MSIL references

Question

In VB.NET you can add file references or project references. For example, if you wanted to use the SQLConnection class, then you would add a reference to System.Data.SQLClient and import the namespace.

How does this work at a lower level in MSIL and assembly language. I assume that the executable would contain all the code from System.Data.SQLClient?

I have inspected some MSIL generated from a VB.NET project, but I am still unclear.

Answer 1

No, the executable would not include all code from System.Data.SQLClient. Instead the .NET runtime loads the assemblies your app references. That allows assemblies to be shared among many apps. So the running process includes the code from System.Data.dll, but your executable does not.

Answer 2

The system.Data assembly is loaded (because your project reference it), you can see this in the MANIFEST (ildasm)

.assembly extern System.Data
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 4:0:0:0
}

Call that uses SqlConnection:

 IL_000f:  newobj     instance void [System.Data]System.Data.SqlClient.SqlConnection::.ctor()

Defnition of .assembly extern:

.assembly extern <assembly name>
   Specifies another assembly that contains items referenced by the current module (in this example, mscorlib).

More information: http://msdn.microsoft.com/en-us/library/ceats605.aspx