… override VisitParameter …

protected override Expression VisitParameter(ParameterExpression node) =>
ReferenceEquals(replacing, node)
? replacement
: base.VisitParameter(node);

… and override VisitLambda …

protected override Expression VisitLambda(Expression node)

… and inside it define the creation of a new instance of the filter …

var newFilterType = Expression.New
(
_filterTypeCtor,
replacement
);

… then we declare a variable to hold the instance of the filter …

var filterInstance = Expression.Variable(typeof(TFilterType), "filterTypeInstance");

… replace the inner filter parameter with the newly created filter instance parameter …

var visitor = new RewriteFilterTypeExpression(node.Parameters.Single(), filterInstance);
var replaced = visitor.Visit(node) as LambdaExpression;

… and then finally create a BlockExpression to wrap this together and return the result of the original expression …

return Expression.Block
(
new[] { filterInstance },
Expression.Assign(filterInstance, newFilterType),
Expression.Condition
(
replaced.Body,
Expression.Constant(true),
Expression.Constant(false)
)
);

The expression block can now be logically combined with the first expression, as now both expect the same input type. Then the resulting expression would look similar to this:

Lambda1(Com.Example.SomeClassToBeFiltered $e)
{
$e.Property1 >= 42 && .Block(Com.Example.MagicFilter $filterTypeInstance)
{
$filterTypeInstance = .New Com.Example.MagicFilter($e);
.If (
(System.Boolean)$filterTypeInstance.IsPresent == True &&
(System.Boolean)$filterTypeInstance.ItemsAvailable > 0
) {
True
} .Else {
False
}
}
}

Once again, with the power of expressions, it is possible to achieve the seemingly impossible …

Ronald Rink

I am a senior auditor and consultant at d-fens for business processes and information systems.

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