This article I have mostly copy / pasted from here.

Here I've already explained how to create custom section in App.config, but now I need more complicated stuff, I need to have more attributes then just key / value pair.

Here is the example:

using System;
using System.Collections.Specialized;
using System.Configuration;

namespace CustomConfig
{
  class Program
  {
    static void Main(string[] args)
    {
      MilosevBlogConfig links = (MilosevBlogConfig)ConfigurationManager.GetSection("milosev.com");
      string homePage = ConfigurationManager.AppSettings.Get("homePage");

      Console.WriteLine("Home page: " + homePage);

      foreach (MilosevBlogCategoriesElement link in links.MilosevBlogInstances)
      {
        Console.WriteLine("Blog category: " + link.BlogCategory);
      }

      Console.WriteLine("Press any key...");
      Console.ReadKey();
    }
  }

  public class MilosevBlogCategoriesElement : ConfigurationElement
  {
    [ConfigurationProperty("BlogCategory", IsKey = true, IsRequired = true)]
    public string BlogCategory
    {
      get
      {
        return (string)base["BlogCategory"];
      }
      set
      {
        base["BlogCategory"] = value;
      }
    }
  }

  public class MilosevBlogCategoriesElementCollection : ConfigurationElementCollection
  {
    public MilosevBlogCategoriesElement this[int index]
    {
      get
      {

        return (MilosevBlogCategoriesElement)BaseGet(index);
      }
      set
      {
        if (BaseGet(index) != null)
          BaseRemoveAt(index);

        BaseAdd(index, value);
      }
    }

    protected override ConfigurationElement CreateNewElement()
    {
      return new MilosevBlogCategoriesElement();
    }

    protected override object GetElementKey(ConfigurationElement element)
    {
      return ((MilosevBlogCategoriesElement)element).BlogCategory;
    }
  }

  public class MilosevBlogConfig : ConfigurationSection
  {
    [ConfigurationProperty("categories")]
    [ConfigurationCollection(typeof(MilosevBlogCategoriesElementCollection))]
    public MilosevBlogCategoriesElementCollection MilosevBlogInstances
    {
      get
      {
        return (MilosevBlogCategoriesElementCollection)this["categories"];
      }
    }
  }
}
App.config should look like this:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>

  <configSections>
    <section name="milosev.com" type="CustomConfig.MilosevBlogConfig, CustomConfig"></section>
  </configSections>

  <milosev.com>
    <categories>
      <add BlogCategory="csharp"/>
      <add BlogCategory="asp.net MVC"/>
    </categories>
  </milosev.com>

  <appSettings>
    <add key="homePage" value="http://milosev.com/" />
  </appSettings>

  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
  </startup>
</configuration>
POI in App.config:
  <configSections>
    <section name="milosev.com" type="CustomConfig.MilosevBlogConfig, CustomConfig"></section>
  </configSections>
Where MilosevBlogConfig looks like:
public class MilosevBlogConfig : ConfigurationSection
{
  [ConfigurationProperty("categories")]
  [ConfigurationCollection(typeof(MilosevBlogCategoriesElementCollection))]
  public MilosevBlogCategoriesElementCollection MilosevBlogInstances
  {
    get
    {
      return (MilosevBlogCategoriesElementCollection)this["categories"];
    }
  }
}
Source code you can download from here.
Interesting (and fast) way to fill the gap between numbers:

static void Main(string[] args)
{
  int[] gapNumbers = new int[] { 1, 3, 4, 7, 8, 10 };

  foreach (int number in FillTheGap(gapNumbers))
  {
	Console.WriteLine(number);
  }

  Console.ReadKey();
}

private static IEnumerable<int> FillTheGap(int[] gapNumbers)
{
  int prevNumber = 1;

  foreach (int number in gapNumbers)
  {
	if (prevNumber == number)
	{
	  yield return number;
	}
	else
	{
	  for (; prevNumber < number; prevNumber++)
	  {
		yield return prevNumber;
	  }
	  yield return number;
	}
	prevNumber++;
  }
}

POI:

foreach (int number in FillTheGap(gapNumbers))
Recently I had to automatically resize images, but I ran into problem that some images were rotated. Problem was the EXIF data on some of the images was altering the orientation of the images. As explained here. So here is my method for image resizing:
    public void ResizeImage(string originalFilename, string saveTo,
                     int canvasWidth, int canvasHeight)
    {
      try
      {
        Image image = Image.FromFile(originalFilename);

        Image thumbnail = new Bitmap(canvasWidth, canvasHeight);
        Graphics graphic = Graphics.FromImage(thumbnail);

        graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
        graphic.SmoothingMode = SmoothingMode.HighQuality;
        graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
        graphic.CompositingQuality = CompositingQuality.HighQuality;

        int originalWidth = image.Width;
        int originalHeight = image.Height;

        double ratioX = canvasWidth / (double)originalWidth;
        double ratioY = canvasHeight / (double)originalHeight;

        double ratio = ratioX < ratioY ? ratioX : ratioY;

        int newHeight = Convert.ToInt32(originalHeight * ratio);
        int newWidth = Convert.ToInt32(originalWidth * ratio);

        int posX = Convert.ToInt32((canvasWidth - (originalWidth * ratio)) / 2);
        int posY = Convert.ToInt32((canvasHeight - (originalHeight * ratio)) / 2);

        graphic.Clear(Color.White); // white padding

        graphic.DrawImage(image, posX, posY, newWidth, newHeight);

