HttpWebRequest.IfModifiedSince 属性
获取或设置 If-Modified-Since HTTP 标头的值。
**命名空间:**System.Net
**程序集:**System（在 system.dll 中）
语法
声明
Public Property IfModifiedSince As DateTime
用法
Dim instance As HttpWebRequest
Dim value As DateTime
value = instance.IfModifiedSince
instance.IfModifiedSince = value
public DateTime IfModifiedSince { get; set; }
public:
property DateTime IfModifiedSince {
DateTime get ();
void set (DateTime value);
}
/** @property */
public DateTime get_IfModifiedSince ()
/** @property */
public void set_IfModifiedSince (DateTime value)
public function get IfModifiedSince () : DateTime
public function set IfModifiedSince (value : DateTime)
属性值
包含 If-Modified-SinceHTTP 标头内容的 DateTime。默认值是当前日期和时间。
备注
IfModifiedSince 属性被假定为本地时间。
提示
此属性的值存储在 WebHeaderCollection 中。如果设置了 WebHeaderCollection，则该属性值将丢失。
示例
下面的代码示例检查 IfModifiedSince 属性。
' Create a new 'Uri' object with the mentioned string.
Dim myUri As New Uri("https://www.contoso.com")
' Create a new 'HttpWebRequest' object with the above 'Uri' object.
Dim myHttpWebRequest As HttpWebRequest = CType(WebRequest.Create(myUri), HttpWebRequest)
' Create a new 'DateTime' object.
Dim today As DateTime = DateTime.Now
If DateTime.Compare(today, myHttpWebRequest.IfModifiedSince) = 0 Then
' Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
Dim myHttpWebResponse As HttpWebResponse = CType(myHttpWebRequest.GetResponse(), HttpWebResponse)
Console.WriteLine("Response headers " + ControlChars.Cr + "{0}" + ControlChars.Cr, myHttpWebResponse.Headers)
Dim streamResponse As Stream = myHttpWebResponse.GetResponseStream()
Dim streamRead As New StreamReader(streamResponse)
Dim readBuff(256) As [Char]
Dim count As Integer = streamRead.Read(readBuff, 0, 256)
Console.WriteLine(ControlChars.Cr + "The contents of Html Page are : " + ControlChars.Cr)
While count > 0
Dim outputData As New [String](readBuff, 0, count)
Console.Write(outputData)
count = streamRead.Read(readBuff, 0, 256)
End While
' Close the Stream object.
streamResponse.Close()
streamRead.Close()
' Release the HttpWebResponse Resource.
myHttpWebResponse.Close()
Console.WriteLine(ControlChars.Cr + "Press 'Enter' key to continue.................")
Console.Read()
Else
Console.WriteLine((ControlChars.Cr + "The page has been modified since " + today))
End If
// Create a new 'Uri' object with the mentioned string.
Uri myUri =new Uri("https://www.contoso.com");
// Create a new 'HttpWebRequest' object with the above 'Uri' object.
HttpWebRequest myHttpWebRequest= (HttpWebRequest)WebRequest.Create(myUri);
// Create a new 'DateTime' object.
DateTime today= DateTime.Now;
if (DateTime.Compare(today,myHttpWebRequest.IfModifiedSince)==0)
{
// Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
HttpWebResponse myHttpWebResponse=(HttpWebResponse)myHttpWebRequest.GetResponse();
Console.WriteLine("Response headers \n{0}\n",myHttpWebResponse.Headers);
Stream streamResponse=myHttpWebResponse.GetResponseStream();
StreamReader streamRead = new StreamReader( streamResponse );
Char[] readBuff = new Char[256];
int count = streamRead.Read( readBuff, 0, 256 );
Console.WriteLine("\nThe contents of Html Page are : \n");
while (count > 0)
{
String outputData = new String(readBuff, 0, count);
Console.Write(outputData);
count = streamRead.Read(readBuff, 0, 256);
}
// Close the Stream object.
streamResponse.Close();
streamRead.Close();
// Release the HttpWebResponse Resource.
myHttpWebResponse.Close();
Console.WriteLine("\nPress 'Enter' key to continue.................");
Console.Read();
}
else
{
Console.WriteLine("\nThe page has been modified since "+today);
}
// Create a new 'Uri' object with the mentioned string.
