我是Xamarin的初学者，我试图学习制作简单的图形元素，例如按钮和图像。我的问题是，当我尝试更改元素的尺寸或位置（在示例中为按钮）时，似乎什么都没有改变。

我已经尝试过类似startButton.WidthRequest = 300;或使用绝对布局AbsoluteLayout.SetLayoutBounds(startButton,new Rectangle(0.5,0.2,200,100));之类的方法，但是没有任何效果，并且在模拟器中的应用程序中，所有内容似乎都在屏幕底部设置了默认高度和宽度屏幕上的。

XAML（也尝试过<AbsoluteLayout>，但遇到了同样的问题，所有内容都设置在屏幕的左上角，并且setLayoutBounds或WidthRequest无效）

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:d="http://xamarin.com/schemas/2014/forms/design"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         mc:Ignorable="d"
         xmlns:local="clr-namespace:TestImages;assembly=TestImages"
         x:Class="TestImages.MainPage">
<StackLayout x:Name="container1">
    <!-- Place new controls here -->
    <Label Text="Welcome to MyApp" HorizontalOptions="Center" VerticalOptions="StartAndExpand" />

    <Image Source="{local:ImageResource TestImages.kids2.png}"/>

    <Button x:Name="startButton"
            Text="START"
            Clicked="goToList" />

</StackLayout>
</Contenpage>

C＃

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Essentials;
using Xamarin.Forms;

namespace TestImages
{
    // Learn more about making custom code visible in the Xamarin.Forms previewer
    // by visiting https://aka.ms/xamarinforms-previewer
    [DesignTimeVisible(false)]
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();

            var mainDisplayInfo = DeviceDisplay.MainDisplayInfo;

        var screenWidth = mainDisplayInfo.Width;
        Console.WriteLine($"Width: {screenWidth}");

        startButton.BackgroundColor = Color.LightGreen;
        startButton.BorderColor = Color.White;
        startButton.CornerRadius = 8;
        startButton.BorderWidth = 1.5;
        startButton.WidthRequest = 300;
        AbsoluteLayout.SetLayoutBounds(startButton,100));

        var bottone2 = new Button { Text = "Start222" };
        container1.Children.Add(bottone2);
        AbsoluteLayout.SetLayoutBounds(bottone2,100));
        bottone2.BackgroundColor = Color.Beige;


        Console.WriteLine($"Button: {startButton.Width}");
    }

    async void goToList(object sender,EventArgs e)
    {
        System.Diagnostics.Debug.Write("Cliccato START");
        Console.Write("Cliccato START");
        await Navigation.PushAsync(new ListPage());

    }
  }
}

我让它打印调试的按钮宽度，它说它是= -1，不知道为什么，这是什么意思。