我有一个案例,Web API调用返回一个非常大的字符串响应。我打电话如下:
var multipartContent = new MultipartFormDataContent();
multipartContent.Add(new ByteArrayContent(blobStream.CopyToBytes()),
"upload", Path.GetFileName(fileName));
var response = await _httpClient.PostAsync("api/v1/textResponse", multipartContent);
int responeLength = response.Content.Headers.ContentLength.HasValue ?
(int)response.Content.Headers.ContentLength.Value : -1;
response.EnsureSuccessStatusCode();
我只需要处理来自响应的第一个1Mb数据,所以如果响应小于1Mb,我会读取所有数据,但如果它更多,我会很难停止读取1Mb。
我正在寻找最有效的方法来做这个阅读。我试过这段代码:
// section above...
response.EnsureSuccessStatusCode();
string contentText = null;
if (responeLength < maxAllowedLimit) // 1Mb
{
// less then limit - read all as string.
contentText = await response.Content.ReadAsStringAsync();
}
else {
var contentStream = await response.Content.ReadAsStreamAsync();
using (var stream = new MemoryStream())
{
byte[] buffer = new byte[5120]; // read in chunks of 5KB
int bytesRead;
while((bytesRead = contentStream.Read(buffer, 0, buffer.Length)) > 0)
{
stream.Write(buffer, 0, bytesRead);
}
contentText = stream.ConvertToString();
}
}
这是最有效的方式,我如何限制读取的数量(其他)。我试过这段代码,它总是返回一个空字符串。还有:
ReadAsStringAsync()
ReadAsByteArrayAsync()
ReadAsStreamAsync()
LoadIntoBufferAsync(int size)
这些方法中的任何一种都更有效吗?
提前感谢任何指针!
我怀疑这样做最有效(但仍然正确)的方式可能是这样的。由于您对读取的字节数有限制,而不是字符数,因此更加复杂,因此我们无法使用StreamReader
。请注意,我们必须注意不要在代码点中间停止读取 - 在许多情况下,使用多个字节表示单个字符,并且在中途停止将是一个错误。
const int bufferSize = 1024;
var bytes = new byte[bufferSize];
var chars = new char[Encoding.UTF8.GetMaxCharCount(bufferSize)];
var decoder = Encoding.UTF8.GetDecoder();
// We don't know how long the result will be in chars, but one byte per char is a
// reasonable first approximation. This will expand as necessary.
var result = new StringBuilder(maxAllowedLimit);
int totalReadBytes = 0;
using (var stream = await response.Content.ReadAsStreamAsync())
{
while (totalReadBytes <= maxAllowedLimit)
{
int readBytes = await stream.ReadAsync(
bytes,
0,
Math.Min(maxAllowedLimit - totalReadBytes, bytes.Length));
// We reached the end of the stream
if (readBytes == 0)
break;
totalReadBytes += readBytes;
int readChars = decoder.GetChars(bytes, 0, readBytes, chars, 0);
result.Append(chars, 0, readChars);
}
}
请注意,你可能想要使用HttpCompletionOption.ResponseHeadersRead
,否则HttpClient
会去下载整个身体。
如果你很乐意限制人物的数量,那么生活会更容易:
string result;
using (var reader = new StreamReader(await response.Content.ReadAsStreamAsync()))
{
char[] chars = new char[maxAllowedLimit];
int read = reader.ReadBlock(chars, 0, chars.Length);
result = new string(chars, 0, read);
}