Custom Panel change the TextBox Width or margin

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WPF

Dear Sir,

I am new to WPF, I try to developing a simple panel. In the panel arranging the textbox and textblock to Left and Right, i cannot reduce the Textbox width according to the form size. help me to improve knowledge and codes are following
thanks

lang="xml"><
Code : 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;

namespace WpfApplication4
{
    public class PanelClass : Panel
    {
        double mHeight = 0.0;
        double mWidth = 0.0;

        protected override Size MeasureOverride(Size availableSize)
        {
            foreach (UIElement child in this.Children)
            {
                child.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
                if (child.DesiredSize.Width > mWidth)
                {
                    mWidth = child.DesiredSize.Width;
                }
                mHeight += child.DesiredSize.Height;

            }

            if (double.IsPositiveInfinity(availableSize.Width))
            {
                return new Size(mWidth, mHeight);
            }
            else
            {
                return new Size(availableSize.Width, mHeight);
            }
        }


protected override Size ArrangeOverride(Size finalSize)
 {
 double x = 0.0;
 double y = 0.0;
 double fw = finalSize.Width;
 double TotRightWidth = 0.0;
 double TotWidth = 0.0;
  
 HorizontalAlignment horizontalAlignment = HorizontalAlignment;
  
 foreach (UIElement child in this.Children)
 {
 if (child is TextBox)
 {
 horizontalAlignment = ((TextBox)child).HorizontalAlignment; //Gets horizontal alignment
 TotWidth += child.DesiredSize.Width;
 }
 if (child is TextBlock)
 {
 horizontalAlignment = ((TextBlock)child).HorizontalAlignment; //Gets horizontal alignment
 TotWidth += child.DesiredSize.Width;
 }
 if (horizontalAlignment == HorizontalAlignment.Right)
 {
 TotRightWidth += child.DesiredSize.Width;
 }
 }
  
 double DiffWidth = (TotWidth - finalSize.Width) / Children.Count;
 fw = finalSize.Width - TotRightWidth;
  
 foreach (UIElement child in Children)
 {
 if (child is TextBox)
 {
 horizontalAlignment = ((TextBox)child).HorizontalAlignment; //Gets horizontal alignment
 //VerticalAlignment verticalAlignment = ((TextBox)child).VerticalAlignment; //Gets vertical alignment
 }
 if (child is TextBlock)
 {
 horizontalAlignment = ((TextBlock)child).HorizontalAlignment; //Gets horizontal alignment
 //VerticalAlignment verticalAlignment = ((TextBlock)child).VerticalAlignment; //Gets vertical alignment
 }
  
 if (horizontalAlignment == HorizontalAlignment.Right)
 {
 Rect rec = new Rect(new Point(fw , y), child.DesiredSize);
 child.Arrange(rec);
 fw += child.DesiredSize.Width; 
}
 else
 { 
Rect rec = new Rect(new Point(x, y), child.DesiredSize); 
child.Arrange(rec);
 x += child.DesiredSize.Width;
 }
 }
 return finalSize;
 }

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WPF Code:

<window x:class=""WpfApplication4.MainWindow"<br" mode="hold" xmlns:x="#unknown" />        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:local="clr-namespace:WpfApplication4"
        Title="MainWindow" Height="350" Width="500">
    <Grid >
        <local:PanelClass>

            <TextBox Height="23" HorizontalAlignment="Left" Margin="0" Name="textBox1" VerticalAlignment="Top" Text="Left1" Width="120" />
            <!--<TextBlock Height="23" Name="textBlock1" Margin="0" Text="TextBlockLeft" />-->
            <TextBox Height="23" HorizontalAlignment="Left" Margin="0" Name="textBox2" VerticalAlignment="Top"  Text="Left2" Width="120" />
            <TextBox Height="23" HorizontalAlignment="Right" Margin="0" Name="textBox3" VerticalAlignment="Top" Text="Right1" Width="120" />
            <TextBox Height="23" HorizontalAlignment="Right" Margin="0" Name="textBox4" VerticalAlignment="Top" Text="Right2" Width="120" />
            <TextBox Height="23" HorizontalAlignment="Right" Margin="0" Name="textBox5" VerticalAlignment="Top" Text="Right3" Width="120" />

        </local:PanelClass>
    </Grid>

</Window>