Merge pull request #3748 from Deadpikle/fix/selected-text-no-clear

Fix TextBox.SelectedText = "" not clearing text

src/Avalonia.Controls/TextBox.cs

@@ -277,13 +277,15 @@ namespace Avalonia.Controls
             get { return GetSelection(); }
             set
             {
-                if (value == null)
+                _undoRedoHelper.Snapshot();
+                if (string.IsNullOrEmpty(value))
                 {
-                    return;
+                    DeleteSelection();
                 }
-
-                _undoRedoHelper.Snapshot();
-                HandleTextInput(value);
+                else
+                {
+                    HandleTextInput(value);
+                } 
                 _undoRedoHelper.Snapshot();
             }
         }

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -425,6 +425,42 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void SelectedText_CanClearText()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var target = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    Text = "0123"
+                };
+                target.SelectionStart = 1;
+                target.SelectionEnd = 3;
+                target.SelectedText = "";
+
+                Assert.True(target.Text == "03");
+            }
+        }
+
+        [Fact]
+        public void SelectedText_NullClearsText()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var target = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    Text = "0123"
+                };
+                target.SelectionStart = 1;
+                target.SelectionEnd = 3;
+                target.SelectedText = null;
+
+                Assert.True(target.Text == "03");
+            }
+        }
+
         [Fact]
         public void CoerceCaretIndex_Doesnt_Cause_Exception_with_malformed_line_ending()
         {