Q:

Draw a circle in WPF using min(width, height)/2 as radius

How I can draw a circle in WPF (without code-behind) using min(width, height)/2 as radius?

A:

you can do it in pure XAML you just need to use Binding for the values.  You also have to make sure that everything is named
  <Grid Name="grdMain"> 
      <Grid.ColumnDefinitions>
         <ColumnDefinition Width="75" Name="Col1" />
         <ColumnDefinition Width="100" Name="Col2" />
      </Grid.ColumnDefinitions>
      <Grid.RowDefinitions>
          <RowDefinition Height="75" Name="Row1" />
                <RowDefinition Height="100" Name="Row2" />
      </Grid.RowDefinitions>

           <Ellipse Grid.Column="1" Grid.Row="1"
                Canvas.Top="50"
                Canvas.Left="50"
                Fill="#FFFFFF00"
                Height="{Binding RowDefinitions/ActualHeight, ElementName=Row1, Mode=OneWay}"
                Width="{Binding ColumnDefinitions/ActualWidth, ElementName=Col1, Mode=OneWay}"
                StrokeThickness="5"
                Stroke="#FF0000FF"/>
   </Grid>

A:

Where does width and height come from?  Example XAML for a circle is:
   <Canvas Background="LightGray"> 
       <Ellipse
          Canvas.Top="50"
          Canvas.Left="50"
          Fill="#FFFFFF00"
          Height="75"
          Width="75"
          StrokeThickness="5"
          Stroke="#FF0000FF"/>
    </Canvas>

A circle is just an Ellipse where Height = Width.

