Eiffel for .NET Integration

Differences between Eiffel and Eiffel for .NET

Limitation of Eiffel for .NET in version 5. 5

Most of the Eiffel mechanisms are supported in 5. 5. All missing features listed below are planned for addition in future releases:

• No creation of Eiffel expanded class support
• Partial implementation of generic conformance (same as what was supported up to and including the 4.2 release of the Eiffel development environment).

Eiffel for .NET supports:

• Multiple Inheritance
• Design By Contract
• Exception handling
• Genericity
• Covariance
• Compilation of any existing Eiffel libraries as long as it does not include C externals that call into the Eiffel Software C runtime

Added to Eiffel and Eiffel for .NET

The following syntax can be used to declare .NET custom attributes on Eiffel entities (features and classes): empty: BOOLEAN note description: "Is Current empty?" metadata: create {OBSOLETE_ATTRIBUTE}.make_obsoleteattribute_1 ("Use `is_empty' instead") end obsolete "Use is_empty instead" do Result := is_empty end

The previous example shows the declaration of the obsolete feature empty . The custom attribute defined by OBSOLETE_ATTRIBUTE is used to ensure that any consumer of the resulting assembly will see the feature as being obsolete. The custom attribute is defined in the note clause metadata. The definition consists of a creation expression that creates the custom attribute with the right parameters.

Using the metadata tag is the most general way of applying a custom attribute. There are however some variations that are explained below:

• metada: most general way, it applies a custom attribute to both the class and interface generated by the Eiffel compiler.
• class_metadata: applies only to the class generated by the Eiffel compiler (mostly for advanced users).
• interface_metadata: applies only to the interface generated by the Eiffel compiler (mostly for advanced users).
• property_metadata: applies a custom attribute to the associated property generated by the Eiffel compiler for a query.
• assembly_metadata: applies a custom attribute for the current assembly. It only works when present in the Eiffel system root class note clause.

Differences between Eiffel for .NET and .NET

Covariance

The CLR (Common Language Runtime) does not support covariance due to a type safety issue that full covariance implies (known as a polymorphic catcall in Eiffel). Although very rare, catcalls are not suitable to .NET where safety is one of the primary goals.

Eiffel for .NET implements a safe variant of covariance that will always perform a check on the types to avoid a catcall. So when a catcall is going to be performed a Invalid Cast Exception will be raised by the CLR instead of an unexpected behavior as is the default behavior in classic Eiffel (i.e., the behavior without catcall detection explicitly enabled).

Another advantage of Eiffel for .NET's implementation of covariance is that it can be easily understood by CLS compliant consumer tools. These tools will actually benefit from the Eiffel for .NET covariance.

Genericity

The CLR does not support generics at all, so that the following Eiffel for .NET classes:

• LIST [ANY]
• LIST [INTEGER]

will actually be generated as:

• LIST_ANY
• LIST_Int32

Meaning that if one wants to reuse an Eiffel generic class from another language than Eiffel for .NET, one has to use either LIST_ANY or LIST_Int32.

Enum types

Eiffel for .NET supports .NET enum types implicitly. From the point of view of Eiffel, they are just considered as expanded classes. The only difference is in the code generation. Eiffel for .NET cannot declare new enum types yet.

ByRef

Eiffel does not have the notion of byref argument passing. At the moment, Eiffel for .NET cannot call nor can it redefine a feature that has a byref argument.