Create APIReview_AllowDrop.md

Author: feich-ms

specs/APIReview_AllowDrop.md

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+# Background
+Currently the drag&drop functionality is default enabled in webview2 and there is no way to disable it. Some developers may want to disbale this functionality in their applications 
+based upon their scenarios. According to such feature requirements, we add the new API to provide developers with the capability to configure the drag&drop functionality.
+
+# Description
+We add a new `AllowDrop` property in `CoreWebView2Controller`. This API allows end developers to toggle the drag&drop functionality easily.
+If it's disabled, any drag&drop actions will keep out of work. By default, it's disabled to accommodate to conventions.
+
+# Examples
+## C++
+
+```cpp
+void ToggleAllowDrop()
+{
+    // Get webView's controller
+    wil::com_ptr<ICoreWebView2Controller> controller = m_appWindow->GetWebViewController();
+    if (controller)
+    {
+        BOOL allowDrop;
+        CHECK_FAILURE(controller->get_AllowDrop(&allowDrop));
+        if (allowDrop)
+        {
+            CHECK_FAILURE(controller->put_AllowDrop(FALSE));
+            MessageBox(
+                nullptr, L"WebView disallows dropping files now.",
+                L"WebView AllowDrop property changed", MB_OK);
+        }
+        else
+        {
+            CHECK_FAILURE(controller->put_AllowDrop(TRUE));
+            MessageBox(
+                nullptr, L"WebView allows dropping files now.",
+                L"WebView AllowDrop property changed", MB_OK);
+        }
+    }
+}
+```
+
+## C#
+```c#
+void ToggleAllowDrop(object target, ExecutedRoutedEventArgs e)
+{
+    // Get webView's controller
+    var controller = _webView.CoreWebView2Controller;
+    if(controller.AllowDrop)
+    {
+        controller.AllowDrop = false;
+    }
+    else
+    {
+        controller.AllowDrop = true;
+    }
+}
+```
+
+# Remarks
+The `AllowDrop` property already exists in some controls of .net UI framework like WPF and WinForms. 
+The .net control wrapper for webview2 natively inherits this property. 
+But actually it doesn't really work until this new API is added.
+When the new API is promoted to public, we will adjust the WPF/WinForms webview2 control to consume the new API accordingly.
+
+
+# API Notes
+See [API Details](#api-details) section below for API reference.
+
+# API Details
+
+## Win32 C++
+```c#
+// This is the ICoreWebView2Controller interface.
+[uuid(6a360a1f-d1cf-4d90-a0ab-ae2e7d1a29f0), object, pointer_default(unique)]
+interface ICoreWebView2Controller : IUnknown {
+  /// Gets the `AllowDrop` property which is used to configure the capability
+  /// that dropping files into webview2 is allowed or permitted.
+  [propget] HRESULT AllowDrop([out, retval] BOOL * allowDrop);
+  /// Sets the `AllowDrop` property which is used to configure the capability
+  /// that dropping files into webview2 is allowed or permitted.
+  [propput] HRESULT AllowDrop([in] BOOL allowDrop);
+}
+```
+
+## .NET and WinRT
+```c#
+namespace Microsoft.Web.WebView2.Core
+{
+    public class CoreWebView2Controller
+    {
+        //
+        // Summary:
+        //     Gets or sets the WebView allow drop property.
+        //
+        // Remarks:
+        //     The AllowDrop is to configure the capability that dropping files into webview2
+        //     is allowed or permitted.
+        public bool AllowDrop { get; set; }
+    }
+}
+```