The .NET Common Language Infrastructure has two different concepts:
- Namespaces: Prefixes for type names, such as
System.Drawing , which are used to distinguish between multiple types that would otherwise have the same name. - Assemblies: Libraries of code that can be deployed, installed, and versioned separately from other assemblies. Types in an assembly can be in any number of namespaces.
Namespaces form a hierarchy based on delimiters with a full stop (period), so you should think that the types in the System.Runtime.InteropServices subordinate to those in the System.Runtime . However, as far as I know, the CLI does not care about the name or hierarchy of your namespaces, unless they make your type names unique.
In addition, an assembly can contain types from several namespaces, even in different hierarchies, and a single namespace can contain types defined in several assemblies. If you look at the MSDN documentation for a type in .NET libraries, it will tell you which particular assembly is included. However, as Paolo Falabella noted, MSDN will not tell you which assembly contains a namespace, since a single namespace can contain types from multiple assemblies.
In your scenario: mscorlib is an assembly that defines some types in the System namespace and many others, such as System.Runtime.InteropServices , as you noted. However, the types that you use in the System.Drawing namespace are located in the System.Drawing assembly.
Because assemblies are a unit of code deployment and reuse, Visual Studio projects reference assemblies, not namespaces, and therefore you must add a reference to the System.Drawing assembly in your Visual Studio project for your program.
The VB.NET Imports (and its C # equivalent, using ) allows you to refer to types in the namespace without having to call the namespace name each time. That is, with Imports System.Drawing , you can write Graphics in your code instead of System.Drawing.Graphics . But this is all the Imports operator. In particular:
Imports System does not automatically create project references for every assembly in the world that defines types in the System namespace.Imports mscorlib does not mean that you refer to each type of assembly "mscorlib" by its short name. This means that you can refer to types in the mscorlib namespace by their short names, which are not only completely different, but are unlikely to be what you want.
Bottom line: if you want to access GDI +, then you use types in the System.Drawing namespace, but this name has nothing to do with the GDI + assembly name. Microsoft chose the name "System.Drawing" for the assembly, which contains GDI + types, but it could choose "gdiplus-cli", "gdi-for-dotnet", or even "Frobinator". No matter what name this assembly has, you must add a link to this assembly. And you do not do this in your source code - you add assembly references in the configuration of your Visual Studio project.
MSDN has an outdated but still good description of assemblies, namespaces, and the differences between them that may be useful.