From c7d565fd0ca12af45ffa99a44b8e9977a1be74e9 Mon Sep 17 00:00:00 2001
From: MonkAlex <3amepob@gmail.com>
Date: Sun, 5 Aug 2018 11:11:39 +0400
Subject: [PATCH 1/2] #1798 Fix binding checkbox with three state.

---
 src/Avalonia.Controls/Primitives/ToggleButton.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Avalonia.Controls/Primitives/ToggleButton.cs b/src/Avalonia.Controls/Primitives/ToggleButton.cs
index dc9b70ab8c..1d2e2f2100 100644
--- a/src/Avalonia.Controls/Primitives/ToggleButton.cs
+++ b/src/Avalonia.Controls/Primitives/ToggleButton.cs
@@ -14,7 +14,7 @@ namespace Avalonia.Controls.Primitives
                 nameof(IsChecked),
                 o => o.IsChecked,
                 (o, v) => o.IsChecked = v,
-                unsetValue: false,
+                unsetValue: null,
                 defaultBindingMode: BindingMode.TwoWay);
 
         public static readonly StyledProperty<bool> IsThreeStateProperty =

From e69f3c7f85f2a751ef9f0a343c5a38753f204714 Mon Sep 17 00:00:00 2001
From: MonkAlex <3amepob@gmail.com>
Date: Sun, 5 Aug 2018 13:36:32 +0400
Subject: [PATCH 2/2] #1798 Test for binding checkbox with three state.

---
 .../Primitives/ToggleButtonTests.cs           | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tests/Avalonia.Controls.UnitTests/Primitives/ToggleButtonTests.cs b/tests/Avalonia.Controls.UnitTests/Primitives/ToggleButtonTests.cs
index ab74d5e7d6..4f4ab47b0a 100644
--- a/tests/Avalonia.Controls.UnitTests/Primitives/ToggleButtonTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/Primitives/ToggleButtonTests.cs
@@ -44,15 +44,41 @@ namespace Avalonia.Controls.Primitives.UnitTests
             Assert.False(toggleButton.IsChecked);
         }
 
+        [Fact]
+        public void ToggleButton_ThreeState_Checked_Binds_To_Nullable_Bool()
+        {
+            var threeStateButton = new ToggleButton();
+            var source = new Class1();
+
+            threeStateButton.DataContext = source;
+            threeStateButton.Bind(ToggleButton.IsCheckedProperty, new Binding(nameof(Class1.NullableFoo)));
+
+            source.NullableFoo = true;
+            Assert.True(threeStateButton.IsChecked);
+
+            source.NullableFoo = false;
+            Assert.False(threeStateButton.IsChecked);
+
+            source.NullableFoo = null;
+            Assert.Null(threeStateButton.IsChecked);
+        }
+
         private class Class1 : NotifyingBase
         {
             private bool _foo;
+            private bool? nullableFoo;
 
             public bool Foo
             {
                 get { return _foo; }
                 set { _foo = value; RaisePropertyChanged(); }
             }
+
+            public bool? NullableFoo
+            {
+                get { return nullableFoo; }
+                set { nullableFoo = value; RaisePropertyChanged(); }
+            }
         }
     }
 }