The wpf-autolayout from trueinteractions

Suggestion: Use Expression<> rather than Strings for declaring constraints?

I was wondering if you would consider extending AddLayoutConstraint to support Expression<TDelegate> rather than using magic-strings for constraint declarations. This has numerous benefits, such as compile-time verification that a layout declaration is type-safe.

I haven't looked too closely at the source-code, but as you'd need to use reflection or WPF's DependencyProperty features then you'd need to use Expression<Func<...>> instead of a naked Func<...>.

An initial API would resemble the current AddLayoutConstraint method, just with generic arguments for type-safety, and using named overloads for =, <, and > instead of using magic-strings:

class AutoLayoutPanel
{
    // A better name might be "ConstrainEquality", perhaps?
    public Int32 AddEqualityConstraint<TLeft,TRight,TProperty>( 
        TLeft left,
        Expression<Func<TLeft ,TProperty>> leftProperty,
        TRight right,
        Expression<Func<TRight,TProperty>> rightProperty
    )
        where TLeft     : UIElement
        where TRight    : UIElement
        where TProperty : IComparable<TProperty>
    {
        // ...
    }

    public Int32 AddLessThanConstraint<TLeft,TRight,TProperty>( 
        TLeft left,
        Expression<Func<TLeft ,TProperty>> leftProperty,
        TRight right,
        Expression<Func<TRight,TProperty>> rightProperty
    )
        where TLeft     : UIElement
        where TRight    : UIElement
        where TProperty : IComparable<TProperty>
    {
        // ...
    }

    public Int32 AddGreaterThanConstraint<TLeft,TRight,TProperty>( 
        TLeft left,
        Expression<Func<TLeft ,TProperty>> leftProperty,
        TRight right,
        Expression<Func<TRight,TProperty>> rightProperty
    )
        where TLeft     : UIElement
        where TRight    : UIElement
        where TProperty : IComparable<TProperty>
    {
        // ...
    }
}

So the example call-sites from the README file would be rewritten as:

panel.AddEqualityConstraint( button, b => b.Left, panel, p => p.Left, multiplier: 1, constant: 0 );
panel.AddEqualityConstraint( button, b => b.Middle, panel, p => p.Middle, multiplier: 1, constant: 0 );

But these can be combined into a single AddLayoutConstraint method if it accepts a more general Expression argument to define the constraint:

    public Int32 AddLayoutConstraint<TLeft,TRight>( 
        TLeft left,
        TRight right,
        Expression<Func<TLeft,TRight,Boolean>> constraint
    )
        where TLeft  : UIElement
        where TRight : UIElement
    {
        // ...
    }

Thus allowing call-sites to be like this:

panel.AddLayoutConstraint( button, panel, ( b, p ) => b.Left == p.Right );

panel.AddLayoutConstraint( button, panel, ( b, p ) => b.Left * 2 + 3 == p.Right * 2 + 3 );

And this way - the constraints could be defined as C# code embedded in XAML that's evaluated when the control is initialized, thus avoiding the need for explicit AddLayoutConstraints entirely.

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Suggestion: Use Expression<> rather than Strings for declaring constraints?

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