters: 8_000,
cancellationToken);
throw new HttpRequestException(
$"Document request failed: {(int)response.StatusCode} " +
$"{response.ReasonPhrase}. {errorBody}",
inner: null,
statusCode: response.StatusCode);
}
if (response.StatusCode == HttpStatusCode.NoContent)
{
throw new InvalidDataException("The server returned 204 No Content; no PDF exists to convert.");
}
string? mediaType = response.Content.Headers.ContentType?.MediaType;
if (!string.Equals(mediaType, "application/pdf", StringComparison.OrdinalIgnoreCase))
{
throw new InvalidDataException(
$"Expected application/pdf but received {mediaType ?? "no Content-Type"}.");
}
Stream networkStream = await response.Content.ReadAsStreamAsync(cancellationToken);
return await CopyAndValidatePdfAsync(networkStream, cancellationToken);
}
private static async Task<string> ReadBoundedTextAsync(
HttpContent content,
int maxCharacters,
CancellationToken cancellationToken)
{
string text = await content.ReadAsStringAsync(cancellationToken);
return text.Length <= maxCharacters
? text
: text[..maxCharacters] + "…";
}
private static async Task<Stream> CopyAndValidatePdfAsync(
Stream source,
CancellationToken cancellationToken)
{
await using (source)
{
MemoryStream copy = new();
await source.CopyToAsync(copy, cancellationToken);
byte[] bytes = copy.ToArray();
if (bytes.Length < 5 ||
bytes[0] != (byte)'%' || bytes[1] != (byte)'P' ||
bytes[2] != (byte)'D' || bytes[3] != (byte)'F' ||
bytes[4] != (byte)'-')
{
copy.Dispose();
throw new InvalidDataException("The successful response does not start with the PDF signature %PDF-.");
}
copy.Position = 0;
return copy;
}
}The example uses ResponseHeadersRead so headers are returned without buffering the entire body. It copies the stream into a seekable MemoryStream because many PDF libraries need seeking. If your library accepts a forward-only stream, pass the response stream directly and dispose it after conversion instead.
Protect logs and exceptions
Error bodies can contain stack traces, account data, tokens, or markup supplied by an attacker. Bound their size, redact secrets, and avoid placing a complete body in a user-facing exception. For structured logging, record the numeric status, reason phrase, request correlation ID, and a safely truncated message.
Use EnsureSuccessStatusCode for exception-only policy
When every non-2xx response should become an exception and the caller does not need the error body, the shorter branch is appropriate:
using HttpResponseMessage response = await httpClient.GetAsync(
uri,
HttpCompletionOption.ResponseHeadersRead,
cancellationToken);
response.EnsureSuccessStatusCode();
if (response.StatusCode == HttpStatusCode.NoContent)
throw new InvalidDataException("Successful response has no PDF body.");
await using Stream pdfStream = await response.Content.ReadAsStreamAsync(cancellationToken);
// Pass pdfStream to your PDF parser or converter here.EnsureSuccessStatusCode throws HttpRequestException when the status is outside 200–299. Inspect or copy the error content before calling it if that content is needed; after throwing, the response may no longer be available to your normal result path.
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Do not accidentally select a convenience method
GetStringAsync, GetStreamAsync, and GetByteArrayAsync do not return an HttpResponseMessage. They implicitly enforce a successful status and throw for non-2xx responses. That behavior is useful when you want a simple exception policy, but it prevents custom handling of an error body and status unless you catch the exception and recover elsewhere. Choose GetAsync or SendAsync when status-aware handling is part of the design.
Validate the body after the status check
Media type
Read response.Content.Headers.ContentType?.MediaType and normally require application/pdf. Some legacy endpoints omit the header or send a vendor type, so make the policy explicit: strict rejection for a controlled API, or signature validation plus parser validation for an unreliable endpoint.
Signature and parser validation
For byte-oriented code, inspect the first five bytes for %PDF- before conversion. This catches common cases where a proxy returns HTML with a 200 status. It does not prove that the entire file is complete or structurally valid; let the PDF library validate cross-reference tables, trailers, encryption, and page objects.
