Check array bounds in expression indexer.

src/Markup/Perspex.Markup/Binding/IndexerNode.cs

@@ -90,7 +90,12 @@ namespace Perspex.Markup.Binding
 
             if (typeInfo.IsArray && _intArgs != null)
             {
-                return ((Array)target).GetValue(_intArgs);
+                var array = (Array)target;
+
+                if (InBounds(_intArgs, array))
+                {
+                    return array.GetValue(_intArgs);
+                }
             }
             else if (target is IList && _intArgs?.Length == 1)
             {
@@ -102,5 +107,25 @@ namespace Perspex.Markup.Binding
 
             return PerspexProperty.UnsetValue;
         }
+
+        private bool InBounds(int[] args, Array array)
+        {
+            if (args.Length == array.Rank)
+            {
+                for (var i = 0; i < args.Length; ++i)
+                {
+                    if (args[i] >= array.GetLength(i))
+                    {
+                        return false;
+                    }
+                }
+
+                return true;
+            }
+            else
+            {
+                return false;
+            }
+        }
     }
 }

tests/Perspex.Markup.UnitTests/Binding/ExpressionObserverTests_Indexer.cs

@@ -32,6 +32,36 @@ namespace Perspex.Markup.UnitTests.Binding
             Assert.Equal("qux", result);
         }
 
+        [Fact]
+        public async void Array_Out_Of_Bounds_Should_Return_UnsetValue()
+        {
+            var data = new { Foo = new[] { "foo", "bar" } };
+            var target = new ExpressionObserver(data, "Foo[2]");
+            var result = await target.Take(1);
+
+            Assert.Equal(PerspexProperty.UnsetValue, result);
+        }
+
+        [Fact]
+        public async void Array_With_Wrong_Dimensions_Should_Return_UnsetValue()
+        {
+            var data = new { Foo = new[] { "foo", "bar" } };
+            var target = new ExpressionObserver(data, "Foo[1,2]");
+            var result = await target.Take(1);
+
+            Assert.Equal(PerspexProperty.UnsetValue, result);
+        }
+
+        [Fact]
+        public async void List_Out_Of_Bounds_Should_Return_UnsetValue()
+        {
+            var data = new { Foo = new List<string> { "foo", "bar" } };
+            var target = new ExpressionObserver(data, "Foo[2]");
+            var result = await target.Take(1);
+
+            Assert.Equal(PerspexProperty.UnsetValue, result);
+        }
+
         [Fact]
         public async void Should_Get_List_Value()
         {