方法一：网络地址存在，有可能可以访问，也有可能不能访问。此方法用来判断地址存在。
static bool urlisexist(string url)
{
system.uri u = null;
try
{
u = new uri(url);
}
catch { return false; }
bool isexist = false;
system.net.httpwebrequest r = system.net.httpwebrequest.create(u) as system.net.httpwebrequest;
r.method = "head";
try
{
system.net.httpwebresponse s = r.getresponse() as system.net.httpwebresponse;
if (s.statuscode == system.net.httpstatuscode.ok)
{
isexist = true;
}
}
catch (system.net.webexception x)
{
try
{
isexist = ((x.response as system.net.httpwebresponse).statuscode != system.net.httpstatuscode.notfound);
}
catch { isexist = (x.status == system.net.webexceptionstatus.success); }
}
return isexist;
} 方法二：地址是否有效，即是网络连接地址是否正常显示。地址有可能存在，也有可能不存在，能正常显示则说明地址是有效的，反正则是无效的。
static bool urlisexist(string url)
{
try
{
system.net.webrequest request = system.net.webrequest.create(url);
request.timeout = 10000;
system.net.webresponse response = request.getresponse();
return true;
}
catch
{
return false;
}
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