Network Programming in .NET

Skype has been around since 2003 and has over 660 million registered users. This is an API wrapper that allows access to the user search facility of the consumer version of skype (i.e. not Skype4business), via a simple interface that allows searching via email address or name

To learn more – go to https://rapidapi.com/infiniteloop/api/skype-graph

This API takes a single parameter, which can be either a name “Robert Smith”, or an email address “robert.smith@hotmail.com“

The response will be in JSON, following a format similar to this;

{
  "requestId": "689067",
  "results": [
    {
      "nodeProfileData": {
        "skypeId": "bobbys88881",
        "name": "Rob Smith",
        "avatarUrl": "https://api.skype.com/users/bobbys88881/profile/avatar",
        "country": "Ireland",
        "countryCode": "ie",
        "contactType": "Skype4Consumer",
        "avatarImageUrl": "https://avatar.skype.com/v1/avatars/bobbys88881/public"
      }
    }
  ]
}	

Above is a sample search for “robert.smith@hotmail.com”

Costa Rica is a central American country with a population of just over 5 million, and half of all households in Costa Rica own at least one vehicle. The number of cars in Costa Rica has more than doubled since 2006 to an automotive park of 1,794,658 vehicles registered up to Feb. 2018. 1,166,042 (65%) corresponds to automotive vehicles; 589,037 motorcycles (33%), 20,918 Micro Buses (1%) and 9,661 Buses (0.5%). The average age of a Costa Rican car is 16 years with 2003 models.

As of today, we have launched an API that allows users search for a vehicle registered in Costa Rica using it’s license plate (like BFH467 in the photo above), via the website http://www.placa.co.cr/

Car registration plates in Costa Rica use the /CheckCostaRica API endpoint and return the following information:

• Make / Model
• Age
• Wheelplan
• Engine size
• Fuel
• Representative image

Sample Registration Number: 

706854

Sample Json:

{

  “Description”: “DAIHATSU TERIOS”,

  “CarMake”: {

    “CurrentTextValue”: “DAIHATSU”

  },

  “CarModel”: {

    “CurrentTextValue”: “TERIOS”

  },

  “MakeDescription”: {

    “CurrentTextValue”: “DAIHATSU”

  },

  “ModelDescription”: {

    “CurrentTextValue”: “TERIOS”

  },

  “EngineSize”: {

    “CurrentTextValue”: “1500”

  },

  “RegistrationYear”: “2016”,

  “Body”: “TODO TERRENO 4 PUERTAS”,

  “Transmission”: “MANUAL”,

  “Fuel”: “GASOLINA”,

  “Cabin”: “CABINA SENCILLA”,

  “WheelPlan”: “4X4”,

  “ImageUrl”: “http://www.placa.co.cr/image.aspx/@REFJSEFUU1UgVEVSSU9T”

}

Cordova hybrid iOS apps are based on the idea of wrapping a web application in a UIWebView in order to serve it as an app resembling a native iOS app. Apple has informed all iOS developers that UIWebView will be deprecated, and that new, or updated apps must use WKWebView instead. The idea that UIWebView could bypass fundamental restrictions that Web users are used to, is certainly problematic, and it’s not surprising that Apple has taken this move.

However, if you’ve got a Cordova based app, and need to make an upgrade, then perhaps you may need to take cognisance of these new limitations, and the upgrade path.

So, first off, to do the basic upgrade, you need to run the following command

cordova plugins add cordova-plugin-wkwebview-engine

Then Make a modification to config.xml as follows;

<platform name="ios">
    <preference name="WKWebViewOnly" value="true" />

    <feature name="CDVWKWebViewEngine">
        <param name="ios-package" value="CDVWKWebViewEngine" />
    </feature>

    <preference name="CordovaWebViewEngine" value="CDVWKWebViewEngine" />
</platform>

Then rebuild the app using

cordova build ios

That’s the basic upgrade path, but let’s dive deeper. One of the abilities of UIWebView was that you could make cross-origin requests without any restrictions. This is no longer possible in WKWebView, so you need to make sure that your AJAX requests are CORS compliant.

Assuming the API you are connecting to is not CORS enabled, and you have no direct control over it to make it CORS enabled, you will need to use a CORS proxy. One of the easiest ways to do this is via an API gateway using AWS.

