Creating a custom Attached Property and setting its value in XAML

[npolyak]

I am trying to create a custom attached property TheWidth: and then refer to it in XAML. Here is my XAML code:

<Grid local:AttachedProperties.TheWidth="250">

And here is my attached property code:

public class AttachedProperties : AvaloniaObject
{
     public static readonly AttachedProperty<double> TheWidthProperty =
           AvaloniaProperty.RegisterAttached<AttachedProperties, Interactive, double>
           (
           "TheWidth",
           default(double),
           false);
 }

The compiler complains about it "Unable to resolve suitable regular or attached property TheWidth on type SimpleAvalonTests:SimpleAvalonTests.AttachedProperties

What am I doing wrong?

[maxkatz6]

You are missing generic parameters.
Let's take an example of Tooltip.Tip:

public static readonly AttachedProperty<object?> TipProperty =
    AvaloniaProperty.RegisterAttached<ToolTip, Control, object?>("Tip");

Here "object?" is the type of value that this property contain.
"ToolTip" is the type of property owner, usually of the class where it will be used and where it was defined.
"Control" is the type of property host, on which you can set or get this property.

See definition of RegisterAttached method https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Base/AvaloniaProperty.cs#L267-L287

[maxkatz6]

So in your case I believe it should be something like this:

public class AttachedProperties : AvaloniaObject
{
    public static readonly AttachedProperty<double> TheWidthProperty =
        AvaloniaProperty.RegisterAttached<AttachedProperties, Control, double>("TheWidth");
} 

(default value and "inherits" are default in your example, so can be skipped)

[npolyak]

I just realized that your code is exactly the same as mine (see above - I fixed the code style), aside from using Control as a second generic argument instead of Interactive. I changed to Control and I still have the same problem. Is something wrong with my XAML? I made it to be the same as in WPF

[jp2masa]

I think you have to define a getter and setter like this:

public static double GetTheWidth(AvaloniaObject obj) => obj.GetValue(TheWidthProperty);

public static void SetTheWidth(AvaloniaObject obj, double value) => obj.SetValue(TheWidthProperty, value);

[npolyak]

Thanks JP! it worked!!!