Implement method->delegate mapping in the XAML compiler for well-known delegate types

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/XamlIlBindingPathHelper.cs

@@ -119,16 +119,18 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                             nodes.Add(new XamlIlAvaloniaPropertyPropertyPathElementNode(avaloniaPropertyFieldMaybe,
                                 XamlIlAvaloniaPropertyHelper.GetAvaloniaPropertyType(avaloniaPropertyFieldMaybe, context.GetAvaloniaTypes(), lineInfo)));
                         }
-                        else
+                        else if (GetAllDefinedProperties(targetType).FirstOrDefault(p => p.Name == propName.PropertyName) is IXamlProperty clrProperty)
                         {
-                            var clrProperty = GetAllDefinedProperties(targetType).FirstOrDefault(p => p.Name == propName.PropertyName);
-
-                            if (clrProperty is null)
-                            {
-                                throw new XamlX.XamlParseException($"Unable to resolve property of name '{propName.PropertyName}' on type '{targetType}'.", lineInfo);
-                            }
                             nodes.Add(new XamlIlClrPropertyPathElementNode(clrProperty));
                         }
+                        else if (GetAllDefinedMethods(targetType).FirstOrDefault(m => m.Name == propName.PropertyName) is IXamlMethod method)
+                        {
+                            nodes.Add(new XamlIlClrMethodPathElementNode(method, context.Configuration.WellKnownTypes.Delegate));
+                        }
+                        else
+                        {
+                            throw new XamlX.XamlParseException($"Unable to resolve property or method of name '{propName.PropertyName}' on type '{targetType}'.", lineInfo);
+                        }
                         break;
                     case BindingExpressionGrammar.IndexerNode indexer:
                         {
@@ -275,6 +277,17 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 }
             }
 
+            static IEnumerable<IXamlMethod> GetAllDefinedMethods(IXamlType type)
+            {
+                foreach (var t in TraverseTypeHierarchy(type))
+                {
+                    foreach (var m in t.Methods)
+                    {
+                        yield return m;
+                    }
+                }
+            }
+
             static IEnumerable<IXamlType> TraverseTypeHierarchy(IXamlType type)
             {
                 if (type.IsInterface)
@@ -529,6 +542,50 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
             public IXamlType Type => _property.Getter?.ReturnType ?? _property.Setter?.Parameters[0];
         }
 
+        class XamlIlClrMethodPathElementNode : IXamlIlBindingPathElementNode
+        {
+            private readonly IXamlMethod _method;
+
+            public XamlIlClrMethodPathElementNode(IXamlMethod method, IXamlType systemDelegateType)
+            {
+                _method = method;
+                Type = systemDelegateType;
+            }
+
+            public IXamlType Type { get; }
+
+            public void Emit(XamlIlEmitContext context, IXamlILEmitter codeGen)
+            {
+                IXamlType specificDelegateType;
+                if (_method.ReturnType == context.Configuration.WellKnownTypes.Void && _method.Parameters.Count <= 8)
+                {
+                    specificDelegateType = context.Configuration.TypeSystem
+                        .GetType($"System.Action`{_method.Parameters.Count}")
+                        .MakeGenericType(_method.Parameters);
+                }
+                else if (_method.Parameters.Count <= 7)
+                {
+                    List<IXamlType> genericParameters = new();
+                    genericParameters.AddRange(_method.Parameters);
+                    genericParameters.Add(_method.ReturnType);
+                    specificDelegateType = context.Configuration.TypeSystem
+                        .GetType($"System.Func`{_method.Parameters.Count}")
+                        .MakeGenericType(genericParameters);
+                }
+                else
+                {
+                    // In this case, we need to emit our own delegate type.
+                    specificDelegateType = null;
+                }
+
+                codeGen
+                    .Ldtoken(_method)
+                    .Ldtoken(specificDelegateType)
+                    .EmitCall(context.GetAvaloniaTypes()
+                        .CompiledBindingPathBuilder.FindMethod(m => m.Name == "Method"));
+            }
+        }
+
         class XamlIlClrIndexerPathElementNode : IXamlIlBindingPathElementNode
         {
             private readonly IXamlProperty _property;

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

@@ -96,7 +96,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
             return this;
         }
 
-        public CompiledBindingPathBuilder Method(RuntimeMethodHandle handle, Type delegateType)
+        public CompiledBindingPathBuilder Method(RuntimeMethodHandle handle, RuntimeTypeHandle delegateType)
         {
             _elements.Add(new MethodAsDelegateElement(handle, delegateType));
             return this;
@@ -190,10 +190,10 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions.CompiledBindings
 
     internal class MethodAsDelegateElement : ICompiledBindingPathElement
     {
-        public MethodAsDelegateElement(RuntimeMethodHandle method, Type delegateType)
+        public MethodAsDelegateElement(RuntimeMethodHandle method, RuntimeTypeHandle delegateType)
         {
             Method = (MethodInfo)MethodBase.GetMethodFromHandle(method);
-            DelegateType = delegateType;
+            DelegateType = Type.GetTypeFromHandle(delegateType);
         }
 
         public MethodInfo Method { get; }