fix window showdialog, and show raising exceptions when incorrect behavior is used.

src/Avalonia.Controls/Window.cs

@@ -641,47 +641,64 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Shows the window as a child of <paramref name="parent"/>.
+        /// Shows the window as a child of <paramref name="owner"/>.
         /// </summary>
-        /// <param name="parent">Window that will be a parent of the shown window.</param>
+        /// <param name="owner">Window that will be the owner of the shown window.</param>
         /// <exception cref="InvalidOperationException">
         /// The window has already been closed.
         /// </exception>
-        public void Show(Window parent)
+        public void Show(Window owner)
         {
-            if (parent is null)
+            if (owner is null)
             {
-                throw new ArgumentNullException(nameof(parent), "Showing a child window requires valid parent.");
+                throw new ArgumentNullException(nameof(owner), "Showing a child window requires valid parent.");
             }
 
-            ShowCore(parent);
+            ShowCore(owner);
         }
 
-        private void ShowCore(Window? parent)
+        private void EnsureStateBeforeShow()
+        {
+            if (PlatformImpl == null)
+            {
+                throw new InvalidOperationException("Cannot re-show a closed window.");
+            }
+            
+            if (_shown)
+            {
+                throw new InvalidOperationException("The window is already being shown.");
+            }
+        }
+        
+        private void EnsureParentStateBeforeShow(Window owner)
+        {
+            if (owner.PlatformImpl == null)
+            {
+                throw new InvalidOperationException("Cannot show a window with a closed owner.");
+            }
+
+            if (owner == this)
+            {
+                throw new InvalidOperationException("A Window cannot be its own owner.");
+            }
+
+            if (!owner.IsVisible)
+            {
+                throw new InvalidOperationException("Cannot show window with non-visible owner.");
+            }
+        }
+
+        private void ShowCore(Window? owner)
         {
             try
             {
                 IgnoreVisibilityChange = true;
-                
-                if (PlatformImpl == null)
-                {
-                    throw new InvalidOperationException("Cannot re-show a closed window.");
-                }
 
-                if (parent != null)
+                EnsureStateBeforeShow();
+                
+                if (owner != null)
                 {
-                    if (parent.PlatformImpl == null)
-                    {
-                        throw new InvalidOperationException("Cannot show a window with a closed parent.");
-                    }
-                    else if (parent == this)
-                    {
-                        throw new InvalidOperationException("A Window cannot be its own parent.");
-                    }
-                    else if (!parent.IsVisible)
-                    {
-                        throw new InvalidOperationException("Cannot show window with non-visible parent.");
-                    }
+                    EnsureParentStateBeforeShow(owner);
                 }
 
                 if (_shown)
@@ -707,15 +724,15 @@ namespace Avalonia.Controls
 
                 LayoutManager.ExecuteInitialLayoutPass();
 
-                if (PlatformImpl != null && parent?.PlatformImpl is not null)
+                if (PlatformImpl != null && owner?.PlatformImpl is not null)
                 {
-                    PlatformImpl.SetParent(parent.PlatformImpl);
+                    PlatformImpl.SetParent(owner.PlatformImpl);
                 }
 
-                Owner = parent;
-                parent?.AddChild(this, false);
+                Owner = owner;
+                owner?.AddChild(this, false);
 
-                SetWindowStartupLocation(parent?.PlatformImpl);
+                SetWindowStartupLocation(owner?.PlatformImpl);
 
                 PlatformImpl?.Show(ShowActivated, false);
                 Renderer?.Start();
@@ -758,26 +775,14 @@ namespace Avalonia.Controls
             {
                 IgnoreVisibilityChange = true;
                 
+                EnsureStateBeforeShow();
+                
                 if (owner == null)
                 {
                     throw new ArgumentNullException(nameof(owner));
                 }
-                else if (owner.PlatformImpl == null)
-                {
-                    throw new InvalidOperationException("Cannot show a window with a closed owner.");
-                }
-                else if (owner == this)
-                {
-                    throw new InvalidOperationException("A Window cannot be its own owner.");
-                }
-                else if (IsVisible)
-                {
-                    throw new InvalidOperationException("The window is already being shown.");
-                }
-                else if (!owner.IsVisible)
-                {
-                    throw new InvalidOperationException("Cannot show window with non-visible parent.");
-                }
+                
+                EnsureParentStateBeforeShow(owner);
 
                 RaiseEvent(new RoutedEventArgs(WindowOpenedEvent));
 
@@ -800,7 +805,7 @@ namespace Avalonia.Controls
 
                 var result = new TaskCompletionSource<TResult>();
 
-                PlatformImpl?.SetParent(owner.PlatformImpl);
+                PlatformImpl?.SetParent(owner.PlatformImpl!);
                 Owner = owner;
                 owner.AddChild(this, true);
 

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -403,7 +403,7 @@ namespace Avalonia.Controls.UnitTests
                 parent.Close();
 
                 var ex = Assert.Throws<InvalidOperationException>(() => target.Show(parent));
-                Assert.Equal("Cannot show a window with a closed parent.", ex.Message);
+                Assert.Equal("Cannot show a window with a closed owner.", ex.Message);
             }
         }
 
@@ -431,7 +431,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new Window();
 
                 var ex = Assert.Throws<InvalidOperationException>(() => target.Show(parent));
-                Assert.Equal("Cannot show window with non-visible parent.", ex.Message);
+                Assert.Equal("Cannot show window with non-visible owner.", ex.Message);
             }
         }
 
@@ -444,7 +444,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new Window();
 
                 var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => target.ShowDialog(parent));
-                Assert.Equal("Cannot show window with non-visible parent.", ex.Message);
+                Assert.Equal("Cannot show window with non-visible owner.", ex.Message);
             }
         }
 
@@ -456,7 +456,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new Window();
 
                 var ex = Assert.Throws<InvalidOperationException>(() => target.Show(target));
-                Assert.Equal("A Window cannot be its own parent.", ex.Message);
+                Assert.Equal("A Window cannot be its own owner.", ex.Message);
             }
         }