dev csharp, wpf

I like scanning all my documents and keep a digital version as well as the dead-tree version just in case. But storing the documents as individual image files is too messy. There are various solutions to merge image files into a single PDF but I don’t want to upload my sensitive documents to unknown parties so I rolled my own: Image2PDF

Implementation

I decided to go with a WPF application rathen than a web-based solution because it would be unnecessary to upload a bunch of image files to cloud and download the PDF back. It’s a lot of network traffic for something that can be handled locally much faster.

Image2PDF a simple WPF application developed in C#. I’ve used iTextSharp library to create the PDFs. It’s a great open-source library with lots of capabilities.

I’ve also started using SyncFusion WPF control library recently. The entire control suite is free of charge (which is always a great price!). It has a Wizard control which I decided to go with. I think using a wizard and is cleaner UI instead of dumping every control on one window.

Usage

As you might expect you provide the list of input images, re-order them if needed, choose the path for the output folder and go!

Step 1: Select input

Page 1: Specify input image files

Step 2: Select output

Page 2: Specify output PDF path

Step 3: Go!

Page 3: Run!

Simples!

Conclusion

In the future I’m planning to add various PDF operarions and enhance the functionality but since shipping is a feature I’ll limit the scope for now. After all, this was the main feature I needed anyway.

Resources

dev csharp, neo4j, cypher, json

I have a simple JSON file that contained a bunch of users with their followers which looked like this:

  {
    "user-id": 2,
    "username": "user_2",
    "avatar": "URL",
    "following": [
      "user_10",
      "user_6",
      "user_1"
    ],
    "followers": [
      "user_10"
    ]
  }

It felt like a good exercise would be to import that data into a graph database as it wasn’t something I had done before.

As my go-to language is C# and I had some experience with Cypher before, my initial instinct was to develop a tool to generate Cypher statements from the JSON.

First I create the nodes:

private void GenerateNodesCypher()
{
    string filename = @"..\..\output\nodes.cql";
    var output = new StringBuilder();

    foreach (var user in _userList)
    {
        string s = $"CREATE ({user.username}:User {{ userid: {user.userid} , username: '{user.username}', avatar: '{user.avatar}'  }} ); ";
        output.AppendLine(s);
    }

    File.WriteAllText(filename, output.ToString());
}

and then the relationships:

private void GenerateRelationshipsCypher()
{
    string filename = @"..\..\output\relationships.cql";
    var output = new StringBuilder();
    int n = 0;
    foreach (var user in _userList)
    {
        foreach (var following in user.following)
        {
            string s = $"MATCH (a), (b) WHERE a.username = '{user.username}' AND b.username = '{following}' CREATE (a)-[:FOLLOWING]->(b); ";
            output.AppendLine(s);
            n++;
        }
    }

    File.WriteAllText(filename, output.ToString());
}

So I ended up with two cql files that looked like this

CREATE (user_1:User { userid: 1 , username: 'user_1', avatar: 'URL' }); 
CREATE (user_2:User { userid: 2 , username: 'user_2', avatar: 'URL' }); 
CREATE (user_3:User { userid: 3 , username: 'user_3', avatar: 'URL' }); 

and

MATCH (a), (b) WHERE a.username = 'user_1' AND b.username = 'user_26' CREATE (a)-[:FOLLOWING]->(b); 
MATCH (a), (b) WHERE a.username = 'user_1' AND b.username = 'user_53' CREATE (a)-[:FOLLOWING]->(b); 
MATCH (a), (b) WHERE a.username = 'user_2' AND b.username = 'user_6' CREATE (a)-[:FOLLOWING]->(b); 

and I used neo4j-shell to execute the files and import the data. But it was no bed of roses. I’ll list the problems I faced along the way and how I got around them so that this experience might be helpful for other people as well:

Issues along the way and lessons learned

Running multiple statements on Neo4J browser

First I tried to run the create statements using the Neo4J browser which turned out to be problematic because it cannot run multiple statements that end with semi-colons. So I removed the semi-colons but then it started giving me this error

WITH is required between CREATE and MATCH

I found a workaround for that on SO. So the following works:

MATCH (a), (b) WHERE a.username = 'user_1' AND b.username = 'user_14' CREATE (a)-[:FOLLOWING]->(b); 
WITH 1 as dummy
MATCH (a), (b) WHERE a.username = 'user_1' AND b.username = 'user_22' CREATE (a)-[:FOLLOWING]->(b); 

Now the problem was, if the data was a bit dirty, for instance if user_14 didn’t exist, it stopped executing the rest and no other relationships were created). I had a few nodes like that so this method didn’t work for me after all.

Starting the shell was not as easy as I’d imagined

I installed Neo4J using the default settings and to start the shell just navigated to that directory and ran the batch file. Got an error instead of my shell:

C:\Program Files\Neo4j Community\bin>Neo4jShell.bat

The system cannot find the path specified.