Log in to AWS, and go to API Gateway. Press “Create API”, then “Build” beside HTTP API, then Add Integration -> HTTP. Set the URL endpoint to the root domain of the destination API that you are trying to call. Give the API a name and press “Next” Set Resource path to $default and press next. Press Next again, then Create.

Once created, click on the API, and press CORS on the left hand side. Press Configure. In Access-Control-Allow-Origin type * and press add. Do the same for Access-Control-Allow-Headers and Access-Control-Expose-Headers. For Access-Control-Allow-Methods select each method in turn, and press Add. Finally, Press Save.

You will now have an CORS-Enabled API endpoint, like https://xxxxxxx.execute-api.yyyyyyyyy.amazonaws.com that you can use to call the target API endpoint.

Then update your app source code, to change the domain to point at the AWS domain rather than the target API.

Before you compile, there is one last step; you have to add the following plugin:

cordova plugin add https://github.com/TheMattRay/cordova-plugin-wkwebviewxhrfix

What this does, is to set the origin header correctly within Cordova WKWebView, such that CORS will see it as a non-null origin, and will actually work.

Now, test your app, and hopefully you can now re-submit updates without apple complaining.

Greece has a population of 10.72 million, and a 5.28 million passenger cars which is a 49% car ownership rate, which is quite average for an EU country. Based on Eurostat data from 2018, graph below.

Today, we’ve launched our Greek API on the domain πινακιδα.gr which means “License plate” in Greek. The API endpoint is available at http://www.πινακιδα.gr/api/reg.asmx?op=CheckGreece

Car registration plates in Greece use the /CheckGreece  endpoint and return the following information:

• Make / Model
• Age
• Doors
• Engine size
• Representative image

Sample Registration Number: 

POT1309

Sample Json:

{
  "Description": "KIA Picanto",
  "CarMake": {
    "CurrentTextValue": "KIA"
  },
  "CarModel": {
    "CurrentTextValue": "Picanto"
  },
  "MakeDescription": {
    "CurrentTextValue": "KIA"
  },
  "ModelDescription": {
    "CurrentTextValue": "Picanto"
  },
  "Variant": "1.0i 12v Lx",
  "VehicleType": "ΕΙΧ",
  "Doors": 5,
  "EngineBand": 7,
  "EngineSize": {
    "CurrentTextValue": 999
  },
  "RegistrationDate": "01/07/2008",
  "Region": "Dodecanese - Rhodes",
  "ImageUrl": "http://www.πινακιδα.gr/image.aspx/@S0lBIFBpY2FudG8="
}

This issue affects Windows users, specifically Visual Studio with the AWS Toolkit installed, and it happens when the AWS Toolkit keeps a copy of old AWS credentials that may no longer be valid.

Here is the scenario.

I set up my AWS credentials using AWS configure, this stores the credentials in a file /user/.aws/credentials – I run Visual Studio, and AWS Toolkit keeps a copy of the credentials. Now, I decide to change the AWS credentials, and using the CLI again, I change the credentials on my machine. The CLI works fine, but Visual Studio is using old credentials, and breaks with an error like;

UnrecognizedClientException – The security token included in the request is invalid

at Amazon.Runtime.Internal.HttpErrorResponseExceptionHandler.HandleException(IExecutionContext executionContext, HttpErrorResponseException exception)
at Amazon.Runtime.Internal.ExceptionHandler`1.Handle(IExecutionContext executionContext, Exception exception)
at Amazon.Runtime.Internal.ErrorHandler.ProcessException(IExecutionContext executionContext, Exception exception)
at Amazon.Runtime.Internal.ErrorHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.CallbackHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.EndpointDiscoveryHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.EndpointDiscoveryHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.CredentialsRetriever.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.RetryHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.RetryHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.CallbackHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.CallbackHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.ErrorCallbackHandler.InvokeAsync[T](IExecutionContext executionContext)
at Amazon.Runtime.Internal.MetricsHandler.InvokeAsync[T](IExecutionContext executionContext)

Well, what how to clear the AWSToolkit cache?

Go to

C:\Users\<your user>\AppData\Local\AWSToolkit

and

erase RegisteredAccounts.json

Then restart Visual Studio, and it will find the correct credentials.

