Fixed CollectionView SelectedItem binding issue on initial loading

src/Controls/src/Core/Handlers/Items/SelectableItemsViewHandler.Windows.cs

@@ -206,6 +206,14 @@ void UpdateItemContentControlSelection()
 			}
 		}
 
+		protected override void UpdateItemsLayout()
+		{
+			_ignorePlatformSelectionChange = true;
+
+			base.UpdateItemsLayout();
+			_ignorePlatformSelectionChange = false;
+		}
+
 		protected override void UpdateItemsSource()
 		{
 			_ignorePlatformSelectionChange = true;

src/Controls/src/Core/Handlers/Items/iOS/SelectableItemsViewController.cs

@@ -40,7 +40,11 @@ internal void SelectItem(object selectedItem)
 
 			if (index.Section > -1 && index.Item > -1)
 			{
-				CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
+                // Ensure the selected index is updated after the collection view's items generation is completed
+				CollectionView.PerformBatchUpdates(null, _ =>
+				{
+					CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
+				});
 			}
 		}
 
@@ -168,4 +172,4 @@ void SynchronizePlatformSelectionWithSelectedItems()
 			}
 		}
 	}
-}
+}

src/Controls/src/Core/Handlers/Items2/iOS/SelectableItemsViewController2.cs

@@ -40,7 +40,11 @@ internal void SelectItem(object selectedItem)
 
 			if (index.Section > -1 && index.Item > -1)
 			{
-				CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
+                // Ensure the selected index is updated after the collection view's items generation is completed
+				CollectionView.PerformBatchUpdates(null, _ =>
+				{
+					CollectionView.SelectItem(index, true, UICollectionViewScrollPosition.None);
+				});
 			}
 		}
 
@@ -168,4 +172,4 @@ void SynchronizePlatformSelectionWithSelectedItems()
 			}
 		}
 	}
-}
+}

src/Controls/src/Core/Platform/Windows/CollectionView/ItemContentControl.cs

@@ -210,6 +210,18 @@ internal void Realize()
 			}
 
 			itemsView?.AddLogicalChild(_visualElement);
+
+			if (itemsView is SelectableItemsView selectableItemsView && selectableItemsView.SelectionMode is not SelectionMode.None)
+			{
+				bool isSelected = false;
+				if (selectableItemsView.SelectionMode == SelectionMode.Single)
+					isSelected = selectableItemsView.SelectedItem == FormsDataContext;
+				else
+					isSelected = selectableItemsView.SelectedItems.Contains(FormsDataContext);
+
+				if (isSelected)
+					UpdateIsSelected(isSelected);
+			}
 		}
 
 		void SetNativeStateConsistent(VisualElement visualElement)

src/Controls/src/Core/PublicAPI/net-windows/PublicAPI.Unshipped.txt

@@ -10,6 +10,7 @@
 *REMOVED*~Microsoft.Maui.Controls.NavigableElement.StyleClass.get -> System.Collections.Generic.IList<string>
 *REMOVED*~Microsoft.Maui.Controls.NavigableElement.StyleClass.set -> void
 Microsoft.Maui.Controls.HybridWebView.SetInvokeJavaScriptTarget<T>(T! target) -> void
+override Microsoft.Maui.Controls.Handlers.Items.SelectableItemsViewHandler<TItemsView>.UpdateItemsLayout() -> void
 ~Microsoft.Maui.Controls.ResourceDictionary.SetAndCreateSource<T>(System.Uri value) -> void
 *REMOVED*~Microsoft.Maui.Controls.ResourceDictionary.SetAndLoadSource(System.Uri value, string resourcePath, System.Reflection.Assembly assembly, System.Xml.IXmlLineInfo lineInfo) -> void
 ~Microsoft.Maui.Controls.WebViewProcessTerminatedEventArgs.PlatformArgs.get -> Microsoft.Maui.Controls.PlatformWebViewProcessTerminatedEventArgs

