blocks|key|1289335|text|是的，那是因为ASCII只有7位--它没有定义任何超过127的值。编码通常将未知的二进制值解码为“？”(尽管这可以使用DecoderFallback进行更改)。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1289336|如果你要提到“扩展ASCII”，我猜你实际上想要的是Encoding.Default，它是“操作系统的默认代码页”……我相信，在大多数西方系统上都有code+page+1252。|style|CODE|1289337|你期望的是什么角色？|1289338|编辑:根据公认的答案(我怀疑问题是在我添加答案后编辑的；我不记得最初看到过任何关于JPEG的内容)，你不应该将二进制数据转换为文本，除非它是真正的编码文本数据。JPEG数据是二进制数据，因此您应该检查实际字节和预期字节。|1289339|任何时候，当您使用“纯”文本编码(如ASCII、UTF-8等)将任意二进制数据(如图像、音乐或视频)转换为文本时，都有丢失数据的风险。如果您必须将其转换为文本，请使用Base64，它既好又安全。但是，如果您只想将其与预期的二进制数据进行比较，最好不要将其转换为文本。|1289340|编辑:好的，这里有一个类来帮助一个给定字节数组的图像检测方法。我还没有使它特定于HTTP；我不完全确定您是否真的应该获取InputStream，只读取其中的一小部分，然后再次获取流。我坚持使用字节数组来避免这个问题:)|1289341|using+System;
using+System.Collections.Generic;
using+System.Collections.ObjectModel;
using+System.Linq;

public+sealed+class+SignatureDetector
{
++++public+static+readonly+SignatureDetector+Png+=
++++++++new+SignatureDetector(0x89,+0x50,+0x4e,+0x47);

++++public+static+readonly+SignatureDetector+Bmp+=
++++++++new+SignatureDetector(0x42,+0x4d);

++++public+static+readonly+SignatureDetector+Gif+=
++++++++new+SignatureDetector(0x47,+0x49,+0x46);

++++public+static+readonly+SignatureDetector+Jpeg+=
++++++++new+SignatureDetector(0xff,+0xd8);

++++public+static+readonly+IEnumerable<SignatureDetector>+Images+=
++++++++new+ReadOnlyCollection<SignatureDetector>(new[]{Png,+Bmp,+Gif,+Jpeg});

++++private+readonly+byte[]+bytes;

++++public+SignatureDetector(params+byte[]+bytes)
++++{
++++++++if+(bytes+==+null)
++++++++{
++++++++++++throw+new+ArgumentNullException("bytes");
++++++++}
++++++++this.bytes+=+(byte[])+bytes.Clone();
++++}

++++public+bool+Matches(byte[]+data)
++++{
++++++++if+(data+==+null)
++++++++{
++++++++++++throw+new+ArgumentNullException("data");
++++++++}
++++++++if+(data.Length+<+bytes.Length)
++++++++{
++++++++++++return+false;
++++++++}
++++++++for+(int+i=0;+i+<+bytes.Length;+i%2B%2B)
++++++++{
++++++++++++if+(data[i]+!=+bytes[i])
++++++++++++{
++++++++++++++++return+false;
++++++++++++}
++++++++}
++++++++return+true;
++++}++++

++++//+Convenience+method
++++public+static+bool+IsImage(byte[]+data)
++++{
++++++++return+Images.Any(detector+=>+detector.Matches(data));
++++}++++++++
}|code-block|syntax|javascript|1289342|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/ASCII|1|http://msdn.microsoft.com/en-us/library/system.text.decoderfallback.aspx|2|http://en.wikipedia.org/wiki/Windows-1252^0|7|5|0|1N|F|1|0|Q|G|22|E|2|0|0|0|0|1O|B|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]|$A|1A|B|1B|1|1C]]|C|$]]|$1|D|3|E|5|6|7|1D|8|@$A|1E|B|1F|F|G]]|9|@$A|1G|B|1H|1|1I]]|C|$]]|$1|H|3|I|5|6|7|1J|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|1K|8|@]|9|@]|C|$]]|$1|L|3|M|5|6|7|1L|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|1M|8|@$A|1N|B|1O|F|G]]|9|@]|C|$]]|$1|P|3|Q|5|R|7|1P|8|@]|9|@]|C|$S|T]]|$1|U|3|-4|5|6|7|1Q|8|@]|9|@]|C|$]]]|V|$W|$5|X|Y|Z|C|$10|11]]|12|$5|X|Y|Z|C|$10|13]]|14|$5|X|Y|Z|C|$10|15]]]]

Yes, that's because <a href="http://en.wikipedia.org/wiki/ASCII" rel="noreferrer">ASCII</a> is only 7-bit - it doesn't define any values above 127. Encodings typically decode unknown binary values to '?' (although this can be changed using <a href="http://msdn.microsoft.com/en-us/library/system.text.decoderfallback.aspx" rel="noreferrer">DecoderFallback</a>).

If you're about to mention "extended ASCII" I suspect you actually want <code>Encoding.Default</code> which is "the default code page for the operating system"... <a href="http://en.wikipedia.org/wiki/Windows-1252" rel="noreferrer">code page 1252</a> on most Western systems, I believe.

What characters were you expecting?