There are many services out there, that can give you a rotating proxy service, using a pool of IP addresses, and some of those pools are probably huge. However, here is an interesting approach discovered by Black Hills Information Security that can use AWS API Gateway as a proxy – A proxy that can only be used for one domain, but it rotates the Egress IP address with every request, and you’re using the IP address pool of an entire AWS Availability zone, and technically, you could set up multiple region end points, if you wanted.

So, the solution is called FireProx, and it’s a Python script that sets up an API gateway for you, such that you can make a request to the AWS gateway URL, and the request will be proxied to the destination,

For example;

https://3eqe4t8sxh.execute-api.eu-west-1.amazonaws.com/fireprox/

Goes to “http://icanhazip.com&#8221; – and nearly demonstrates the rotating proxy IP.

To run it, in Windows (Assuming you have Python and VirtualEnv already installed, and AWS already configured):

git clone https://github.com/ustayready/fireprox
cd fireprox
py -m virtualenv -p python3 .
env\Scripts\activate
pip install -r requirements.txt
python fire.py

Then you can create an api gateway using;

python fire.py --command create --url http://icanhazip.com

The GitHub repo is here; so you can verify the source yourself; https://github.com/ustayready/fireprox

If you need a quick check to determine the country of origin of your website visitor using Javascript, then you can use this little snippet of code using the ip-api service;

  var s = document.createElement("script");
  s.src = "http://ip-api.com/json/?callback=ipFound";
  document.body.appendChild(s);
  function ipFound(ipInfo)
  {
	  alert("You are from " + ipInfo.countryCode);
  }

It’s free to use (for non-commercial purposes), and uses no API Key, It’s using JSONP which is supported on most browsers.

Evidently, if you want more foolproof check, then a server-side check would be better, but it’s good to help guide your users to the right country, or preselect some options for them.

The AWS Toolkit for Visual Studio is great, when it works, but when it breaks, it’s a headache. The AWS Toolkit allows you publish your .NET code to AWS Lambda, with a API gateway already setup for you. It does this with a Cloud Formation stack under the hood, and this encapsulates the steps required to publish the Lambda, connect and configure the API gateway, and give you a public URL to play with.

Now, as of the 1st of March 2021, AWS deprecated the policy AWSLambdaFullAccess and replaced it with the similarly named AWSLambda_FullAccess – which is one underscore, but perhaps an hour of work to change.

First off, the error isn’t very helpful;

AspNetCoreFunctionRole

UPDATE_FAILED

1 validation error detected: Value ‘AWSLambdaFullAccess‘ at ‘policyArn’ failed to satisfy constraint: Member must have length greater than or equal to 20 (Service: AmazonIdentityManagement; Status Code: 400; Error Code: ValidationError; Request ID: cec22c28-2aae-4de8-9ef7-51880070081d; Proxy: null)

What does that mean, well, after googling, I found out that the policy had been deprecated, and set about editing the cloudformation stack.

So, I went into AWS, found the cloud formation stack, and tried to edit the template. But I couldn’t edit it, just make a copy of it (perhaps there is a better way here!, please leave a comment!) – and then I created a new Cloud Formation Stack, with a new name, but with the policy AWSLambdaFullAccess changed to AWSLambda_FullAccess

Back in Visual Studio, I clicked on the “Publish to AWS Lambda”, then selected the new stack from the drop down list, and pressed publish. Finally it worked!

It gave me a new API endpoint URL, which I didn’t want, and I had to reconfigure some permissions and the VPC that I had applied to the lambda manually.

Not a good experience at all, and I would really like to slap the AWS engineer that for making that change.

---

Following up from this post yesterday, I figured out how to update the existing Cloud Formation Stack, which importantly, allowed me to keep the same URL, and maintained the VPC and custom permissions.

So, First off, you go into the stack – press the Template tab, then View in Designer.

If you flick the language settings from JSON to YAML and back again, you get the JSON in a pretty format, so you can search for the AWSLambdaFullAccess and replace it with AWSLambda_FullAccess – Once finished, you can save the template locally, which you will upload again later., by pressing File > Save

Back in the Stack, press the Change Sets tab, and press Create Change set, select “Replace current template”, then “upload a template file”, and upload the file from the previous step. Press Next > Next > Next, then create change set.

Once you’ve done that, you’ll need to execute the change set.

