当我尝试达到Rectangle时为空值

Question

I have a uwp project that i have a booking function in and would be pleased to have some help with one thing.

So i'm trying to reach a Rectangle in my UserControl from my Main.xaml.cs file, so i can set the Rectangles fill property.

This is how i have done it:

Method in Main.xaml.cs for finding the child property:

private DependencyObject FindChildControl<T>(DependencyObject control, string ctrlName)
        {
            int childNumber = VisualTreeHelper.GetChildrenCount(control);
            for (int i = 0; i < childNumber; i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(control, i);
                FrameworkElement fe = child as FrameworkElement;
                // Not a framework element or is null
                if (fe == null) return null;

                if (child is T && fe.Name == ctrlName)
                {
                    // Found the control so return
                    return child;
                }
                else
                {
                    // Not found it - search children
                    DependencyObject nextLevel = FindChildControl<T>(child, ctrlName);
                    if (nextLevel != null)
                        return nextLevel;
                }
            }
            return null;
        }

Inside the function where i want to call the Rectangle that is named StatusColor I do this:

Rectangle Rec = FindChildControl<Rectangle>(GridView1, "StatusColor") as Rectangle;

But somehow when I try to execute the code I get this error for the variable Rec :

System.NullReferenceException: 'Object reference not set to an instance of an object.'

I know there are many posts about this exception but i still not getting any smarter by looking at them...

But as i said, I would be very pleased with someones help

Thanks in advance!

EDIT:

My xaml in Main.xaml to declare the UserControl :

<GridView x:Name="GridView1" ItemContainerStyle="{StaticResource testgrid}" ItemsSource="{x:Bind Rooms}" Width="1740" Height="835" IsHitTestVisible="False" ScrollViewer.HorizontalScrollBarVisibility="Hidden" Margin="75,190,75,100" FontFamily="Segoe MDL2 Assets" IsDoubleTapEnabled="False" IsHoldingEnabled="False" IsRightTapEnabled="False" IsTapEnabled="False">
            <GridView.ItemTemplate>
                <DataTemplate x:DataType="data:Room">
                    <local:TemplateGrid x:Name="TemplateGrid"/>
                </DataTemplate>
            </GridView.ItemTemplate>
        </GridView>

And this is how my code looks like for the Rectangle :

<Rectangle x:Name="StatusColor" x:FieldModifier="public" Margin="0,0,0,0" Height="218" VerticalAlignment="Top">
                    <Rectangle.Fill>
                        <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                            <GradientStop Color="#00000000" Offset="0.901"/>
                            <GradientStop Color="#CC17FF00" Offset="0"/>
                        </LinearGradientBrush>
                    </Rectangle.Fill>
                </Rectangle>

Answer 1

It is these lines:

 // Not a framework element or is null
 if (fe == null) return null;

You are exiting the loop too early. When the element i is not a framework element, you should just ignore it and continue on to the next one.

So instead of exiting when you encounter a non-framework element, just execute the next lines only when it is one:

 if (fe != null)
 {
    // rest of checking
 }

or alternatively:

 if (fe == null) continue;

---

EDIT
A slight reworking of your code to:

    private T FindChildControl<T>(DependencyObject control, string ctrlName) 
         where T: FrameworkElement
    {
        int childNumber = VisualTreeHelper.GetChildrenCount(control);
        for (int i = 0; i < childNumber; i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(control, i);
            FrameworkElement fe = child as FrameworkElement;

            // Not a framework element or is null
            if (fe == null)
            {
                continue;
            }

            if (child is T && fe.Name == ctrlName)
            {
                // Found the control so return
                return (T)child;
            }
            else
            {
                // Not found it - search children
                T nextLevel = this.FindChildControl<T>(child, ctrlName);
                if (nextLevel != null)
                {
                    return nextLevel;
                }
            }
        }

        return null;
    }

should be called with

Rectangle rec = FindChildControl<Rectangle>(GridView1, "StatusColor");

to give the correct result. At least it did in my test situation (with adjusted parameters).

The only real change was the use of continue instead of return . Furthermore I made use of generics to immediately return the correct type, no casting needed.