C# 0 The Complete Reference

HttpWebRequest

and

HttpWebResponse

.

Handling Network Errors

Although the program in the preceding section is correct, it is not resilient. Even the 

simplest network error will cause it to end abruptly. Although this isn’t a problem for the 

example programs shown in this chapter, it is something that must be avoided in real-world 

applications. To fully handle all network exceptions that the program might generate, you 

must monitor calls to 

Create( )

,

GetResponse( )

, and 

GetResponseStream( )

. It is important 

to understand that the exceptions that can be generated depend upon the protocol being 

used. The following discussion describes the errors possible when using HTTP.

Exceptions Generated by Create( )

The

Create( )

 method defined by 

WebRequest

 can generate four exceptions. If the protocol 

specified by the URI prefix is not supported, then 

NotSupportedException

 is thrown. If the 

URI format is invalid, 

UriFormatException

 is thrown. If the user does not have the proper 

authorization, a 

System.Security.SecurityException

 will be thrown. 

Create( )

 can also 

throw an 

ArgumentNullException

 if it is called with a null reference, but this is not an 

error generated by networking.

Exceptions Generated by GetReponse( )

A number of errors can occur when obtaining an HTTP response by calling 

GetResponse( )

.

These are represented by the following exceptions: 

InvalidOperationException

,

ProtocolViolationException

,

NotSupportedException

, and 

WebException

. Of these, 

the one of most interest is 

WebException

.

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