HttpWebRequest.IfModifiedSince 属性

定义

命名空间:

System.Net

程序集:

System.Net.Requests.dll

Source:

HttpWebRequest.cs

获取或设置 If-Modified-Since HTTP 标头的值。

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

DateTime

包含 If-Modified-Since HTTP 标头内容的 DateTime。

示例

下面的代码示例检查 IfModifiedSince 属性。

// 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

谨慎

WebRequest、HttpWebRequest、ServicePoint和 WebClient 已过时，不应将其用于新开发。 请改用 HttpClient。

如果 If-Modified-Since 标头 null，则返回值将设置为 DateTime.MinValue。

IfModifiedSince 属性是一个标准 System.DateTime 对象，可以包含 DateTimeKind.Local、DateTimeKind.Utc或 DateTimeKind.Unspecified的 System.DateTimeKind 字段。 使用 IfModifiedSince 属性时，可以设置任何类型的时间。 如果设置或检索 DateTimeKind.Unspecified，则假定 IfModifiedSince 属性 DateTimeKind.Local（本地时间）。

System.Net 命名空间中的类始终使用 GMT （Utc） 格式以标准形式在传输过程中将其写出网络上 IfModifiedSince 属性。

如果 IfModifiedSince 属性设置为 DateTime.MinValue，则会从 Headers 属性和 WebHeaderCollection中删除 If-Modified-Since HTTP 标头。

如果 IfModifiedSince 属性 DateTime.MinValue，则表示 Headers 属性和 WebHeaderCollection中不包含 If-Modified-Since HTTP 标头。

注意

此属性的值存储在 WebHeaderCollection中。 如果设置了 WebHeaderCollection，则属性值将丢失。

如果设置了 IfModifiedSince 属性并返回 304（未修改）状态代码，则 BeginGetResponse、GetResponse和 EndGetResponse 方法将引发 System.Net.WebException。