This project was made more than two years ago when I started learning WPF. All I can say is that the code is a mess and it's impossible to refactor without re-writing the whole application. So take this into consideration before wasting your time reading the code.
UPDATE: If you'd like to create your own node based editor, have a look at Nodify
Visual Programming Editor is a node based editor that tries to mimic the way you write code in Unreal Engine's 4 Blueprints editor. It features an infinite canvas where you can place nodes and connect their input and output pins to create logically connected sequences of actions. It pretty much functions like a procedural programming language, where you chain function calls starting from an entry point. Each function has its own entry point (input) and exit point (output) and can call other functions and declare and use variables to store temporary data.
The code is all C# having the UI written in WPF. The theme, the window and the dialog controls are built using the MahApps library.
When the application is first launched, it will display the Start Page which has options for creating a new project or opening an existing one. It also displays the most recent used projects. After creating a project, the project explorer becomes available.
Here you can add, remove or rename files or folders by right clicking on them. You can open files by double clicking on them or from the context menu. The file will open in a new tab using a custom editor for that file type.
Creating a new graph file will open in a custom editor which contains a canvas. The canvas can be navigated by holding the right mouse button and dragging. Canvas elements can be selected by holding the left mouse button and dragging or by clicking an item holding the CTRL button to add or remove it from the selection. Nodes can be created using the Actions Menu made visible by clicking the right mouse button on the canvas. Most of the elements show a context menu when you right click on them.