Not throw exception in last or first position.

src/Avalonia.Controls/GridSplitter.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.VisualTree;
@@ -12,16 +11,16 @@ using Avalonia.VisualTree;
 namespace Avalonia.Controls
 {
     /// <summary>
-    /// Represents the control that redistributes space between columns or rows of a Grid control.
+    ///     Represents the control that redistributes space between columns or rows of a Grid control.
     /// </summary>
     /// <remarks>
-    /// Unlike WPF GridSplitter, Avalonia GridSplitter has only one Behavior, GridResizeBehavior.PreviousAndNext.
+    ///     Unlike WPF GridSplitter, Avalonia GridSplitter has only one Behavior, GridResizeBehavior.PreviousAndNext.
     /// </remarks>
     public class GridSplitter : Thumb
     {
         private List<DefinitionBase> _definitions;
 
-        protected Grid _grid;
+        private Grid _grid;
 
         private DefinitionBase _nextDefinition;
 
@@ -31,13 +30,13 @@ namespace Avalonia.Controls
 
         private void GetDeltaConstraints(out double min, out double max)
         {
-            double prevDefinitionLen = GetActualLength(_prevDefinition);
+            var prevDefinitionLen = GetActualLength(_prevDefinition);
-            double prevDefinitionMin = GetMinLength(_prevDefinition);
+            var prevDefinitionMin = GetMinLength(_prevDefinition);
-            double prevDefinitionMax = GetMaxLength(_prevDefinition);
+            var prevDefinitionMax = GetMaxLength(_prevDefinition);
 
-            double nextDefinitionLen = GetActualLength(_nextDefinition);
+            var nextDefinitionLen = GetActualLength(_nextDefinition);
-            double nextDefinitionMin = GetMinLength(_nextDefinition);
+            var nextDefinitionMin = GetMinLength(_nextDefinition);
-            double nextDefinitionMax = GetMaxLength(_nextDefinition);
+            var nextDefinitionMax = GetMaxLength(_nextDefinition);
             // Determine the minimum and maximum the columns can be resized
             min = -Math.Min(prevDefinitionLen - prevDefinitionMin, nextDefinitionMax - nextDefinitionLen);
             max = Math.Min(prevDefinitionMax - prevDefinitionLen, nextDefinitionLen - nextDefinitionMin);
@@ -69,26 +68,32 @@ namespace Avalonia.Controls
 
         private double GetActualLength(DefinitionBase definition)
         {
+            if (definition == null)
+                return 0;
             var columnDefinition = definition as ColumnDefinition;
-            return columnDefinition?.ActualWidth ?? ((RowDefinition)definition).ActualHeight;
+            return columnDefinition?.ActualWidth ?? ((RowDefinition) definition).ActualHeight;
         }
 
         private double GetMinLength(DefinitionBase definition)
         {
+            if (definition == null)
+                return 0;
             var columnDefinition = definition as ColumnDefinition;
-            return columnDefinition?.MinWidth ?? ((RowDefinition)definition).MinHeight;
+            return columnDefinition?.MinWidth ?? ((RowDefinition) definition).MinHeight;
         }
 
         private double GetMaxLength(DefinitionBase definition)
         {
+            if (definition == null)
+                return 0;
             var columnDefinition = definition as ColumnDefinition;
-            return columnDefinition?.MaxWidth ?? ((RowDefinition)definition).MaxHeight;
+            return columnDefinition?.MaxWidth ?? ((RowDefinition) definition).MaxHeight;
         }
 
         private bool IsStar(DefinitionBase definition)
         {
             var columnDefinition = definition as ColumnDefinition;
-            return columnDefinition?.Width.IsStar ?? ((RowDefinition)definition).Height.IsStar;
+            return columnDefinition?.Width.IsStar ?? ((RowDefinition) definition).Height.IsStar;
         }
 
         private void SetLengthInStars(DefinitionBase definition, double value)
@@ -100,7 +105,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                ((RowDefinition)definition).Height = new GridLength(value, GridUnitType.Star);
+                ((RowDefinition) definition).Height = new GridLength(value, GridUnitType.Star);
             }
         }
 
@@ -111,28 +116,32 @@ namespace Avalonia.Controls
 
             _orientation = DetectOrientation();
 
-            int pos;
+            int defenitionIndex; //row or col
             if (_orientation == Orientation.Vertical)
             {
                 Cursor = new Cursor(StandardCursorType.SizeWestEast);
-                pos = GetValue(Grid.ColumnProperty);
                 _definitions = _grid.ColumnDefinitions.Cast<DefinitionBase>().ToList();
+                defenitionIndex = GetValue(Grid.ColumnProperty);
                 PseudoClasses.Add(":vertical");
             }
             else
             {
                 Cursor = new Cursor(StandardCursorType.SizeNorthSouth);
-                pos = GetValue(Grid.RowProperty);
+                defenitionIndex = GetValue(Grid.RowProperty);
                 _definitions = _grid.RowDefinitions.Cast<DefinitionBase>().ToList();
                 PseudoClasses.Add(":horizontal");
             }
-            _prevDefinition = _definitions[pos - 1];
+
-            _nextDefinition = _definitions[pos + 1];
+            if (defenitionIndex > 0)
+                _prevDefinition = _definitions[defenitionIndex - 1];
+
+            if (defenitionIndex < _definitions.Count - 1)
+                _nextDefinition = _definitions[defenitionIndex + 1];
         }
 
         private Orientation DetectOrientation()
         {
-            if(!_grid.ColumnDefinitions.Any())
+            if (!_grid.ColumnDefinitions.Any())
                 return Orientation.Horizontal;
             if (!_grid.RowDefinitions.Any())
                 return Orientation.Vertical;
@@ -141,10 +150,6 @@ namespace Avalonia.Controls
             var row = GetValue(Grid.RowProperty);
             var width = _grid.ColumnDefinitions[col].Width;
             var height = _grid.RowDefinitions[row].Height;
-            if (!width.IsAuto && !height.IsAuto)
-            {
-                throw new InvalidOperationException("Whether RowDefenition or ColumnDefenition matched with the GridSplitter should have Auto size");
-            }
             if (width.IsAuto && !height.IsAuto)
             {
                 return Orientation.Vertical;
@@ -155,7 +160,7 @@ namespace Avalonia.Controls
             }
             if (_grid.Children.OfType<Control>() // Decision based on other controls in the same column
                 .Where(c => Grid.GetColumn(c) == col)
-                .Any(c => c.GetType() != typeof(GridSplitter)))
+                .Any(c => c.GetType() != typeof (GridSplitter)))
             {
                 return Orientation.Horizontal;
             }