将json post从xcode发送到wep.api

向Wep.api应用程序发送post消息时遇到问题。我已经在google chrome postman应用程序上试用了我的wep.api，它运行得非常好。我希望xcode中的代码将此json消息发送到我的wep.api

Content-Type: application/json; charset=utf-8
{ UserName: "fjkdlajfka", DeviceID: "1568948" , Password: "fkjdalfda"}

但每次我发送消息时，参数总是为空。这是我的xcode消息：

NSString *JsonMsg = [NSString stringWithFormat:@"{UserName: \"%@\",DeviceID: \"%@\",Password: \"%@\"}",
                     Username.text,@"123456789",Password.text];
NSURL *url = [NSURL URLWithString: @"http://192.168.xxx.xxxx:53913/api/LogIn"];
NSMutableURLRequest *theRequest = [NSMutableURLRequest requestWithURL:url];
NSString *msgLength = [NSString stringWithFormat:@"%d", [JsonMsg length]];

[theRequest setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
[theRequest setValue:@"application/json" forHTTPHeaderField:@"Accept"];
[theRequest setValue:msgLength forHTTPHeaderField:@"Content-Length"];
[theRequest setHTTPMethod:@"POST"];
[theRequest setHTTPBody:[JsonMsg dataUsingEncoding:NSUTF16BigEndianStringEncoding allowLossyConversion:YES]];

conn = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];

if ( conn ) {
    webData = [NSMutableData data];

}
else {
    NSLog(@"theConnection is NULL");
}

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有人知道这里的错误是什么吗？

您的JSON无效，缺少引号

NSString *JsonMsg = [NSString stringWithFormat:@"{"UserName": \"%@\",
                                                  "DeviceID": \"%@\",
                                                  "Password": \"%@\"}",
                     Username.text,@"123456789",Password.text];