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Generics in .NET programming refers to a class that is not related to any specific Type, but is still used in a Type-Safe manner. A perfect example of where Generics are useful is with collections of items (integers, strings, Orders etc.). We can create a generic collection than can handle any Type in a generic and Type-Safe manner. For example, we can have a single array class that we can use to store a list of Users or even a list of Products, and when we actually use it, we will be able to access the items in the collection directly as a list of Users or Products, and not as objects.

The use of generic classes is supported in Clarion#.

To use generic types/methods, specify generic arguments (types) in angle brackets.( <;'>).

Examples:

There are two restrictions:

1. Arrays can't be used as a generic argument

2. All .Net reference types are considered as reference and .Net value-types are considered as values. Using of types with &amp; (such as &amp;LONG) in generic arguments are not supported

Declaration of generics

To declare a generic CLASS, STRUCT, or INTERFACE you should add generic parameters in angle brackets (e.g., <; >) just after the appropriate keyword. Constraints for generic parameters can be specified with a WHERE(') clause. The list of WHERE clauses (without any delimiters) should be placed before all attributes.

The syntax of the WHERE clause is as follows:

WHERE(<;generic argument name>=(constraint1,constraint2',constraintN)

Where constraint has the following syntax:

   NEW|CLASS|STRUCT|<;type>

In the generic method definition the class label uses the following syntax:

Notes:

1.CLASS and STRUCT constraints can't be used simultaneously.

2.If <;type> is a STRUCT or CLASS, then STRUCT and CLASS constraints can't be used.

3.When calling the generic method, the explicit <;type> is superfluous, if the passed parameter is already of the desired class.

In the method definition for a Generic the class label has the following syntax:

Name <;generic parameter1,', generic parameterN>

Example:

PROGRAM

NAMESPACE qqq

USING System

           

           

aa CLASS<;T,V> WHERE(T=ICloneable)

mm  PROCEDURE

bb  PROCEDURE(T b)

vv  PROCEDURE(T b,V e)

    INLINE

     CODE

      MESSAGE(b.Clone())

    END

  END

 CODE

aa<;T,V>.mm   PROCEDURE

ee  &amp;ICloneable

bb1 T

 CODE

 ee = bb1

 SELF.bb(bb1)

 MESSAGE(bb1.Clone())

aa<;T,V>.bb   PROCEDURE(T b)

 CODE

To declare generic methods place the generic parameters of the procedure in <;'> and add constraints (WHERE clause before any attributes of the procedure)

Example:

Bb    PROCEDURE<;T>(T arg) WHERE(T=NEW)

Current restrictions in using generic parameters:

1. If T is a generic parameter then the following declarations are equivalent:

No instance is created and no constructor is called. It's just a declaration of a variable.

The following declaration syntax:

Aa &amp;T

is currently not supported.

2. If the left or right part of the assignment statement has a type T, where T is a generic parameter, you need to use the = assignment operator (not &amp;.

For example:

Aa  T

Bb  T

  CODE

  AA = Bb

DEFAULTOF

You can also generate a default value for a type in a generic declaration.

The special expression:

 DEFAULTOF (<;type>)

is used for this purpose.

The expression generates a default value for the corresponding type. For value-types, it's value with zeros (0) and for reference types it's just a null reference. For more information see the default keyword in C# documentation.

Example:

A  T

    CODE

    A=DEFAULTOF(T)

See Also: Variable Declaration outside of the DATA section