[[Property:title|Eiffel for .NET Integration]]
[[Property:weight|3]]
[[Property:uuid|fe8a6a7d-4590-0db2-d59a-307082b18ecc]]
==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):
<code>
    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
</code>

The previous example shows the declaration of the obsolete feature <code> empty </code> . The custom attribute defined by <code>OBSOLETE_ATTRIBUTE</code> 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 <code>note</code> clause <code>metadata</code>. The definition consists of a creation expression that creates the custom attribute with the right parameters. 

Using the <code>metadata</code> tag is the most general way of applying a custom attribute. There are however some variations that are explained below:
*<code>metada</code>: most general way, it applies a custom attribute to both the class and interface generated by the Eiffel compiler.
*<code>class_metadata</code>: applies only to the class generated by the Eiffel compiler (mostly for advanced users).
*<code>interface_metadata</code>: applies only to the interface generated by the Eiffel compiler (mostly for advanced users).
*<code>property_metadata</code>: applies a custom attribute to the associated property generated by the Eiffel compiler for a query.
*<code>assembly_metadata</code>: applies a custom attribute for the current assembly. It only works when present in the Eiffel system root class <code>note</code> clause.

==Differences between Eiffel for .NET and .NET==

===Covariance===

The CLR (Common Language Runtime) does not support [[ET: Inheritance#Covariance and anchored declarations|covariance]] due to a type safety issue that full covariance implies (known as a polymorphic [[ET: Inheritance#Catcalls|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:
*  <code>LIST [ANY]</code> 
*  <code>LIST [INTEGER]</code> 

will actually be generated as:
*  <code>LIST_ANY</code> 
*  <code>LIST_Int32</code> 

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

===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.