src/Controls/tests/TestCases.HostApp/Issues/Issue25551.xaml

@@ -0,0 +1,100 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue25551"
+             xmlns:local="clr-namespace:Maui.Controls.Sample.Issues">
+
+  <VerticalStackLayout>
+    <CollectionView AutomationId="SingleSelection"
+    Margin="15,0,0,15"
+    SelectionMode="Single"   
+    HeightRequest="300"
+    ItemsSource="{Binding Items}"
+    SelectedItem="{Binding SelectedItem}"
+    VerticalOptions="Center">
+      <CollectionView.ItemsLayout>
+        <LinearItemsLayout Orientation="Horizontal" ItemSpacing="10"/>
+      </CollectionView.ItemsLayout>
+      <CollectionView.ItemTemplate>
+        <DataTemplate>
+          <Grid RowDefinitions="Auto,4" RowSpacing="5">
+            <VisualStateManager.VisualStateGroups>
+              <VisualStateGroup x:Name="CommonStates">
+                <VisualState x:Name="Normal">
+                  <VisualState.Setters>
+                    <Setter Property="BackgroundColor" Value="Transparent"/>
+                    <Setter TargetName="SelectedIndicator" Property="BoxView.IsVisible" Value="False"/>
+                  </VisualState.Setters>
+                </VisualState>
+                <VisualState x:Name="Selected">
+                  <VisualState.Setters>
+                    <Setter Property="BackgroundColor" Value="Transparent"/>
+                    <Setter Property="Background" Value="Transparent"/>
+                    <Setter TargetName="SelectedIndicator" Property="BoxView.IsVisible" Value="True"/>
+                  </VisualState.Setters>
+                </VisualState>
+              </VisualStateGroup>
+            </VisualStateManager.VisualStateGroups>
+
+            <Label 
+              Text="{Binding .}" 
+              x:Name="ItemName"
+              FontAttributes="Bold"
+              FontSize="14"
+              VerticalOptions="Center"
+              HorizontalOptions="Center"
+              TextColor="Green"/>
+            <BoxView x:Name="SelectedIndicator" Color="blue" HeightRequest="4" HorizontalOptions="Fill" Grid.Row="1"/>
+          </Grid>
+        </DataTemplate>
+      </CollectionView.ItemTemplate>
+
+    </CollectionView>
+
+    <CollectionView AutomationId="MultipleSelection"
+    Margin="15,0,0,15"
+    SelectionMode="Multiple"   
+    HeightRequest="300"
+    ItemsSource="{Binding Items}"
+    SelectedItems="{Binding SelectedItems}"
+    VerticalOptions="Center">
+      <CollectionView.ItemsLayout>
+        <LinearItemsLayout Orientation="Horizontal" ItemSpacing="10"/>
+      </CollectionView.ItemsLayout>
+      <CollectionView.ItemTemplate>
+        <DataTemplate>
+          <Grid RowDefinitions="Auto,4" RowSpacing="5">
+            <VisualStateManager.VisualStateGroups>
+              <VisualStateGroup x:Name="CommonStates">
+                <VisualState x:Name="Normal">
+                  <VisualState.Setters>
+                    <Setter Property="BackgroundColor" Value="Transparent"/>
+                    <Setter TargetName="SelectedIndicator" Property="BoxView.IsVisible" Value="False"/>
+                  </VisualState.Setters>
+                </VisualState>
+                <VisualState x:Name="Selected">
+                  <VisualState.Setters>
+                    <Setter Property="BackgroundColor" Value="Transparent"/>
+                    <Setter Property="Background" Value="Transparent"/>
+                    <Setter TargetName="SelectedIndicator" Property="BoxView.IsVisible" Value="True"/>
+                  </VisualState.Setters>
+                </VisualState>
+              </VisualStateGroup>
+            </VisualStateManager.VisualStateGroups>
+
+            <Label 
+              Text="{Binding .}" 
+              x:Name="ItemName"
+              FontAttributes="Bold"
+              FontSize="14"
+              VerticalOptions="Center"
+              HorizontalOptions="Center"
+              TextColor="Green"/>
+            <BoxView x:Name="SelectedIndicator" Color="blue" HeightRequest="4" HorizontalOptions="Fill" Grid.Row="1"/>
+          </Grid>
+        </DataTemplate>
+      </CollectionView.ItemTemplate>
+
+    </CollectionView>
+  </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue25551.xaml.cs

@@ -0,0 +1,26 @@
+using Microsoft.Maui.Controls;
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 25551, "CollectionView SelectedItem binding issue on initial loading",
+		PlatformAffected.All)]
+	public partial class Issue25551 : ContentPage
+	{
+		public string SelectedItem { get; set; }
+
+		public ObservableCollection<string> Items { get; set; }
+		public List<object> SelectedItems { get; set; }
+
+		public Issue25551()
+		{
+			InitializeComponent();
+			SelectedItems = new List<object>();
+			Items = new ObservableCollection<string> { "A", "B", "C", "D", "E", "F" };
+			SelectedItem = Items[0];
+			SelectedItems.Add(Items[1]);
+			SelectedItems.Add(Items[3]);
+			BindingContext = this;
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25551.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using NUnit.Framework.Legacy;
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Tests.Issues
+{
+	internal class Issue25551 : _IssuesUITest
+	{
+		public override string Issue => "CollectionView SelectedItem binding issue on initial loading";
+
+		public Issue25551(TestDevice device) : base(device)
+		{
+		}
+
+		[Test]
+		[Category(UITestCategories.CollectionView)]
+
+		public void CollectionViewPreSelectionShouldUpdate()
+		{
+			App.WaitForElement("SingleSelection");
+			VerifyScreenshot();
+			App.WaitForElement("MultipleSelection");
+			VerifyScreenshot();
+		}
+	}
+}