{"id":8425,"date":"2017-08-28T09:00:28","date_gmt":"2017-08-28T16:00:28","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vbteam\/?p=8425"},"modified":"2024-07-05T12:36:22","modified_gmt":"2024-07-05T19:36:22","slug":"dependency-injection-with-visual-basic-net-part-1","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/vbteam\/dependency-injection-with-visual-basic-net-part-1\/","title":{"rendered":"Dependency Injection with Visual Basic .NET – Part 1"},"content":{"rendered":"

This post was authored by guest blogger Andr\u00e9 Obelink, a Visual Basic MVP, and published by the VBTeam on his behalf.<\/em>\nIn this first blog post of a series of two, I explain what dependency injection (DI) is and why you might want to use this design principle in your software. The target audience of this post is the junior \/ medium experienced software developer, with no knowledge of dependency injection or related techniques. In the second post, I\u2019ll describe the use of Inversion of Control Containers (IoC containers), to use dependency injection in a much more flexible way.<\/p>\n

What is dependency injection?<\/h2>\n

Dependency injection isn\u2019t new. It\u2019s been around since the early 90s. Because it\u2019s a design principle, it isn\u2019t directly related to a specific programming language. For example, you can use this principle in Java, C#, or Visual Basic .NET. It\u2019s one of the principles of SOLID<\/a><\/span>. SOLID is a mnemonic acronym for the five basic principles of object-oriented programming and design. In my daily life as a software developer, I try to commit myself to apply the principles of SOLID. They were, years ago, defined by Robert C. Martin (also known as \u2018Uncle Bob\u2019). He\u2019s the author of many books, including \u2018Clean Code\u2019. A highly recommended book!\nLet\u2019s start from the beginning by defining the term \u2018dependency injection\u2019. However, this is easier said than done. Wikipedia says: \u201cIn software engineering, dependency injection is a technique whereby one object supplies the dependencies of another object. A dependency is an object that can be used (a service). An injection is the passing of a dependency to a dependent object (a client) that would use it. The service is made part of the client’s state. Passing the service to the client, rather than allowing a client to build or find the service, is the fundamental requirement of the pattern<\/em>.\u201d\nIf you\u2019re new to this subject, you may read this definition more than once and still have no clue what exactly is meant. I think an example from real life might help.\nWhen you were a child, and you asked your mother to give you something to drink, you were happy when she handed you a cup of milk, a glass of orange juice, or a bottle of apple juice, for example. Your mother was in charge. You asked for a drink, but you didn\u2019t know what you\u2019d eventually get. The fact that you knew that you\u2019d get some drink, without knowing exactly which drink it would be, describes more-or-less the core concept of dependency injection. You don\u2019t know the exact type you\u2019ll get, but you know about, for example, the methods on the interface.\nLet\u2019s look at another example from daily life, more related to computer software. Perhaps you\u2019re familiar with Adobe Photoshop and their mechanism of plug-ins. You want to apply a filter to a photo. You can install a filter or plug-in, published by a third-party vendor. Adobe Photoshop doesn\u2019t know anything about that filter. However, when this plug-in is installed, it can execute the filter logic, and apply that specific implementation of the filter to the active photo. Again, there\u2019s a concept of not knowing about the filter that is installed, and the ability to apply that \u2018unknown\u2019 filter.\nIn my daily life as a developer, I use dependency injection a lot. Imagine, for example, the logging components we\u2019re using in our software, like Log4net. Based on configuration, it logs to different – or a combination of – output destinations. In our code, we call something general like _logger.Log(\u201cHello!\u201d). But when writing that code, we don\u2019t know where this text will end up. Is it in a text file or is logged to the Windows Event log? Or both? Or will an email be sent containing the message?<\/p>\n

Four roles<\/h2>\n

It\u2019s important to understand that dependency injection involves four roles:<\/p>\n