WPF : How do I set width to Auto in code behind?

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C#

WPF

How do I set the Width property of a UserControl in the code behind for the window I want it in to Auto?

I have a user control called Project, I have a MainWindow that uses Project.
The issue is that I need Project to have a Windows Forms-like Docking ability, which "Auto" fits in WPF for Width and Height instead of Fill in Windows Forms.
I've tried using Double.NaN to set the Width of Project in the MainWindow and it doesn't work.

My Code:

public MainWindow()
{
            InitializeComponent();
            Current_User_Project = new ProjectControl();
            Current_User_Project.HorizontalAlignment = HorizontalAlignment.Stretch;
            Current_User_Project.VerticalAlignment = VerticalAlignment.Stretch;
            Current_User_Project.Width = Double.NaN;
            Canvas c = new Canvas();
            c.HorizontalAlignment = HorizontalAlignment.Stretch;
            c.VerticalAlignment = VerticalAlignment.Stretch;
            c.Children.Add(Current_User_Project);
            Project_Tab.Content = c;
}