Error 400 when calling a .net service on the iOS app - How to solve it?

I managed to resolve. The problem was in the composition of the soap message. Replace the two double quotation marks ("") with a single quotation mark (').

- (BOOL)validateCode:(NSString *)code {
    NSString *soapMessage = @"<?xml version='1.0' encoding='UTF-8'?><soap:Envelope xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'><soap:Body><GetSystemStatus xmlns='http://www.cgsapi.com/'/></soap:Body></soap:Envelope>";

    NSURL *url = [NSURL URLWithString:@"http://www.cgsapi.com/CGSWebService.asmx"];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    NSURLSession *session = [NSURLSession sharedSession];
    NSError *error;

    request.HTTPMethod = @"POST";
    request.HTTPBody = [soapMessage dataUsingEncoding:NSUTF8StringEncoding];

    [request addValue:@"www.cgsapi.com" forHTTPHeaderField:@"Host"];
    [request addValue:@"text/xml; charset=utf-8" forHTTPHeaderField:@"Content-Type"];
    [request addValue:[NSString stringWithFormat:@"%i", soapMessage.length] forHTTPHeaderField:@"Content-Length"];
    [request addValue:@"http://www.cgsapi.com/GetSystemStatus" forHTTPHeaderField:@"SOAPAction"];

    NSLog(@"request: %@", request);

    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        NSLog(@"response: %@", response);
        NSString *output = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

        NSLog(@"output: %@", output);
        if (error !=nil) {
            NSLog(@"error: %i %@", error.code, error.description);
        }
    }];

    [task resume];

    return true;
}

no .h

 NSMutableData *myData; 

no .m

NSString *jsonPost = [NSString stringWithFormat:@""];
NSURL *url = [NSURL URLWithString:@"http://www.cgsapi.com/CGSWebService.asmx"];

NSData *postData = [jsonPost dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:NO];

NSString *postLength = [NSString stringWithFormat:@"%lu",(unsigned long)[postData length]];

NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
request.timeoutInterval = 7.0;

[request setURL:url];
[request setHTTPMethod:@"POST"];
[request setValue:postLength forHTTPHeaderField:@"Content-Length"];
[request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Current-Type"];
[request setValue:@"text/xml" forHTTPHeaderField:@"Content-Type"];
[request setHTTPBody:postData];
myData = [[NSMutableData alloc] initWithLength:0];
NSURLConnection *conn = [[NSURLConnection alloc]initWithRequest:request delegate:self startImmediately:YES];

    if(conn){ 
        NSLog(@"iniciando envio");
        }



- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData*)data {    
    [myData appendData:data];

}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {  
    NSLog(@"terminei! estou alocado em myData");
  }

Apparently the problem is in the Content-Type you are using, I think the server is configured differently.

Try this with your same code: [request addValue:@"application/soap+xml; charset=utf-8" forHTTPHeaderField:@"Content-Type"];