Extract WindowLong parameter into enum

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -558,7 +558,18 @@ namespace Avalonia.Win32.Interop
         {
             DIB_RGB_COLORS = 0,     /* color table in RGBs */
             DIB_PAL_COLORS          /* color table in palette indices */
-        };
+        }
+
+        public enum WindowLongParam
+        {
+            GWL_WNDPROC = -4,
+            GWL_HINSTANCE = -6,
+            GWL_HWNDPARENT = -8,
+            GWL_ID = -12,
+            GWL_STYLE = -16,
+            GWL_EXSTYLE = -20,
+            GWL_USERDATA = -21
+        }
 
         [StructLayout(LayoutKind.Sequential)]
         public struct RGBQUAD

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -78,8 +78,8 @@ namespace Avalonia.Win32
         {
             get
             {
-                var style = UnmanagedMethods.GetWindowLong(_hwnd, -16);
-                var exStyle = UnmanagedMethods.GetWindowLong(_hwnd, -20);
+                var style = UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
+                var exStyle = UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_EXSTYLE);
                 var padding = new UnmanagedMethods.RECT();
 
                 if (UnmanagedMethods.AdjustWindowRectEx(ref padding, style, false, exStyle))
@@ -236,7 +236,7 @@ namespace Avalonia.Win32
                 return;
             }
 
-            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, -16);
+            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
 
             var systemDecorationStyles = UnmanagedMethods.WindowStyles.WS_OVERLAPPED
                 | UnmanagedMethods.WindowStyles.WS_CAPTION
@@ -258,7 +258,7 @@ namespace Avalonia.Win32
             Rect newRect;
             var oldThickness = BorderThickness;
 
-            UnmanagedMethods.SetWindowLong(_hwnd, -16, (uint)style);
+            UnmanagedMethods.SetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE, (uint)style);
 
             if (value)
             {
@@ -805,7 +805,7 @@ namespace Avalonia.Win32
 
         public void ShowTaskbarIcon(bool value)
         {
-            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, -20);
+            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_EXSTYLE);
 
             style &= ~(UnmanagedMethods.WindowStyles.WS_VISIBLE);
 
@@ -820,7 +820,7 @@ namespace Avalonia.Win32
             {
                 //Toggle to make the styles stick
                 UnmanagedMethods.ShowWindow(_hwnd, ShowWindowCommand.Hide);
-                UnmanagedMethods.SetWindowLong(_hwnd, -20, (uint)style);
+                UnmanagedMethods.SetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_EXSTYLE, (uint)style);
                 UnmanagedMethods.ShowWindow(_hwnd, windowPlacement.ShowCmd);
             }
         }
@@ -832,14 +832,14 @@ namespace Avalonia.Win32
                 return;
             }
 
-            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, -16);
+            var style = (UnmanagedMethods.WindowStyles)UnmanagedMethods.GetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE);
             
             if (value)
                 style |= UnmanagedMethods.WindowStyles.WS_SIZEFRAME;
             else
                 style &= ~(UnmanagedMethods.WindowStyles.WS_SIZEFRAME);
             
-            UnmanagedMethods.SetWindowLong(_hwnd, -16, (uint)style);
+            UnmanagedMethods.SetWindowLong(_hwnd, (int)UnmanagedMethods.WindowLongParam.GWL_STYLE, (uint)style);
             _resizable = value;
         }
     }