Filters

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Filters are a powerful boolean algebra (AND-OR-NOT) based mechanism, which enables the composition of virtually any type of logical expression (predicate). The abstraction for a filter is defined by the INFilter interface, a core implementation of which can be found in the NFilter class.

The INFilter interface has only one method - Filter. It accepts a single parameter - the object which must be filtered. If the specified object satisfies the filter criteria this method must return true. On failure the method must return false.

Primitive Filters

Primitive filters are filters, which do not aggregate other filters. The Nevron System Layer contains the following set of predefined primitive filters:

Filter type	Description
Object Type Filters
NAssignableFromTypeFilter	return true, if the specified object type can be assigned to the target type
NExactTypeMatchFilter	returns true, if the specified object type is the target type
NInstanceOfTypeFilter	returns true, if the specified object is an instance of the target type
Numeric Filters
NDoubleInRangeFilter	returns true, if the specified object can be converted to double and is in the range [begin, end]
NInt32InRangeFilter	returns true, if the specified object can be converted to int and is in the range [begin, end]

Each individual Nevron product typically defines additional primitive filters.

Filter Expressions

Since filter logic is AND-OR-NOT based, filter expressions are constructed with the help of the NAndFilter, NOrFilter and NNotFilter classes. The result of the first two is a logical AND/OR of the result of the filters they contain. If an AND or OR filter does not contain any filters it will always evaluate to false. The NNotFilter inverts the result of the filter it contains.

The following example demonstrates a simple type filter and an AND expression filter:

C#	Copy Code
// filters only objects derived from MyBaseType, which do not implement MyInterface NAndFilter andFilter = new NAndFilter(); andFilter.Add(new NInstanceOfTypeFilter(typeof(MyBaseType))); andFilter.Add(new NNotFilter(new NInstanceOfTypeFilter(typeof(MyInterface))));

Copy Code

// filters only objects derived from MyBaseType, which do not implement MyInterface
NAndFilter andFilter = new NAndFilter();
andFilter.Add(new NInstanceOfTypeFilter(typeof(MyBaseType)));
andFilter.Add(new NNotFilter(new NInstanceOfTypeFilter(typeof(MyInterface))));

Visual Basic	Copy Code
' filters only objects derived from MyBaseType, which do not implement MyInterface Dim andFilter As New NAndFilter andFilter.Add(New NInstanceOfTypeFilter(GetType(MyBaseType))) andFilter.Add(New NNotFilter(New NInstanceOfTypeFilter(GetType(MyInterface))))

Visual Basic

Copy Code

' filters only objects derived from MyBaseType, which do not implement MyInterface
Dim andFilter As New NAndFilter
andFilter.Add(New NInstanceOfTypeFilter(GetType(MyBaseType)))
andFilter.Add(New NNotFilter(New NInstanceOfTypeFilter(GetType(MyInterface))))