How i finding Internate network error using Try..........Catch in iphone?

Question

Hi, I developing an application in which i calling web services on iphone. I want to implement Try.......Catch in that code for catching internet and GPRS connection error.The code is as follow,

NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];
[request setURL:[NSURL URLWithString:mainURL5]];
[request setHTTPMethod:@"POST"];

NSHTTPURLResponse* urlResponse = nil;  
NSError *error = [[[NSError alloc] init] autorelease];  
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error];  
result5 = [[NSMutableString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];  

I was using Try......catch but it didn't work that code as follows,

@try
{
//prepar request
NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];
[request setURL:[NSURL URLWithString:mainURL5]];
[request setHTTPMethod:@"POST"];

NSHTTPURLResponse* urlResponse = nil;  
NSError *error = [[[NSError alloc] init] autorelease];  
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error];  
result5 = [[NSMutableString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
}
@catch(NSException * e)
{
    [clsMessageBox ShowMessageOK:@"some text" :[e reason]];
}

Answer 1

In Objective-C exceptions are different from errors. Try this:

NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];
[request setURL:[NSURL URLWithString:mainURL5]];
[request setHTTPMethod:@"POST"];

NSHTTPURLResponse* urlResponse = nil;  
NSError *error = nil;
NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error];  
if (error) {
    [clsMessageBox ShowMessageOK:@"ERROR" :[error description]];
} else {
    result5 = [[NSMutableString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
}

edit: also, unless you are doing this from a background thread, -[NSURLConnection sendSynchronousRequest:returningResponse:error:] is not recommended.

Answer 2

This method doesn't throw exceptions, so using try/catch is not going to help you. After the method returns, if responseData is nil, you have an error and you should check the value of error. You may also have an HTTP error, which will be part of the response data. Also, there is no reason to initialize error to some empty error object. Just initialize it to nil.

So:

NSError* error = nil;
NSData* responseData = nil;
if( !(responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&urlResponse error:&error]) ) {
  NSLog(@"Oops: %@", error);
}