"Visual Basic .NET includes data type conversion keywords, many of which are carried over from Visual Basic 6. But unlike the Visual Basic 6 functions, these keywords are not function calls but intrinsic language features. The keywords CBool, CByte, CChar, CShort, CInt, CLng, CSng, CDbl, CDec, CDate, CObj, and CStr map to Visual Basic Runtime method calls, .NET Framework class library method calls, or IL type conversion instructions. The exact method call or IL instructions generated depends on the expression against which the conversion is being applied. Some conversions are optimized away, such as CInt(123.45) which is replaced with the integer constant 123 in the IL. This is an example where using the Visual Basic Runtime results in better performance than using the System namespace. CInt("123") becomes a call that leads to calls to System.Double.Parse then System.Math.Round."
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dv_vstechart/html/vbtchmicrosoftvisualbasicnetinternals.asp