Implement the runtime side of the "method->Delegate" binding conversion for compiled bindings.

src/Avalonia.Base/Data/Core/Plugins/MethodAccessorPlugin.cs

@@ -22,19 +22,9 @@ namespace Avalonia.Data.Core.Plugins
 
             var method = GetFirstMethodWithName(instance.GetType(), methodName);
 
-            if (method != null)
+            if (method is not null)
             {
-                var parameters = method.GetParameters();
-
-                if (parameters.Length + (method.ReturnType == typeof(void) ? 0 : 1) > 8)
-                {
-                    var exception = new ArgumentException(
-                        "Cannot create a binding accessor for a method with more than 8 parameters or more than 7 parameters if it has a non-void return type.",
-                        nameof(methodName));
-                    return new PropertyError(new BindingNotification(exception, BindingErrorType.Error));
-                }
-
-                return new Accessor(reference, method, parameters);
+                return new Accessor(reference, method);
             }
             else
             {
@@ -82,7 +72,7 @@ namespace Avalonia.Data.Core.Plugins
 
         private sealed class Accessor : PropertyAccessorBase
         {
-            public Accessor(WeakReference<object?> reference, MethodInfo method, ParameterInfo[] parameters)
+            public Accessor(WeakReference<object?> reference, MethodInfo method)
             {
                 _ = reference ?? throw new ArgumentNullException(nameof(reference));
                 _ = method ?? throw new ArgumentNullException(nameof(method));
@@ -90,10 +80,13 @@ namespace Avalonia.Data.Core.Plugins
                 var returnType = method.ReturnType;
                 bool hasReturn = returnType != typeof(void);
 
+                var parameters = method.GetParameters();
+
                 var signatureTypeCount = (hasReturn ? 1 : 0) + parameters.Length;
 
                 var paramTypes = new Type[signatureTypeCount];
 
+
                 for (var i = 0; i < parameters.Length; i++)
                 {
                     ParameterInfo parameter = parameters[i];
@@ -105,13 +98,10 @@ namespace Avalonia.Data.Core.Plugins
                 {
                     paramTypes[paramTypes.Length - 1] = returnType;
 
-                    PropertyType = Expression.GetFuncType(paramTypes);
-                }
-                else
-                {
-                    PropertyType = Expression.GetActionType(paramTypes);
                 }
 
+                PropertyType = Expression.GetDelegateType(paramTypes);
+
                 if (method.IsStatic)
                 {
                     Value = method.CreateDelegate(PropertyType);

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -20,6 +20,7 @@
         <Compile Include="MarkupExtensions\CompiledBindings\ArrayElementPlugin.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\CompiledBindingPath.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\FindVisualAncestorNode.cs" />
+        <Compile Include="MarkupExtensions\CompiledBindings\MethodAccessorPlugin.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\ObservableStreamPlugin.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\PropertyInfoAccessorFactory.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\PropertyInfoAccessorPlugin.cs" />

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/CompiledBindingPath.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using Avalonia.Controls;
 using Avalonia.Data.Core;
 using Avalonia.Data.Core.Plugins;
@@ -36,6 +37,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
                     case PropertyElement prop:
                         node = new PropertyAccessorNode(prop.Property.Name, enableValidation, new PropertyInfoAccessorPlugin(prop.Property, prop.AccessorFactory));
                         break;
+                    case MethodAsDelegateElement methodAsDelegate:
+                        node = new PropertyAccessorNode(methodAsDelegate.Method.Name, enableValidation, new MethodAccessorPlugin(methodAsDelegate.Method, methodAsDelegate.DelegateType));
+                        break;
                     case ArrayElementPathElement arr:
                         node = new PropertyAccessorNode(CommonPropertyNames.IndexerName, enableValidation, new ArrayElementPlugin(arr.Indices, arr.ElementType));
                         break;
@@ -92,6 +96,12 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
             return this;
         }
 
+        public CompiledBindingPathBuilder Method(RuntimeMethodHandle handle, Type delegateType)
+        {
+            _elements.Add(new MethodAsDelegateElement(handle, delegateType));
+            return this;
+        }
+
         public CompiledBindingPathBuilder StreamTask<T>()
         {
             _elements.Add(new TaskStreamPathElement<T>());
@@ -178,6 +188,19 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
             => _isFirstElement ? Property.Name : $".{Property.Name}";
     }
 
+    internal class MethodAsDelegateElement : ICompiledBindingPathElement
+    {
+        public MethodAsDelegateElement(RuntimeMethodHandle method, Type delegateType)
+        {
+            Method = (MethodInfo)MethodBase.GetMethodFromHandle(method);
+            DelegateType = delegateType;
+        }
+
+        public MethodInfo Method { get; }
+
+        public Type DelegateType { get; }
+    }
+
     internal interface IStronglyTypedStreamElement : ICompiledBindingPathElement
     {
         IStreamPlugin CreatePlugin();

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/CompiledBindings/MethodAccessorPlugin.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Reflection;
+using System.Text;
+using Avalonia.Data;
+using Avalonia.Data.Core.Plugins;
+
+#nullable enable
+
+namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
+{
+    internal class MethodAccessorPlugin : IPropertyAccessorPlugin
+    {
+        private MethodInfo _method;
+        private readonly Type _delegateType;
+
+        public MethodAccessorPlugin(MethodInfo method, Type delegateType)
+        {
+            _method = method;
+            _delegateType = delegateType;
+        }
+
+        public bool Match(object obj, string propertyName)
+        {
+            throw new InvalidOperationException("The MethodAccessorPlugin does not support dynamic matching");
+        }
+
+        public IPropertyAccessor Start(WeakReference<object?> reference, string propertyName)
+        {
+            Debug.Assert(_method.Name == propertyName);
+            return new Accessor(reference, _method, _delegateType);
+        }
+
+        private sealed class Accessor : PropertyAccessorBase
+        {
+            public Accessor(WeakReference<object?> reference, MethodInfo method, Type delegateType)
+            {
+                _ = reference ?? throw new ArgumentNullException(nameof(reference));
+                _ = method ?? throw new ArgumentNullException(nameof(method));
+
+                PropertyType = delegateType;
+
+                if (method.IsStatic)
+                {
+                    Value = method.CreateDelegate(PropertyType);
+                }
+                else if (reference.TryGetTarget(out var target))
+                {
+                    Value = method.CreateDelegate(PropertyType, target);
+                }
+            }
+
+            public override Type? PropertyType { get; }
+
+            public override object? Value { get; }
+
+            public override bool SetValue(object? value, BindingPriority priority) => false;
+
+            protected override void SubscribeCore()
+            {
+                try
+                {
+                    PublishValue(Value);
+                }
+                catch { }
+            }
+
+            protected override void UnsubscribeCore()
+            {
+            }
+        }
+    }
+}