Why am I seeing a "member is not recognized or is not accessible" error on my WPF User Control?

Question

I've got a custom user control with a public property that I'd like to be able to set in XAML. Here it is below.

TestControl.xaml

<UserControl x:Class="Scale.Controls.TestControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">

TestControl.xaml.cs

using System.Windows.Controls;

namespace MyProject.Controls
{
    public partial class TestControl : UserControl
    {
        public string TestMe { get; set; }
        public TestControl()
        {
            InitializeComponent();
        }
    }
}

Then, in my MainWindow.xaml file, I try to include this:

<controls:TestControl TestMe="asdf" />

However, even though Visual Studio autocompletes the TestMe property, I then see things with a squiggly underline that says "The member "Test Me" is not recognized or is not accessible," as seen below.

I could have sworn I've done something like this before in other projects. How can I access (i.e. set) the public properties via XAML like this?

Answer 1

You need to declare your property as Dependency Properties

namespace MyProject.Controls
{
    public partial class TestControl : UserControl
    {
        //Register Dependency Property

        public static readonly DependencyProperty TestMeDependency = DependencyProperty.Register("MyProperty", typeof(string), typeof(TestControl));

        public string MyCar
        {
            get
            {

                return (string)GetValue(TestMeDependency);

            }
            set
            {
                SetValue(TestMeDependency, value);
            }
        }

        public TestControl()
        {
            InitializeComponent();
        }
    }
}