Error: Could not find or load main class org.neo4j.shell.StartClient

Apparently the way to run the shell is using the Neo4J server application and clicking Options -> Command Prompt

Neo4J command prompt

This launches the Neo4J Command Prompt then the rest is easy:

neo4jshell -file nodes.cql

neo4jshell -file relationships.cql

“Exiting with unterminated multi-line input”

Final issue was the statements in a sql file must be terminated with a semi-colon in order for the shell to execute them. Otherwise it gives the above warning and quits

So after all this hassle my data is in the Neo4J database ready to be queried to death!:

Next I’ll investigate converting the data into CSV and achieve the same results by using LOAD CSV command and look into visualization of this data.

Resources

dev csharp, asp_net, mvc

In ASP.NET, TempData is one of the mechanisms used to pass data from controller to the view. In this post I’ll dive into its source code to investigate its behaviour.

What is TempData

TempData is an instance of TempDataDictionary that is used to pass data from the controller to the view.

Lifespan

The lifespan of TempData is rather unusual: It lives for one request only. In order to achieve this it maintains two HashSets to manage keys as well as the data dictionary:

private Dictionary<string, object> _data;
private HashSet<string> _initialKeys = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
private HashSet<string> _retainedKeys = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

When we read some data using an indexer or TryGetValue method it removes that key from _initalKeys collection.

public bool TryGetValue(string key, out object value)
{
    _initialKeys.Remove(key);
    return _data.TryGetValue(key, out value);
}

The actual dictionary that holds the data is intact at this point. That’s why we can read same data consecutively without any issues. It only removes the key from _initialKeys collection, basically marking it to be deleted when the data is persisted.

Peek and keep

If we want the values in TempData last longer we can use Peek and Keep methods. What Peek does is return the value without removing it from the _initialKeys:

public object Peek(string key)
{
    object value;
    _data.TryGetValue(key, out value);
    return value;
}

Alternatively, we can call Keep method. Similarly it doesn’t manipulate the data directly but just marks the key to be persisted by adding it to the _retainedKeys collection.

public void Keep(string key)
{
    _retainedKeys.Add(key);
}

Parameterless overload of Keep method add all keys in the _data dictionary to _retainedKeys.

Which keys to persist

So as seen above when we get values and call Peek/Keep methods, operations are carried out on _initialKeys and _retainedKeys collections and nothing happens to the actual data. These operations take effect when the _data is actually saved:

public void Save(ControllerContext controllerContext, ITempDataProvider tempDataProvider)
{
    _data.RemoveFromDictionary((KeyValuePair<string, object> entry, TempDataDictionary tempData) =>
        {
            string key = entry.Key;
            return !tempData._initialKeys.Contains(key) 
                && !tempData._retainedKeys.Contains(key);
        }, this);

    tempDataProvider.SaveTempData(controllerContext, _data);
}

Before the data is passed on to the provider it’s pruned. The keys that don’t exist in the _retainedKeys (the keys we explicitly told to keep) and _initialKeys (the keys that have not been touched so far or accessed through Peek method) collections are removed.

Providers

By default, TempData uses session variables to persist data from one request to the next. Serializing and deserializing data is carried out via an object implementing ITempDataProvider. By default SessionStateTempDataProvider class is used to provide this functionality. It occurs in the CreateTempDataProvider method in Controller.cs class:

protected virtual ITempDataProvider CreateTempDataProvider()
{
    return Resolver.GetService<ITempDataProvider>() ?? new SessionStateTempDataProvider();
}

This also means we can replace the provider with our own custom class. For demonstration purposes I wrote my own provider which uses a simple text file to persist TempData:

public class TextFileTempDataProvider : ITempDataProvider
{
    internal readonly string FileName = Path.Combine(HttpContext.Current.Server.MapPath(@"~/App_Data"), @"TempData.txt");

    public IDictionary<string, object> LoadTempData(ControllerContext controllerContext)
    {
        if (File.Exists(FileName))
        {
            string json = File.ReadAllText(FileName);
            return Newtonsoft.Json.JsonConvert.DeserializeObject<Dictionary<string, object>>(json);
        }

        return new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
    }
    
    public void SaveTempData(ControllerContext controllerContext, IDictionary<string, object> values)
    {
        string json = Newtonsoft.Json.JsonConvert.SerializeObject(values);
        File.WriteAllText(FileName, json);
    }
}

In order to use this class it needs to be assigned to TempDataProvider in the controller’s constructor

public FirstController()
{
    TempDataProvider = new TextFileTempDataProvider();
}

Of course it’s not a bright idea to use disk for such operations, this is just for demonstration purposes and makes it easier to observe the behaviour.

Conclusion

Often times I’ve found knowledge about the internals of a construct useful. Applications and frameworks are getting more complex each day, adding more layers and hiding the complexity from the consumers. It’s great because we can focus on the actual business logic and application we are building but when we get stuck it takes quite a while to figure out what’s going on. Having in-depth knowledge on the internals can save a lot if time.