Similar Types Occurring in Both Libraries
Whose String is it anyway?
Over the last 15 years or so, the Eiffel class libraries have been a source for reusable software components for developers.
The Eiffel Base library contains classes for commonly used objects like different kinds of numbers, strings, files, and data structures.
But there are also libaries of Eiffel classes for sophisticated purposes like lexical analysis and parsing, data access, and graphical user interface development.
Likewise .NET is delivered with assemblies containing thousands of powerful types with similar purposes.
Working in Both Worlds
When we build software that has access to both the rich Eiffel libraries and the many useful .NET types, we inevitably run into types from both worlds that have similar names and purposes, but are still different types with different semantics.
The Case of Strings
The example of these similar types which will almost certainly get in your face is the string types. You may remember that we looked briefly at the case of the string types in Naming Conventions and Name Handling.
The Eiffel Base Library contains class
STRING; the .NET assembly contains type
System.String, which Eiffel for .NETusers see as
SYSTEM_STRING. At an abstract level both of these model sequences of characters. But they are not the same type. In fact, they are different in some important ways. For example, instances of
System.String are immutable. So you cannot append to an instance of
System.String. If you want to build a string by appending, you should use an instance of
System.Text.StringBuilder to do the appending, then extract the instance of
System.String from it. With
STRING it is permissible to append, so you don't need a helper like the
So the two types are similar at an abstract level, but different semantically. There are reasonable arguments for the design of each.
Many types in the delivered assemblies have properties which are strings or methods which return or take strings as arguments. In all these cases, the strings in question are instances of
Many classes in the delivered Eiffel libraries have features involving strings, that is attributes which are strings or routines which return or take strings as arguments. In all these cases, the strings are instances of
STRING (except for those designed for .NET compliance).
In C# and VB.NET, if you specify a quoted string like
"Hello World!" in your code, that string will conform to type
System.String. If you do the same in Eiffel, then
"Hello World!" will be an instance of
STRING. In Eiffel terminology,
"Hello World!" appearing in source code is a manifest string.
What all this means to you is that you cannot use an instance of
System.String when an instance of
STRING is called for, and vice versa. Three out of four of the executable lines in the following code sample are invalid:
my_system_string := "Hello World!" -- Invalid
my_string := "Hello World!" -- Valid
my_string := my_system_string -- Invalid
my_system_string := my_string -- Invalid
To handle this issue, the Eiffel for .NET class
STRING has two features which can be used when a string of the other type is needed.
The first of these features is a query
to_cil which returns an object of type
System.String which has a sequence of characters equivalent to that of the
STRING to which
to_cil is applied. The
to_cil can be applied to manifest strings by enclosing the manifest string in parentheses.
The other feature is a creation procedure named
make_from_cil which takes as an argument an instance of
System.String and initializes its target
STRING with a sequence of characters equivalent to that of the argument.
In the following sample, we use these features of
STRING to make all the lines from the previous sample valid.
my_system_string := ("Hello World!").to_cil -- Valid
my_string := "Hello World!" -- Valid
my_string.make_from_cil (my_system_string) -- Valid
my_system_string := my_string.to_cil -- Valid
Other Similar Types
There are many other cases of types available from the .NET assemblies which have similar purpose and semantics to those found in the Eiffel libraries. Fortunately, there is none that you will have to deal with as often as strings.