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An empty body is not a usable PDF even when the status is 200. Treat a known zero Content-Length, an empty stream, or a parser “no pages” error as a document failure. Chunked responses may not provide a length, so check the bytes actually read.
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Network failures are not HTTP error statuses
DNS failures, connection refusal, TLS errors, cancellation, and timeouts can throw before an HTTP response exists. There is then no status code or error body to inspect. Catch HttpRequestException, TaskCanceledException (while distinguishing caller cancellation from a configured timeout), and PDF-specific exceptions separately so diagnostics identify the failing layer.
try
{
await DownloadPdfAsync(client, uri, cancellationToken);
}
catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
{
// The caller cancelled; do not retry automatically.
throw;
}
catch (HttpRequestException ex)
{
// Transport failure or an HTTP status rejected by your branch.
logger.LogWarning(ex, "Document HTTP request failed for {Uri}", uri);
throw;
}
catch (InvalidDataException ex)
{
// A successful HTTP response was not a usable PDF.
logger.LogError(ex, "Document payload was not a valid PDF");
throw;
}Retries: only retry the right failures
Retry policy belongs around the HTTP operation, not around PDF conversion. A transient 408, 429, or selected 5xx response may be retryable according to the service contract; a 400, 401, 403, 404, or a deterministic malformed PDF generally is not. Honor Retry-After when supplied, cap attempts and total delay, use jitter, and ensure the request is safe to repeat. Do not retry caller cancellation. If the server returned a non-PDF 2xx body, fix the upstream contract or authentication rather than repeatedly parsing it.
Resource lifetime and buffering choices
- Dispose every
HttpResponseMessage. Ausingdeclaration keeps the connection lifecycle clear. - Dispose or asynchronously dispose response streams after the PDF library finishes.
- Use
ReadAsByteArrayAsyncfor small documents or APIs that require a byte array; use streaming for large files. - Set a maximum acceptable document size before buffering. A declared length is only a hint, so enforce limits while copying streamed data.
- Reuse a configured
HttpClientrather than creating one per request; configure its timeout, decompression, proxy, and handler policies centrally.
Common symptoms and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| PDF parser reports invalid header | HTML, JSON, or a login page was sent to it | Keep the response, branch on status, inspect media type, and check %PDF-. |
Only GetStreamAsync throws |
That convenience method enforces 2xx status | Switch to GetAsync with ResponseHeadersRead for custom error handling. |
| Error details disappear | EnsureSuccessStatusCode ran before reading content |
Read and safely bound the error body first, or return a structured failure. |
| 200 response has no pages | Empty, truncated, or non-PDF body despite a successful status | Check bytes read, signature, content type, and parser diagnostics. |
| Requests hang or cancel | Timeout, stalled server, or caller cancellation | Set an explicit timeout, pass a cancellation token, and distinguish cancellation from retryable transport errors. |
| Works in a browser but not in code | Missing cookies, authorization, user agent, or redirect assumptions | Inspect redirects and response headers; configure authentication deliberately and never log credentials. |
Testing the boundary
Unit-test the HTTP handling with a fake HttpMessageHandler that returns representative responses: 200 with valid PDF bytes, 200 with HTML, 204, 400 with JSON, 500 with a large body, and a thrown timeout. Assert that the converter is called only for an acceptable success payload, that error text is bounded, and that status codes are preserved. Integration tests should also cover redirects, authentication expiry, chunked transfer, cancellation, and a genuinely malformed PDF.
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Recommended decision rule
Use an explicit IsSuccessStatusCode branch when status distinctions, error content, custom results, or retry decisions matter. Use EnsureSuccessStatusCode when non-2xx responses should simply become exceptions. In both cases, keep PDF conversion behind the success boundary and validate the returned bytes as a separate document check.
Frequently Asked Questions
Does a 2xx status guarantee that the body is a PDF?
No. It only reports HTTP success. Check the media type and, when useful, the %PDF- signature, then let the PDF parser validate the document structure.
Should I retry a PDF conversion exception?
Usually no. Retry the HTTP request only for failures your service identifies as transient; a malformed or non-PDF body normally requires correcting the request or upstream response.
When should I use ResponseHeadersRead?
Use it when you want to process large responses as streams and avoid buffering the complete body before your status and content checks.
Quick Recap
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