Add tests for TreeNodeCollection Find method (#3497)

src/System.Windows.Forms/src/System/Windows/Forms/TreeNodeCollection.cs

@@ -269,6 +269,11 @@ namespace System.Windows.Forms
 
         public TreeNode[] Find(string key, bool searchAllChildren)
         {
+            if (string.IsNullOrEmpty(key))
+            {
+                throw new ArgumentNullException(nameof(key), SR.FindKeyMayNotBeEmptyOrNull);
+            }
+
             ArrayList foundNodes = FindInternal(key, searchAllChildren, this, new ArrayList());
 
             //

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ControlTests.ControlCollection.cs

@@ -1452,7 +1452,7 @@ namespace System.Windows.Forms.Tests
             // Don't search all children.
             Assert.Equal(new Control[] { child2, child3 }, collection.Find(key, searchAllChildren: false));
 
-            // Call again..
+            // Call again.
             Assert.Equal(new Control[] { child2, child3 }, collection.Find(key, searchAllChildren: false));
         }
 

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/TreeNodeCollectionTests.cs

@@ -79,5 +79,96 @@ namespace System.Windows.Forms.Tests
             collection.Add("text");
             Assert.Throws<ArgumentOutOfRangeException>("index", () => collection[index] = new TreeNode());
         }
+
+        [WinFormsTheory]
+        [InlineData("name2")]
+        [InlineData("NAME2")]
+        public void TreeNodeCollection_Find_InvokeKeyExists_ReturnsExpected(string key)
+        {
+            using var treeView = new TreeView();
+            var child1 = new TreeNode
+            {
+                Name = "name1"
+            };
+            var child2 = new TreeNode
+            {
+                Name = "name2"
+            };
+            var child3 = new TreeNode
+            {
+                Name = "name2"
+            };
+
+            var grandchild1 = new TreeNode
+            {
+                Name = "name1"
+            };
+            var grandchild2 = new TreeNode
+            {
+                Name = "name2"
+            };
+            var grandchild3 = new TreeNode
+            {
+                Name = "name2"
+            };
+            child3.Nodes.Add(grandchild1);
+            child3.Nodes.Add(grandchild2);
+            child3.Nodes.Add(grandchild3);
+            TreeNodeCollection collection = treeView.Nodes;
+            collection.Add(child1);
+            collection.Add(child2);
+            collection.Add(child3);
+
+            // Search all children.
+            Assert.Equal(new TreeNode[] { child2, child3, grandchild2, grandchild3 }, collection.Find(key, searchAllChildren: true));
+
+            // Call again.
+            Assert.Equal(new TreeNode[] { child2, child3, grandchild2, grandchild3 }, collection.Find(key, searchAllChildren: true));
+
+            // Don't search all children.
+            Assert.Equal(new TreeNode[] { child2, child3 }, collection.Find(key, searchAllChildren: false));
+
+            // Call again.
+            Assert.Equal(new TreeNode[] { child2, child3 }, collection.Find(key, searchAllChildren: false));
+        }
+
+        [WinFormsTheory]
+        [InlineData("NoSuchName")]
+        [InlineData("abcd")]
+        [InlineData("abcde")]
+        [InlineData("abcdef")]
+        public void TreeNodeCollection_Find_InvokeNoSuchKey_ReturnsEmpty(string key)
+        {
+            using var treeView = new TreeView();
+            var child1 = new TreeNode
+            {
+                Name = "name1"
+            };
+            var child2 = new TreeNode
+            {
+                Name = "name2"
+            };
+            var child3 = new TreeNode
+            {
+                Name = "name2"
+            };
+            TreeNodeCollection collection = treeView.Nodes;
+            collection.Add(child1);
+            collection.Add(child2);
+            collection.Add(child3);
+
+            Assert.Empty(collection.Find(key, searchAllChildren: true));
+            Assert.Empty(collection.Find(key, searchAllChildren: false));
+        }
+
+        [WinFormsTheory]
+        [CommonMemberData(nameof(CommonTestHelper.GetNullOrEmptyStringTheoryData))]
+        public void TreeNodeCollection_Find_NullOrEmptyKey_ThrowsArgumentNullException(string key)
+        {
+            using var treeView = new TreeView();
+            var collection = treeView.Nodes;
+            Assert.Throws<ArgumentNullException>("key", () => collection.Find(key, searchAllChildren: true));
+            Assert.Throws<ArgumentNullException>("key", () => collection.Find(key, searchAllChildren: false));
+        }
     }
 }