Programming Visual Basic .NET

Metadata

Source code consists of some constructs that are procedural in nature and others that are declarative in nature. An example of a procedural construct is:

someObject.SomeMember = 5

This is procedural because it compiles into executable code that performs an action at runtime. Namely, it assigns the value 5 to the SomeMember member of the someObject object.

In contrast, here is a declarative construct:

Dim someObject As SomeClass

This is declarative because it doesn’t perform an action. It states that the symbol someObject is a variable that holds a reference to an object of type SomeClass.

In the past, declarative information typically was used only by the compiler and did not compile directly into the executable. In the CLR, however, declarative information is everything! The CLR uses type and signature information to ensure that memory is always referenced in a safe way. The JIT compiler uses type and signature information to resolve method calls to the appropriate target code at JIT compile time. The only way for this to work is for this declarative information to be included alongside its associated procedural information. Compilers that target the CLR therefore store both procedural and declarative information in the resulting .exe or .dll file. The procedural information is stored as IL, and the declarative information is stored as metadata. Metadata is just the CLI’s name for declarative information.

The CLI has a mechanism that allows programmers to include arbitrary ...