EDIT: As per the accepted answer (I suspect the question was edited after I added my answer; I don't recall seeing anything about JPEG originally) you shouldn't convert binary data to text unless it's genuinely encoded text data. JPEG data is binary data - so you should be checking the actual bytes against the expected bytes.

Any time you convert arbitrary binary data (such as images, music or video) into text using a "plain" text encoding (such as ASCII, UTF-8 etc) you risk data loss. If you have to convert it to text, use Base64 which is nice and safe. If you just want to compare it with expected binary data, however, it's best not to convert it to text at all.

EDIT: Okay, here's a class to help image detection method for a given byte array. I haven't made it HTTP-specific; I'm not entirely sure whether you should really fetch the <code>InputStream</code>, read just a bit of it, and then fetch the stream again. I've ducked the issue by sticking to byte arrays :)

<pre><code>using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;

public sealed class SignatureDetector
{
 public static readonly SignatureDetector Png =
 new SignatureDetector(0x89, 0x50, 0x4e, 0x47);

 public static readonly SignatureDetector Bmp =
 new SignatureDetector(0x42, 0x4d);

 public static readonly SignatureDetector Gif =
 new SignatureDetector(0x47, 0x49, 0x46);

 public static readonly SignatureDetector Jpeg =
 new SignatureDetector(0xff, 0xd8);

 public static readonly IEnumerable&lt;SignatureDetector&gt; Images =
 new ReadOnlyCollection&lt;SignatureDetector&gt;(new[]{Png, Bmp, Gif, Jpeg});

 private readonly byte[] bytes;

 public SignatureDetector(params byte[] bytes)
 {
 if (bytes == null)
 {
 throw new ArgumentNullException("bytes");
 }
 this.bytes = (byte[]) bytes.Clone();
 }

 public bool Matches(byte[] data)
 {
 if (data == null)
 {
 throw new ArgumentNullException("data");
 }
 if (data.Length &lt; bytes.Length)
 {
 return false;
 }
 for (int i=0; i &lt; bytes.Length; i++)
 {
 if (data[i] != bytes[i])
 {
 return false;
 }
 }
 return true;
 } 

 // Convenience method
 public static bool IsImage(byte[] data)
 {
 return Images.Any(detector =&gt; detector.Matches(data));
 } 
}
</code></pre>

blocks|key|4069506|text|你确定“？”结果是什么？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4069507|以下结果是什么：|4069508|(int)ascii[0]
(int)ascii[1]|code-block|syntax|javascript|4069509|另一方面，纯ASCII仅为0-127+...|4069510|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Are you sure "????" is the result?

What is the result of:

<pre><code>(int)ascii[0]
(int)ascii[1]
</code></pre>

On the other hand, pure ASCII is 0-127 only...

blocks|key|851664|text|如果你随后写道：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|851665|Console.WriteLine(ascii)|code-block|syntax|javascript|851666|并期望打印出"FFD8“，但这不是GetString的工作方式。为此，您需要：|851667|+string+ascii+=+String.Format("{0:X02}{1:X02}",+header[0],+header[1]);+++|851668|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

If you then wrote:

<pre><code>Console.WriteLine(ascii)
</code></pre>

And expected "FFD8" to print out, that's not the way GetString work. For that, you would need:

<pre><code> string ascii = String.Format("{0:X02}{1:X02}", header[0], header[1]); 
</code></pre>

blocks|key|851728|text|在这种情况下，最好是比较字节数组，而不是转换为字符串。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|851729|如果您必须转换为字符串，我建议使用编码拉丁语-1又称ISO-8859-1又名代码页28591编码，因为此编码将把所有十六进制值在0-255范围内的字节映射到具有相同十六进制值的Unicode字符-这对此场景很方便。以下任一项都将获得此编码：|851730|Encoding.GetEncoding(28591)
Encoding.GetEncoding("Latin1")
Encoding.GetEncoding("ISO-8859-1")|code-block|syntax|javascript|851731|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

In this case you'd be better to compare the byte arrays rather than converting to string.

If you must convert to string, I suggest using the encoding Latin-1 aka ISO-8859-1 aka Code Page 28591 encoding, as this encoding will map all bytes with hex values are in the range 0-255 to the Unicode character with the same hex value - convenient for this scenario. Any of the following will get this encoding:

<pre><code>Encoding.GetEncoding(28591)
Encoding.GetEncoding("Latin1")
Encoding.GetEncoding("ISO-8859-1")
</code></pre>

blocks|key|4069617|text|我曾经写过一个自定义的编码器/解码器，它将字节0-255编码为unicode字符0-255，然后再编码回来。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4069618|只有在实际不是字符串的东西上使用字符串函数时，它才真正有用。|4069619|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

I once wrote a custom encoder/decoder that encoded bytes 0-255 to unicode characters 0-255 and back again.

It was only really useful for using string functions on something that isn't actually a string.

<pre><code>byte[] header = new byte[]{255, 216}; 

string ascii = Encoding.ASCII.GetString(header);
</code></pre>

I expect ASCII to be equal to be FFD8 (JPEG SOI marker)

Instead I get "????"

c# and Encoding.ASCII.GetString

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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byte[] header = new byte[]{255, 216}; string ascii =  Encoding.ASCII.GetString(header);我希望ASCII等于FFD8 (JPEG SOI标记)但我得到的是“？”

问c#和Encoding.ASCII.GetString
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