Switched order of attribute initialization and DOM addition.

src/Components/Web.JS/src/Rendering/BrowserRenderer.ts

@@ -238,7 +238,8 @@ export class BrowserRenderer {
       document.createElementNS('http://www.w3.org/2000/svg', tagName) :
       document.createElement(tagName);
     const newElement = toLogicalElement(newDomElementRaw);
-    insertLogicalChild(newDomElementRaw, parent, childIndex);
+
+    let inserted = false;
 
     // Apply attributes
     const descendantsEndIndexExcl = frameIndex + frameReader.subtreeLength(frame);
@@ -247,24 +248,49 @@ export class BrowserRenderer {
       if (frameReader.frameType(descendantFrame) === FrameType.attribute) {
         this.applyAttribute(batch, componentId, newDomElementRaw, descendantFrame);
       } else {
+        insertLogicalChild(newDomElementRaw, parent, childIndex);
+        inserted = true;        
         // As soon as we see a non-attribute child, all the subsequent child frames are
         // not attributes, so bail out and insert the remnants recursively
         this.insertFrameRange(batch, componentId, newElement, 0, frames, descendantIndex, descendantsEndIndexExcl);
         break;
       }
     }
 
-    // We handle setting 'value' on a <select> in two different ways:
-    // [1] When inserting a corresponding <option>, in case you're dynamically adding options
+    // this element did not have any children, so it's not inserted yet.
+    if (!inserted) {
+        insertLogicalChild(newDomElementRaw, parent, childIndex);
+    }
+
+    // We handle setting 'value' on a <select> in three different ways:
+    // [1] When inserting a corresponding <option>, in case you're dynamically adding options.
+    //     This is the case below.
     // [2] After we finish inserting the <select>, in case the descendant options are being
-    //     added as an opaque markup block rather than individually
-    // Right here we implement [2]
-    if (newDomElementRaw instanceof HTMLSelectElement && selectValuePropname in newDomElementRaw) {
+    //     added as an opaque markup block rather than individually. This is the other case below.
+    // [3] In case the the value of the select and the option value is changed in the same batch.
+    //     We just receive an attribute frame and have to set the select value afterwards.     
+
+    if (newDomElementRaw instanceof HTMLOptionElement)
+    {
+      // Situation 1
+      this.trySetSelectValueFromOptionElement(newDomElementRaw);
+    } else if (newDomElementRaw instanceof HTMLSelectElement && selectValuePropname in newDomElementRaw) {
+      // Situation 2
       const selectValue: string | null = newDomElementRaw[selectValuePropname];
       setSelectElementValue(newDomElementRaw, selectValue);
     }
   }
 
+  private trySetSelectValueFromOptionElement(optionElement: HTMLOptionElement) {
+    const selectElem = this.findClosestAncestorSelectElement(optionElement);
+    if (selectElem && (selectValuePropname in selectElem) && selectElem[selectValuePropname] === optionElement.value) {
+      setSelectElementValue(selectElem, optionElement.value);
+      delete selectElem[selectValuePropname];
+      return true;
+    }
+    return false;
+  }
+
   private insertComponent(batch: RenderBatch, parent: LogicalElement, childIndex: number, frame: RenderTreeFrame) {
     const containerElement = createAndInsertLogicalContainer(parent, childIndex);
 
@@ -385,13 +411,10 @@ export class BrowserRenderer {
         } else {
           element.removeAttribute('value');
         }
+
         // See above for why we have this special handling for <select>/<option>
-        // Note that this is only one of the two cases where we set the value on a <select>
-        const selectElem = this.findClosestAncestorSelectElement(element);
-        if (selectElem && (selectValuePropname in selectElem) && selectElem[selectValuePropname] === value) {
-          this.tryApplyValueProperty(batch, selectElem, attributeFrame);
-          delete selectElem[selectValuePropname];
-        }
+        // Situation 3
+        this.trySetSelectValueFromOptionElement(<HTMLOptionElement>element);
         return true;
       }
       default:

src/Components/test/E2ETest/Tests/BindTest.cs

@@ -216,6 +216,10 @@ public void CanBindSelect()
             Browser.Equal("Fourth", () => boundValue.Text);
             Assert.Equal("Fourth choice", target.SelectedOption.Text);
 
+            // verify that changing an option value and selected value at the same time works.
+            Browser.FindElement(By.Id("change-variable-value")).Click();
+            Browser.Equal("Sixth", () => boundValue.Text);
+
             // Verify we can select options whose value is empty
             // https://github.com/dotnet/aspnetcore/issues/17735
             target.SelectByText("Empty value");

src/Components/test/E2ETest/Tests/EventTest.cs

@@ -288,6 +288,17 @@ public void EventDuringBatchRendering_CannotTriggerJSInterop()
             Browser.Contains(expectedMessage, () => errorLog.Text);
         }
 
