HttpWebRequestheader设置

在HttpWebRequest中，有一些header为限制级的，对于这些header，你不能直接来通过headers.Add方法来添加，比如Host、Connection、User-Agent、Referer、Range、Content-Type、Content-Length、Expect、Proxy-Connection、If-Modified-Since等header. 这些header都是通过属性来设置的 。

添加的具体步骤如下

1，测试是否能够为请求设置指定的 HTTP 标头；

WebHeaderCollection.IsRestricted(string headerName, bool response);

2, 添加头字符串；

WebHeaderCollection headers = request.Headers;
headers.Add("Accept-Language: en-us;q=0.5,en;q=0.3");

参考源码：

internal class HeaderInfo
{
    // Fields
    internal readonly bool AllowMultiValues;
    internal readonly string HeaderName;
    internal readonly bool IsRequestRestricted;
    internal readonly bool IsResponseRestricted;
    internal readonly HeaderParser Parser;

    // Methods
    internal HeaderInfo(string name, bool requestRestricted, bool responseRestricted, bool multi, HeaderParser p)
    {
        this.HeaderName = name;
        this.IsRequestRestricted = requestRestricted;
        this.IsResponseRestricted = responseRestricted;
        this.Parser = p;
        this.AllowMultiValues = multi;
    }
}


internal class HeaderInfoTable
{
    // Fields
    private static Hashtable HeaderHashTable;
    private static HeaderParser MultiParser = new HeaderParser(HeaderInfoTable.ParseMultiValue);
    private static HeaderParser SingleParser = new HeaderParser(HeaderInfoTable.ParseSingleValue);
    private static HeaderInfo UnknownHeaderInfo = new HeaderInfo(string.Empty, false, false, false, SingleParser);

    // Methods
    static HeaderInfoTable()
    {
        HeaderInfo[] infoArray = new HeaderInfo[] { 
            new HeaderInfo("Age", false, false, false, SingleParser),
            new HeaderInfo("Allow", false, false, true, MultiParser),
            new HeaderInfo("Accept", true, false, true, MultiParser), 
            new HeaderInfo("Authorization", false, false, true, MultiParser), 
            new HeaderInfo("Accept-Ranges", false, false, true, MultiParser), 
            new HeaderInfo("Accept-Charset", false, false, true, MultiParser),
            new HeaderInfo("Accept-Encoding", false, false, true, MultiParser), 
            new HeaderInfo("Accept-Language", false, false, true, MultiParser),
            new HeaderInfo("Cookie", false, false, true, MultiParser), 
            new HeaderInfo("Connection", true, false, true, MultiParser), 
            new HeaderInfo("Content-MD5", false, false, false, SingleParser), 
            new HeaderInfo("Content-Type", true, false, false, SingleParser),
            new HeaderInfo("Cache-Control", false, false, true, MultiParser), 
            new HeaderInfo("Content-Range", false, false, false, SingleParser),
            new HeaderInfo("Content-Length", true, true, false, SingleParser), 
            new HeaderInfo("Content-Encoding", false, false, true, MultiParser), 
            new HeaderInfo("Content-Language", false, false, true, MultiParser), 
            new HeaderInfo("Content-Location", false, false, false, SingleParser), 
            new HeaderInfo("Date", true, false, false, SingleParser), 
            new HeaderInfo("ETag", false, false, false, SingleParser),
            new HeaderInfo("Expect", true, false, true, MultiParser),
            new HeaderInfo("Expires", false, false, false, SingleParser),
            new HeaderInfo("From", false, false, false, SingleParser), 
            new HeaderInfo("Host", true, false, false, SingleParser), 
            new HeaderInfo("If-Match", false, false, true, MultiParser),
            new HeaderInfo("If-Range", false, false, false, SingleParser),
            new HeaderInfo("If-None-Match", false, false, true, MultiParser),
            new HeaderInfo("If-Modified-Since", true, false, false, SingleParser), 
            new HeaderInfo("If-Unmodified-Since", false, false, false, SingleParser),
            new HeaderInfo("Keep-Alive", false, true, false, SingleParser), 
            new HeaderInfo("Location", false, false, false, SingleParser), 
            new HeaderInfo("Last-Modified", false, false, false, SingleParser), 
            new HeaderInfo("Max-Forwards", false, false, false, SingleParser), 
            new HeaderInfo("Pragma", false, false, true, MultiParser), 
            new HeaderInfo("Proxy-Authenticate", false, false, true, MultiParser), 
            new HeaderInfo("Proxy-Authorization", false, false, true, MultiParser), 
            new HeaderInfo("Proxy-Connection", true, false, true, MultiParser), 
            new HeaderInfo("Range", true, false, true, MultiParser), 
            new HeaderInfo("Referer", true, false, false, SingleParser), 
            new HeaderInfo("Retry-After", false, false, false, SingleParser), 
            new HeaderInfo("Server", false, false, false, SingleParser), 
            new HeaderInfo("Set-Cookie", false, false, true, MultiParser),
            new HeaderInfo("Set-Cookie2", false, false, true, MultiParser), 
            new HeaderInfo("TE", false, false, true, MultiParser), 
            new HeaderInfo("Trailer", false, false, true, MultiParser), 
            new HeaderInfo("Transfer-Encoding", true, true, true, MultiParser), 
            new HeaderInfo("Upgrade", false, false, true, MultiParser), 
            new HeaderInfo("User-Agent", true, false, false, SingleParser), 
            new HeaderInfo("Via", false, false, true, MultiParser), 
            new HeaderInfo("Vary", false, false, true, MultiParser),
            new HeaderInfo("Warning", false, false, true, MultiParser), 
            new HeaderInfo("WWW-Authenticate", false, true, true, SingleParser)
         };
        HeaderHashTable = new Hashtable(infoArray.Length * 2, CaseInsensitiveAscii.StaticInstance);
        for (int i = 0; i < infoArray.Length; i++)
        {
            HeaderHashTable[infoArray[i].HeaderName] = infoArray[i];
        }
    }

    private static string[] ParseMultiValue(string value)
    {
        StringCollection strings = new StringCollection();
        bool flag = false;
        int length = 0;
        char[] chArray = new char[value.Length];
        for (int i = 0; i < value.Length; i++)
        {
            if (value[i] == '"')
            {
                flag = !flag;
            }
            else if ((value[i] == ',') && !flag)
            {
                string str = new string(chArray, 0, length);
                strings.Add(str.Trim());
                length = 0;
                continue;
            }
            chArray[length++] = value[i];
        }
        if (length != 0)
        {
            strings.Add(new string(chArray, 0, length).Trim());
        }
        string[] array = new string[strings.Count];
        strings.CopyTo(array, 0);
        return array;
    }

    private static string[] ParseSingleValue(string value)
    {
        return new string[] { value };
    }

    // Properties
    internal HeaderInfo this[string name]
    {
        get
        {
            HeaderInfo info = (HeaderInfo) HeaderHashTable[name];
            if (info == null)
            {
                return UnknownHeaderInfo;
            }
            return info;
        }
    }
}

 
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