Uri^ myUri = gcnew Uri( "https://www.contoso.com" );
// Create a new 'HttpWebRequest' object with the above 'Uri' object.
HttpWebRequest^ myHttpWebRequest = dynamic_cast<HttpWebRequest^>(WebRequest::Create( myUri ));
// Create a new 'DateTime' object.
DateTime today = DateTime::Now;
if ( DateTime::Compare( today, myHttpWebRequest->IfModifiedSince ) == 0 )
{
// Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
HttpWebResponse^ myHttpWebResponse = dynamic_cast<HttpWebResponse^>(myHttpWebRequest->GetResponse());
Console::WriteLine( "Response headers \n {0}\n", myHttpWebResponse->Headers );
Stream^ streamResponse = myHttpWebResponse->GetResponseStream();
StreamReader^ streamRead = gcnew StreamReader( streamResponse );
array<Char>^readBuff = gcnew array<Char>(256);
int count = streamRead->Read( readBuff, 0, 256 );
Console::WriteLine( "\nThe contents of Html Page are : \n" );
while ( count > 0 )
{
String^ outputData = gcnew String( readBuff,0,count );
Console::Write( outputData );
count = streamRead->Read( readBuff, 0, 256 );
}
streamResponse->Close();
streamRead->Close();
// Release the HttpWebResponse Resource.
myHttpWebResponse->Close();
Console::WriteLine( "\nPress 'Enter' key to continue................." );
Console::Read();
}
else
{
Console::WriteLine( "\nThe page has been modified since {0}", today );
}
}
catch ( WebException^ e )
{
Console::WriteLine( "\nWebException Caught!" );
Console::WriteLine( "Source : {0}", e->Source );
Console::WriteLine( "Message : {0}", e->Message );
}
catch ( Exception^ e )
{
Console::WriteLine( "\nException raised!" );
Console::WriteLine( "Source : {0}", e->Source );
Console::WriteLine( "Message : {0}", e->Message );
}
}
// Create a new 'Uri' object with the mentioned string.
Uri myUri = new Uri("https://www.contoso.com");
//Create a new 'HttpWebRequest' object with the above 'Uri' object.
HttpWebRequest myHttpWebRequest = (HttpWebRequest)
(WebRequest.Create(myUri));
// Create a new 'DateTime' object.
DateTime today = DateTime.get_Now();
if (DateTime.Compare(today,
myHttpWebRequest.get_IfModifiedSince()) == 0) {
// Assign the response object of 'HttpWebRequest' to a
// 'HttpWebResponse' variable.
HttpWebResponse myHttpWebResponse = (HttpWebResponse)
(myHttpWebRequest.GetResponse());
Console.WriteLine("Response headers \n{0}\n",
myHttpWebResponse.get_Headers());
Stream streamResponse = myHttpWebResponse.GetResponseStream();
StreamReader streamRead = new StreamReader(streamResponse);
char readBuff[] = new char[256];
int count = streamRead.Read(readBuff, 0, 256);
Console.WriteLine("\nThe contents of Html Page are : \n");
while (count > 0) {
String outputData = new String(readBuff, 0, count);
Console.Write(outputData);
count = streamRead.Read(readBuff, 0, 256);
}
// Close the Stream object.
streamResponse.Close();
streamRead.Close();
// Release the HttpWebResponse Resource.
myHttpWebResponse.Close();
Console.WriteLine("\nPress 'Enter' key to continue"
+ ".................");
Console.Read();
}
else {
Console.WriteLine("\nThe page has been modified since "
+ today);
}
平台
Windows 98、Windows 2000 SP4、Windows CE、Windows Millennium Edition、Windows Mobile for Pocket PC、Windows Mobile for Smartphone、Windows Server 2003、Windows XP Media Center Edition、Windows XP Professional x64 Edition、Windows XP SP2、Windows XP Starter Edition
.NET Framework 并不是对每个平台的所有版本都提供支持。有关受支持版本的列表，请参见系统要求。
版本信息
.NET Framework
受以下版本支持：2.0、1.1、1.0
.NET Compact Framework
受以下版本支持：2.0、1.0