Cogs.ActiveExpressions 1.17.1

This library accepts a LambdaExpression and arguments to pass to it, dissects the LambdaExpression's body, and hooks into change notification events for properties (INotifyPropertyChanged), collections (INotifyCollectionChanged), and dictionaries (Cogs.Collections.INotifyDictionaryChanged).

// Employee implements INotifyPropertyChanged
var elizabeth = Employee.GetByName("Elizabeth");
var expr = ActiveExpression.Create(e => e.Name.Length, elizabeth);
// expr subscribed to elizabeth's PropertyChanged

Then, as changes involving any elements of the expression occur, a chain of automatic re-evaluation will get kicked off, possibly causing the active expression's Value property to change.

var elizabeth = Employee.GetByName("Elizabeth");
var expr = ActiveExpression.Create(e => e.Name.Length, elizabeth);
// expr.Value == 9
elizabeth.Name = "Lizzy";
// expr.Value == 5

Also, since exceptions may be encountered after an active expression was created due to subsequent element changes, active expressions also have a Fault property, which will be set to the exception that was encountered during evaluation.

var elizabeth = Employee.GetByName("Elizabeth");
var expr = ActiveExpression.Create(e => e.Name.Length, elizabeth);
// expr.Fault is null
elizabeth.Name = null;
// expr.Fault is NullReferenceException

Active expressions raise property change events of their own, so listen for those (kinda the whole point)!

var elizabeth = Employee.GetByName("Elizabeth");
var expr = ActiveExpression.Create(e => e.Name.Length, elizabeth);
expr.PropertyChanged += (sender, e) =>
{
    if (e.PropertyName == "Fault")
    {
        // Whoops
    }
    else if (e.PropertyName == "Value")
    {
        // Do something
    }
};

When you dispose of your active expression, it will disconnect from all the events.

var elizabeth = Employee.GetByName("Elizabeth");
using (var expr = ActiveExpression.Create(e => e.Name.Length, elizabeth))
{
    // expr subscribed to elizabeth's PropertyChanged
}
// expr unsubcribed from elizabeth's PropertyChanged

Active expressions will also try to automatically dispose of disposable objects they create in the course of their evaluation when and where it makes sense. Use the ActiveExpressionOptions class for more direct control over this behavior.

You can use the static property Optimizer to specify an optimization method to invoke automatically during the active expression creation process. We recommend Tuomas Hietanen's Linq.Expression.Optimizer, the utilization of which would like like so:

ActiveExpression.Optimizer = ExpressionOptimizer.tryVisit;

var a = Expression.Parameter(typeof(bool));
var b = Expression.Parameter(typeof(bool));

var lambda = Expression.Lambda<Func<bool, bool, bool>>
(
    Expression.AndAlso
    (
        Expression.Not(a),
        Expression.Not(b)
    ),
    a,
    b
); // lambda explicitly defined as (a, b) => !a && !b

var expr = ActiveExpression.Create<bool>(lambda, false, false);
// optimizer has intervened and defined expr as (a, b) => !(a || b)
// (because Augustus De Morgan said they're essentially the same thing, but this involves less steps)