Type t = typeof(HttpWebRequest);
object[] args = new object[3];
req = (HttpWebRequest)HttpWebRequest.Create(txtURL.Text);
args[0] = "bytes";
args[1]=start.ToString();
args[2]=end.ToString();
MethodInfo[] mi = t.GetMethods(BindingFlags.Instance | BindingFlags.NonPublic);
for (MethodI = 0; MethodI < mi.Length; MethodI++)
{
       if (mi[MethodI].Name == "AddRange") break;
}
mi[MethodI].Invoke(req, args);

public delegate void _AddRange(HttpWebRequest httpWebRequest, string rangeSpecifier, string from, string to);
static public _AddRange delegateAddRange;

static AddRangeCodeGenerator()
{
     Type stringType = typeof(string);
     DynamicMethod dynamicMethod = new DynamicMethod("AddRange", typeof(void), new Type[] {
                typeof (HttpWebRequest), stringType, stringType, stringType },
                typeof(AddRangeCodeGenerator), true);
     ILGenerator ilGenerator = dynamicMethod.GetILGenerator();

ilGenerator.Emit(OpCodes.Ldarg_0);
     ilGenerator.Emit(OpCodes.Ldarg_1);
     ilGenerator.Emit(OpCodes.Ldarg_2);
     ilGenerator.Emit(OpCodes.Ldarg_3);
     ilGenerator.Emit(OpCodes.Call, typeof(System.Net.HttpWebRequest).GetMethod("AddRange", 
                BindingFlags.Instance | BindingFlags.NonPublic));
     ilGenerator.Emit(OpCodes.Pop);
     ilGenerator.Emit(OpCodes.Ret);

delegateAddRange = dynamicMethod.CreateDelegate(typeof(_AddRange)) as _AddRange;
}

AddRange的实现方法如下：

private bool AddRange(string rangeSpecifier, string from, string to)
{
    string str = this._HttpRequestHeaders["Range"];
    if ((str == null) || (str.Length == 0))
    {
        str = rangeSpecifier + "=";
    }
    else
    {
        if (string.Compare(str.Substring(0, str.IndexOf('=')), rangeSpecifier, 5) != 0)
        {
            return false;
        }
        str = string.Empty;
    }
    str = str + from.ToString();
    if (to != null)
    {
        str = str + "-" + to;
    }
    this._HttpRequestHeaders.SetAddVerified("Range", str);
    return true;
}

这个方法，实现设置HTTP 的Range 标头。具体规范参考HTTP1.1协议规范Header Field Definitions

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

rangeSpecifier：指的是规范中的bytes-unit，一般是bytes
比如：从第1024字节开始下载，则请求报文如下

Range:bytes=1024-

一般用在断点续传和多线程下载。一些多线程下载的软件你可以看到分块，每个分块从不同的位置下载