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[Charlie Poole <charlie@xxxxxxxxx>]

** Tags added: feature

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https://bugs.launchpad.net/bugs/901192

Title:
  Feature request: Provide a strongly typed variant of the Property
  constraint

Status in NUnit V2 Test Framework:
  Triaged

Bug description:
  Currently, NUnit supports a number of generic constraints that define
  the type of the tested expression at compile time (if they succeed):
  Is.TypeOf<T>(), Is.AssignableTo<T>(), and Is.InstanceOf<T>(). (Note
  that Is.AssignableFrom<T>() is not a candidate for this feature
  request because it only defines a derived type!)

  Those constraint factory methods currently return instances of non-
  generic expression types (e.g., ExactTypeConstraint, etc.) that
  inherit the ordinary, untyped constraint expression factory methods,
  so that you can write, for example: Is.TypeOf<object>().With.Property
  ("P").EqualTo (42). Note that when "With" is called, the type
  information specified at the TypeOf<T> call gets lost and the Property
  constraint needs to take the property name by string rather than, for
  example, by expression.

  Passing the property name as string is bad as an invalid name will not
  trigger a compile-time error and as automated refactorings (e.g.,
  renaming the property) will not take notice of the property name.

  Therefore, change TypeOf<T>, AssignableTo<T>, and InstanceOf<T> to
  return generic subclasses of the contraint types (e.g.,
  ExactTypeConstraint<T>) that redefine Constraint.With to retain the
  type information. Provide generic variants of ConstraintExpression and
  ResolvableConstraintExpression to allow for nested strongly typed
  constraints. Then provide a generic version of the Property constraint
  factory method.

  Here is what the code could look like for NUnit 2.6 (with non-breaking
  changes).

  public class ExactTypeConstraint<T> : ExactTypeConstraint
  {
    public new ConstraintExpression<T> With { get { ... } }
  }

  public class ConstraintExpression<T> : ConstraintExpression 
  {
    public new ConstraintExpression<T> With { get { ... } }
    public new ConstraintExpression<T> And { get { ... } }
    // ...

    public ResolvableConstraintExpression<T> Property<TR> (Expression<Func<T, TR>> propertyAccessor);
  }

  public class ResolvableConstraintExpression<T> : ResolvableConstraintExpression
  {
    public new ConstraintExpression<T> With { get { ... } }
    public new ConstraintExpression<T> And { get { ... } }
  }

  That way, one could write constraints similar to:
  Is.TypeOf<object>().With.Property (o => o.P).EqualTo (42).And.Property (o => o.P2).EqualTo (43)

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