如何限制asp.net中上传文件的大小的代码

<system.web><httpRuntime executionTimeout="9999" maxRequestLength="2097151"/></system.web>

iserviceprovider provider = (iserviceprovider) httpcontext.current;
httpworkerrequest wr = (httpworkerrequest) provider.getservice(typeof(httpworkerrequest));
byte[] bs = wr.getpreloadedentitybody();
....
if (!wr.isentireentitybodyispreloaded())
{
int n = 1024;
byte[] bs2 = new byte[n];
while (wr.readentitybody(bs2,n) >0)
{
.....
}
}