#478 – Two Out of Four Margin Values Are Used for Elements in a Canvas

When adding child elements to a Canvas panel, you can specify at most two of the four attached properties: Top, Left, Bottom, Right.  This places the child element relative to one of the four corners of the Canvas.

Child elements can have values for their Margin property, but only two of the four possible sub-values are used.  The two parts of the margin used are based on the two Canvas attached properties that are being used.

For example, if you position a child element using the Canvas.Bottom and Canvas.Left properties, then only the Bottom and Left portions of the child element’s Margin are used.  The Top and Right margins are ignored.

Margin values that are used are added to the canvas positioning properties.

<Canvas>
    <Button Content="Left=10, Margin.Left=0" Canvas.Left="10" Canvas.Top="10" Margin="0,0,0,0"/>
    <Button Content="Left=10, Margin.Left=15" Canvas.Left="10" Canvas.Top="40" Margin="15,0,0,0"/>
</Canvas>

#475 – Child Elements in Canvas Can Overlap

It’s possible for child elements of a Canvas to overlap each other, depending on their position.  In the example below, the Canvas contains four Button controls, each located relative to one of the four corners of the Canvas.  As the parent window is resized, it eventually gets small enough for the child elements to overlap.

When child elements in a Canvas overlap, their order is based on the order in which they were added to the Canvas.  If the elements were all specified in XAML, the first elements listed will be at the bottom of overlapped elements and the last elements will be at the top.

    <Canvas>
        <Button Content="1 - Lft10,Top10" Canvas.Left="10" Canvas.Top="10"/>
        <Button Content="2 - Rt10,Top10" Canvas.Right="10" Canvas.Top="15"/>
        <Button Content="3 - Lft10,Bott10..." Canvas.Left="15" Canvas.Bottom="15"/>
        <Button Content="4 - Rt10,Bott10" Canvas.Right="10" Canvas.Bottom="8"/>
    </Canvas>

#473 – Positioning Child Elements in a Canvas

Child elements are position in a Canvas panel by specifying the location of the child element in WPF units.

You can position all elements by specifying values for Left and Top properties only.  This will position each element relative to the upper left corner of the container.

These child elements will then retain the same position from the upper left corner of the container as it is resized.

You can specify a child element’s position relative to any of the four corners of the container, depending on which two of the four positioning properties you specify.  Child elements will then retain their position relative to that corner as the container is resized.

• Left/Top – upper left corner
• Right/Top – upper right corner
• Right/Bottom – lower right corner
• Left/Bottom – lower bottom corner

    <Canvas>
        <Button Content="Left=10,Top=10" Canvas.Left="10" Canvas.Top="10"/>
        <Button Content="Right=10,Top=10" Canvas.Right="10" Canvas.Top="10"/>
        <Button Content="Right=10,Bottom=10" Canvas.Right="10" Canvas.Bottom="10"/>
        <Button Content="Left=10,Bottom=10" Canvas.Left="10" Canvas.Bottom="10"/>
    </Canvas>

#393 – Canvas Element

The Canvas panel serves as a container for a collection of child elements and allows positioning its children using exact coordinates (in WPF Units).

You can specify the following attached properties for a child element of the Canvas, to control how the child element is positioned.

• Left – Distance between left side of element and left side of canvas
• Top – Distance between top side of element and top side of canvas
• Right – Distance between right side of element and right side of canvas
• Bottom – Distance between bottom side of element and bottom of canvas

If you don’t specify values for any of these properties for a child element, it will be positioned in the upper left corner of the Canvas.

    <Canvas>
        <Label Content="Pease Porridge Hot"/>
        <Label Content="Pease Porridge Cold" Canvas.Left="99" Canvas.Top="23" />
        <Button Content="In the Pot" Canvas.Left="55" Canvas.Top="63" />
        <TextBox Text="Nine Days Old" Canvas.Left="161" Canvas.Top="111" />
    </Canvas>