        ImageCodecInfo[] info = ImageCodecInfo.GetImageEncoders();
        EncoderParameters encoderParameters;
        encoderParameters = new EncoderParameters(1);
        encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality,
                         100L);

        int OrientationKey = 0x0112;
        const int NotSpecified = 0;
        const int NormalOrientation = 1;
        const int MirrorHorizontal = 2;
        const int UpsideDown = 3;
        const int MirrorVertical = 4;
        const int MirrorHorizontalAndRotateRight = 5;
        const int RotateLeft = 6;
        const int MirorHorizontalAndRotateLeft = 7;
        const int RotateRight = 8;

        if (image.PropertyIdList.Contains(OrientationKey))
        {
          if (image.PropertyIdList.Contains(OrientationKey))
          {
            var orientation = (int)image.GetPropertyItem(OrientationKey).Value[0];
            switch (orientation)
            {
              case NotSpecified: // Assume it is good.
              case NormalOrientation:
                // No rotation required.
                break;
              case MirrorHorizontal:
                thumbnail.RotateFlip(RotateFlipType.RotateNoneFlipX);
                break;
              case UpsideDown:
                thumbnail.RotateFlip(RotateFlipType.Rotate180FlipNone);
                break;
              case MirrorVertical:
                thumbnail.RotateFlip(RotateFlipType.Rotate180FlipX);
                break;
              case MirrorHorizontalAndRotateRight:
                thumbnail.RotateFlip(RotateFlipType.Rotate90FlipX);
                break;
              case RotateLeft:
                thumbnail.RotateFlip(RotateFlipType.Rotate90FlipNone);
                break;
              case MirorHorizontalAndRotateLeft:
                thumbnail.RotateFlip(RotateFlipType.Rotate270FlipX);
                break;
              case RotateRight:
                thumbnail.RotateFlip(RotateFlipType.Rotate270FlipNone);
                break;
              default:
                throw new NotImplementedException("An orientation of " + orientation + " isn't implemented.");
            }
          }
        }

        //thumbnail.RotateFlip(RotateFlipType.Rotate90FlipNone);
        thumbnail.Save(saveTo, info[1], encoderParameters);
        Debug.WriteLine($"Thumbnail from file: {originalFilename} created in {saveTo}");
      }
      catch (Exception e)
      {
        Debug.WriteLine($"Error creating thumbnail: {e.Message}");
      }
    }
POI:
int OrientationKey = 0x0112;
const int NotSpecified = 0;
const int NormalOrientation = 1;
const int MirrorHorizontal = 2;
const int UpsideDown = 3;
const int MirrorVertical = 4;
const int MirrorHorizontalAndRotateRight = 5;
const int RotateLeft = 6;
const int MirorHorizontalAndRotateLeft = 7;
const int RotateRight = 8;

if (image.PropertyIdList.Contains(OrientationKey))
{
  if (image.PropertyIdList.Contains(OrientationKey))
  {
	var orientation = (int)image.GetPropertyItem(OrientationKey).Value[0];
	switch (orientation)
	{
	  case NotSpecified: // Assume it is good.
	  case NormalOrientation:
		// No rotation required.
		break;
	  case MirrorHorizontal:
		thumbnail.RotateFlip(RotateFlipType.RotateNoneFlipX);
		break;
	  case UpsideDown:
		thumbnail.RotateFlip(RotateFlipType.Rotate180FlipNone);
		break;
	  case MirrorVertical:
		thumbnail.RotateFlip(RotateFlipType.Rotate180FlipX);
		break;
	  case MirrorHorizontalAndRotateRight:
		thumbnail.RotateFlip(RotateFlipType.Rotate90FlipX);
		break;
	  case RotateLeft:
		thumbnail.RotateFlip(RotateFlipType.Rotate90FlipNone);
		break;
	  case MirorHorizontalAndRotateLeft:
		thumbnail.RotateFlip(RotateFlipType.Rotate270FlipX);
		break;
	  case RotateRight:
		thumbnail.RotateFlip(RotateFlipType.Rotate270FlipNone);
		break;
	  default:
		throw new NotImplementedException("An orientation of " + orientation + " isn't implemented.");
	}
  }
}
With this code I was checking if image has to be rotated. Example application (at this moment still under development), you can see on my GitHub.
One example of custom section in configuration file.

App.Config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  
  <configSections>     
    <section name="links" type="System.Configuration.NameValueSectionHandler">
    </section>
  </configSections> 
  
  <links>
      <add key="link1" value="http://milosev.com/25-c.html" />
      <add key="link2" value="http://milosev.com/84-asp-net-mvc-3.html" />
  </links>
  
  <appSettings>
      <add key="homePage" value="http://milosev.com/" />
  </appSettings>    
  
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
    </startup>
</configuration>
.NET code:
using System;
using System.Collections.Specialized;
using System.Configuration;

namespace CustomConfig
{
    class Program
    {
        static void Main(string[] args)
        {
            NameValueCollection links = ConfigurationManager.GetSection("links") as NameValueCollection;
            string homePage = ConfigurationManager.AppSettings.Get("homePage");

            Console.WriteLine("Home page: " + homePage);

            foreach (string link in links)
            {
                Console.WriteLine("Link: " + links.Get(link));
            }

            Console.WriteLine("Press any key...");
            Console.ReadKey();
        }
    }
}
In reference list you will need to add System.Configuration.

POI:

  <configSections>     
    <section name="links" type="System.Configuration.NameValueSectionHandler">
    </section>
  </configSections> 

Here is more about configSection.

Also notice line:

Console.WriteLine("Link: " + links.Get(link));