HttpWebRequest.IfModifiedSince 属性

定义

获取或设置 If-Modified-Since HTTP 标头的值。Gets or sets the value of the If-Modified-Since HTTP header.

public:
 property DateTime IfModifiedSince { DateTime get(); void set(DateTime value); };
public DateTime IfModifiedSince { get; set; }
member this.IfModifiedSince : DateTime with get, set
Public Property IfModifiedSince As DateTime

属性值

包含 HTTP If-Modified-Since 标头内容的 DateTimeA DateTime that contains the contents of the If-Modified-Since HTTP header.

示例

下面的代码示例检查 IfModifiedSince 属性。The following code example checks the IfModifiedSince property.

// Create a new 'Uri' object with the mentioned string.
Uri^ myUri = gcnew Uri( "http://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 targetDate = DateTime::Now;
// Set a target date of a week ago
targetDate.AddDays(-7.0);
myHttpWebRequest->IfModifiedSince = targetDate;

try
{
     
  // 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();
}
catch ( WebException^ e )
{
  if (e->Response)
  {
    if ( ((HttpWebResponse ^)e->Response)->StatusCode == HttpStatusCode::NotModified)
      Console::WriteLine("\nThe page has not been modified since {0}", targetDate);
    else
      Console::WriteLine("\nUnexpected status code = {0}", ((HttpWebResponse ^)e->Response)->StatusCode);  
  }
  else
    Console::WriteLine("\nUnexpected Web Exception {0}" + e->Message); 
}
// Create a new 'Uri' object with the mentioned string.
Uri myUri =new Uri("http://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 targetDate = DateTime.Now;
// Set a target date of a week ago
targetDate.AddDays(-7.0);
myHttpWebRequest.IfModifiedSince = targetDate;

try   
{
  // Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
  HttpWebResponse myHttpWebResponse=(HttpWebResponse)myHttpWebRequest.GetResponse();
  Console.WriteLine("Response headers for recently modified page\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();
}
catch(WebException e)
{
  if (e.Response != null)
  {
    if ( ((HttpWebResponse)e.Response).StatusCode == HttpStatusCode.NotModified)
      Console.WriteLine("\nThe page has not been modified since "+targetDate);
    else
      Console.WriteLine("\nUnexpected status code = " + ((HttpWebResponse)e.Response).StatusCode);  
  }
  else
            {
                Console.WriteLine("\nUnexpected Web Exception " + e.Message);
            }
        }
' Create a new 'Uri' object with the mentioned string.
Dim myUri As New Uri("http://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 targetDate As DateTime = DateTime.Now
targetDate.AddDays(-7.0)
myHttpWebRequest.IfModifiedSince = targetDate

Try
  ' Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
  Dim myHttpWebResponse As HttpWebResponse = CType(myHttpWebRequest.GetResponse(), HttpWebResponse)
  Console.WriteLine("Response headers for recently modified page" + 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()
Catch e As WebException
  If e.Response IsNot Nothing
  
    If CType(e.Response,HttpWebResponse).StatusCode = HttpStatusCode.NotModified
      Console.WriteLine((ControlChars.Cr + "The page has not been modified since " + targetDate))
    Else
      Console.WriteLine(ControlChars.Cr + "Unexpected status code = " + Ctype(e.Response,HttpWebResponse).StatusCode)
    End If
  Else
    Console.WriteLine(ControlChars.Cr + "Unexpected Web Exception " + e.Message) 
  End If
End Try  

注解

如果 null``If-Modified-Since 标题,则返回值将设置为 "DateTime.MinValue"。If the If-Modified-Since header is null, then the return value will be set to DateTime.MinValue.

IfModifiedSince 属性是标准 System.DateTime 对象,可包含 DateTimeKind.LocalDateTimeKind.UtcDateTimeKind.UnspecifiedSystem.DateTimeKind 字段。The IfModifiedSince property is a standard System.DateTime object and can contain a System.DateTimeKind field of DateTimeKind.Local, DateTimeKind.Utc, or DateTimeKind.Unspecified. 使用 IfModifiedSince 属性时,可以设置任意类型的时间。Any kind of time can be set when using the IfModifiedSince property. 如果设置或检索 DateTimeKind.Unspecified,则假定 IfModifiedSince 属性为 DateTimeKind.Local (本地时间)。If DateTimeKind.Unspecified is set or retrieved, the IfModifiedSince property is assumed to be DateTimeKind.Local (local time).

System.Net 命名空间中的类始终会在传输时,使用 GMT (Utc)格式将其写出网络上的 IfModifiedSince 属性。The classes in the System.Net namespace always write it out the IfModifiedSince property on the wire during transmission in standard form using GMT (Utc) format.

如果 IfModifiedSince 属性设置为 DateTime.MinValue,则从 Headers 属性和 WebHeaderCollection中删除 If-Modified-Since HTTP 标头。If the IfModifiedSince property is set to DateTime.MinValue, then the If-Modified-Since HTTP header is removed from the Headers property and the WebHeaderCollection.

如果 DateTime.MinValueIfModifiedSince 属性,则表示 If-Modified-Since 的 HTTP 标头未包含在 Headers 属性和 WebHeaderCollection中。If the IfModifiedSince property is DateTime.MinValue, this indicates that the If-Modified-Since HTTP header is not included in the Headers property and the WebHeaderCollection.

备注

此属性的值存储在 WebHeaderCollection 中。The value for this property is stored in WebHeaderCollection. 如果设置了 WebHeaderCollection,则该属性值将丢失。If WebHeaderCollection is set, the property value is lost.

如果设置了 IfModifiedSince 属性并且返回了304(未修改)状态代码,则 BeginGetResponseGetResponseEndGetResponse 方法将引发 System.Net.WebExceptionIf the IfModifiedSince property is set and 304 (Not Modified) status code is returned, then a System.Net.WebException will be thrown by the BeginGetResponse, GetResponse, and EndGetResponse methods.

适用于