However, if you’re like me, your stack will be in an invalid state, which you have to fix first, so you need to rollback the changeset, while ignoring the error that stopped the automatic rollback in the first place.

(from: https://aws.amazon.com/premiumsupport/knowledge-center/cloudformation-update-rollback-failed/)

If you want to skip FAILED resources during rollback, complete the following:

1.    From the Stack name column, select the stack that’s stuck in UPDATE_ROLLBACK_FAILED status.

2.    Choose Stack Actions, and then choose Continue update rollback.

3.    In the Continue update rollback dialog box, expand Advanced troubleshooting.

4.    In the Resources to skip – optional section, select the resources that you want to skip.

5.    Choose Continue update rollback.

Once you’ve successfully rolled back the stack, then you can execute the change set, and your cloudformation stack should be back working again, and you can deploy again from Visual Studio.

Templating is extremely common, and is used to separate the data (often in JSON) from the presentation layer, which can either be on the client side (Mustache, Angular, Vue, React) , or on the server side (ASP.NET, Razor, etc.)

The template is typically hard-coded so that the data changes, but the template used to render that data in a presentable way is fixed. However, in certain circumstances, you may want to offer the ability to the user to change the template.

This could be in a situation where you want to offer full flexibility in presentation to the end-customer. Perhaps you want them to be able to style their own webpage, user portal, or report, or pdf. In this case, allowing both a dynamic template applied to dynamic data can offer much more flexibility.

This approach demonstrates the use of Razor, which is used commonly in modern .NET web apps, although it requires a very technical user, and could potentially cause some security issues, if the end user were to inject malicious code into the template.

So, without further Ado, let’s take a look at this project.

First, I’ll define some data as follows;

{
  "Company": "Microsoft",
  "Directors": [
    "Bill Gates",
    "Steve Ballmer"
  ]
}

Which I save as data.json, and set the property to “Copy always” in Visual Studio.

Then I define a template like so;

@Model.Company, has @Model.Directors.Count directors which are: @foreach (var Director in Model.Directors) { @String.Concat(Director," ") }

Which is designed to read “Microsoft has 2 directors which are Bill Gates Steve Ballmer”, this would be saved as “template.html” in the project.

Now, I Install the relevant NuGet packages, and usings;

using Newtonsoft.Json.Linq;  // Install-Package Newtonsoft.JSON
using RazorEngine; // Install-Package RazorEngine.NetCore
using RazorEngine.Templating;

And finally, put it all together as follows;

var data = File.ReadAllText("data.json");
var template = File.ReadAllText("template.html");
dynamic model = JObject.Parse(data);
var result =
Engine.Razor.RunCompile(
    template,
    "template",
    null,
    (object)model);
Console.WriteLine(result);

Viola!

The github repo for this project is available here: https://github.com/infiniteloopltd/RazorTemplating

TL; DR; Here is the github repo: https://github.com/infiniteloopltd/hello-world-pi/blob/master/program.cs

This was an academic exercise for me, but in case you ever wanted to generate PDFs on a Raspberry pi, here is some code to do so in C# (.NET Core).

The core of the operation is wkhtmltopdf, which you can install on Raspberry pi (ubuntu), by simply typing

"apt-get wkhtmltopdf"

Then I created a C# app, based on the code from WkHtmlDriver that simply opens the executable, and redirects the standard input / output and error streams to the host application, and feeds it HTML to be encoded, and captures the output stream.

The code then converts the output stream to a byte array, and then dumps this to disk.

Here’s the final code:

var proc = new Process {
    StartInfo = new ProcessStartInfo {
        FileName = "wkhtmltopdf",
            Arguments = "-q - -",
            UseShellExecute = false,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            RedirectStandardInput = true,
            CreateNoWindow = true
    }
};
proc.Start();
using(var sIn = proc.StandardInput) {
    sIn.WriteLine("hello world");
}
var ms = new MemoryStream();
var sOut = proc.StandardOutput.BaseStream;
sOut.CopyTo(ms);
if (ms.Length == 0) {
    string error = proc.StandardError.ReadToEnd();
    throw new Exception(error);
}
proc.WaitForExit();
Console.WriteLine(ms.Length + " bytes written");
File.WriteAllBytes("hello.pdf", ms.ToArray());