Avoid boxing in EffectiveValue<T>.SetAndRaise.

Added `IValueEntry<T>` interface back in and use that if present to get the value from the entry.

src/Avalonia.Base/PropertyStore/BindingEntryBase.cs

@@ -1,12 +1,11 @@
 using System;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
 using System.Reactive.Disposables;
 using Avalonia.Data;
 
 namespace Avalonia.PropertyStore
 {
-    internal abstract class BindingEntryBase<TValue, TSource> : IValueEntry,
+    internal abstract class BindingEntryBase<TValue, TSource> : IValueEntry<TValue>,
         IObserver<TSource>,
         IObserver<BindingValue<TSource>>,
         IDisposable
@@ -58,6 +57,14 @@ namespace Avalonia.PropertyStore
             BindingCompleted();
         }
 
+        public TValue GetValue()
+        {
+            Start(produceValue: false);
+            if (!_hasValue)
+                throw new AvaloniaInternalException("The binding entry has no value.");
+            return _value!;
+        }
+
         public void Start() => Start(true);
 
         public void OnCompleted() => BindingCompleted();

src/Avalonia.Base/PropertyStore/EffectiveValue`1.cs

@@ -55,7 +55,8 @@ namespace Avalonia.PropertyStore
         {
             Debug.Assert(priority != BindingPriority.LocalValue);
             UpdateValueEntry(value, priority);
-            SetAndRaiseCore(owner, (StyledPropertyBase<T>)value.Property, (T)value.GetValue()!, priority);
+
+            SetAndRaiseCore(owner,  (StyledPropertyBase<T>)value.Property, GetValue(value), priority);
         }
 
         public void SetLocalValueAndRaise(
@@ -144,6 +145,14 @@ namespace Avalonia.PropertyStore
 
         protected override object? GetBoxedValue() => Value;
         
+        private static T GetValue(IValueEntry entry)
+        {
+            if (entry is IValueEntry<T> typed)
+                return typed.GetValue();
+            else
+                return (T)entry.GetValue()!;
+        }
+
         private void SetAndRaiseCore(
             ValueStore owner,
             StyledPropertyBase<T> property,

src/Avalonia.Base/PropertyStore/IValueEntry`1.cs

@@ -0,0 +1,16 @@
+namespace Avalonia.PropertyStore
+{
+    /// <summary>
+    /// Represents a typed value entry in a <see cref="ValueFrame"/>.
+    /// </summary>
+    internal interface IValueEntry<T> : IValueEntry
+    {
+        /// <summary>
+        /// Gets the value associated with the entry.
+        /// </summary>
+        /// <exception cref="AvaloniaInternalException">
+        /// The entry has no value.
+        /// </exception>
+        new T GetValue();
+    }
+}

src/Avalonia.Base/PropertyStore/ImmediateValueEntry.cs

@@ -2,7 +2,7 @@
 
 namespace Avalonia.PropertyStore
 {
-    internal class ImmediateValueEntry<T> : IValueEntry, IDisposable
+    internal class ImmediateValueEntry<T> : IValueEntry<T>, IDisposable
     {
         private readonly ImmediateValueFrame _owner;
         private readonly T _value;
@@ -26,5 +26,6 @@ namespace Avalonia.PropertyStore
         public void Dispose() => _owner.OnEntryDisposed(this);
 
         object? IValueEntry.GetValue() => _value;
+        T IValueEntry<T>.GetValue() => _value;
     }
 }