Add delegate -> command conversion as a default value conversion.

src/Markup/Avalonia.Markup/AlwaysEnabledDelegateCommand.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Windows.Input;
+
+namespace Avalonia.Markup
+{
+    class AlwaysEnabledDelegateCommand : ICommand
+    {
+        private readonly Delegate action;
+
+        public AlwaysEnabledDelegateCommand(Delegate action)
+        {
+            this.action = action;
+        }
+
+        public event EventHandler CanExecuteChanged;
+
+        public bool CanExecute(object parameter)
+        {
+            return true;
+        }
+
+        public void Execute(object parameter)
+        {
+            if (action.Method.GetParameters().Length == 0)
+            {
+                action.DynamicInvoke();
+            }
+            else
+            {
+                action.DynamicInvoke(parameter); 
+            }
+        }
+    }
+}

src/Markup/Avalonia.Markup/DefaultValueConverter.cs

@@ -5,6 +5,7 @@ using System;
 using System.Globalization;
 using Avalonia.Data;
 using Avalonia.Utilities;
+using System.Windows.Input;
 
 namespace Avalonia.Markup
 {
@@ -34,6 +35,11 @@ namespace Avalonia.Markup
                 return AvaloniaProperty.UnsetValue;
             }
 
+            if (typeof(ICommand).IsAssignableFrom(targetType) && value is Delegate d)
+            {
+                return new AlwaysEnabledDelegateCommand(d);
+            }
+
             if (TypeUtilities.TryConvert(targetType, value, culture, out object result))
             {
                 return result;

tests/Avalonia.Markup.UnitTests/DefaultValueConverterTests.cs

@@ -5,6 +5,8 @@ using System.Globalization;
 using Avalonia.Controls;
 using Avalonia.Data;
 using Xunit;
+using System.Windows.Input;
+using System;
 
 namespace Avalonia.Markup.UnitTests
 {
@@ -118,6 +120,42 @@ namespace Avalonia.Markup.UnitTests
             Assert.IsType<BindingNotification>(result);
         }
 
+        [Fact]
+        public void Can_Convert_From_Delegate_To_Command()
+        {
+            int commandResult = 0;
+
+            var result = DefaultValueConverter.Instance.Convert(
+                (Action<int>)((int i) => { commandResult = i; }),
+                typeof(ICommand),
+                null,
+                CultureInfo.InvariantCulture);
+
+            Assert.IsAssignableFrom<ICommand>(result);
+
+            (result as ICommand).Execute(5);
+
+            Assert.Equal(5, commandResult);
+        }
+
+        [Fact]
+        public void Can_Convert_From_Delegate_To_Command_No_Parameters()
+        {
+            int commandResult = 0;
+
+            var result = DefaultValueConverter.Instance.Convert(
+                (Action)(() => { commandResult = 1; }),
+                typeof(ICommand),
+                null,
+                CultureInfo.InvariantCulture);
+
+            Assert.IsAssignableFrom<ICommand>(result);
+
+            (result as ICommand).Execute(null);
+
+            Assert.Equal(1, commandResult);
+        }
+
         private enum TestEnum
         {
             Foo,