+        [Fact]
+        public void RenderAttributesBeforeConnectedCallBack()
+        {
+            Browser.MountTestComponent<RenderAttributesBeforeConnectedCallback>();
+            var element = Browser.FindElement(By.TagName("custom-web-component-data-from-attribute"));
+
+            var expectedContent = "success";
+
+            Browser.Contains(expectedContent, () => element.Text);
+        }
+
         void SendKeysSequentially(IWebElement target, string text)
         {
             // Calling it for each character works around some chars being skipped

src/Components/test/testassets/BasicTestApp/BindCasesComponent.razor

@@ -242,6 +242,7 @@
         <optgroup label="Some choices">
             <!-- Show it also works with optgroup -->
             <option value=@string.Empty>Empty value</option>
+            <option value="@variableValue">Variable value</option>
             <option value=@SelectableValue.First>First choice</option>
             <option value=@SelectableValue.Second>Second choice</option>
             <option value=@SelectableValue.Third>Third choice</option>
@@ -253,6 +254,7 @@
     </select>
     <span id="select-box-value">@selectValue</span>
     <button id="select-box-add-option" @onclick="AddAndSelectNewSelectOption">Add and select new item</button>
+    <button id="change-variable-value" @onclick="ChangeVariableValueToSixth">Change variable value to Sixth.</button>
 </p>
 
 <h2>Select (markup block)</h2>
@@ -383,13 +385,21 @@
     DateTime? timeStepTextboxNullableDateTimeValue = null;
 
     bool includeFourthOption = false;
-    enum SelectableValue { First, Second, Third, Fourth }
+    enum SelectableValue { First, Second, Third, Fourth, Fifth, Sixth }
     SelectableValue? selectValue = SelectableValue.Second;
     SelectableValue? selectMarkupValue = SelectableValue.Second;
 
+    SelectableValue variableValue = SelectableValue.Fifth;
+
     void AddAndSelectNewSelectOption()
     {
         includeFourthOption = true;
         selectValue = SelectableValue.Fourth;
     }
+
+    void ChangeVariableValueToSixth()
+    {
+        selectValue = SelectableValue.Sixth;
+        variableValue = SelectableValue.Sixth;
+    }
 }

src/Components/test/testassets/BasicTestApp/Index.razor

@@ -70,6 +70,7 @@
         <option value="BasicTestApp.RedTextComponent">Red text</option>
         <option value="BasicTestApp.ReliabilityComponent">Server reliability component</option>
         <option value="BasicTestApp.RenderFragmentToggler">Render fragment renderer</option>
+        <option value="BasicTestApp.RenderAttributesBeforeConnectedCallback">Render attributes before ConnectedCallback</option>
         <option value="BasicTestApp.ReorderingFocusComponent">Reordering focus retention</option>
         <option value="BasicTestApp.RouterTest.NavigationManagerComponent">NavigationManager Test</option>
         <option value="BasicTestApp.RouterTest.TestRouter">Router</option>

src/Components/test/testassets/BasicTestApp/RenderAttributesBeforeConnectedCallback.razor

@@ -0,0 +1,14 @@
+<h1>Render Attributes Before connectedCallback</h1>
+
+<p>
+    When the connectedcallback event fires, it's nice to have the attributes of the HTML element set already,
+    as data is usually passed to the component using custom attributes.
+</p>
+
+
+<custom-web-component-data-from-attribute myattribute="@attributeString"></custom-web-component-data-from-attribute>
+
+
+@code {
+    string attributeString = "success";
+}

src/Components/test/testassets/BasicTestApp/wwwroot/index.html

@@ -23,6 +23,7 @@
 
     <!-- Used for specific test cases -->
     <script src="js/jsinteroptests.js"></script>
+    <script src="js/renderattributestest.js"></script>
     <script src="js/webComponentPerformingJsInterop.js"></script>
 
     <script>

src/Components/test/testassets/BasicTestApp/wwwroot/js/renderattributestest.js

@@ -0,0 +1,9 @@
+// This web component is used from the RenderAttributesBeforeConnectedCallback test case
+
+window.customElements.define('custom-web-component-data-from-attribute', class extends HTMLElement {
+    connectedCallback() {
+        let myattribute = this.getAttribute('myattribute') || 'failed';
+
+        this.innerHTML = myattribute;
+    }
+});

src/Components/test/testassets/TestServer/Pages/_ServerHost.cshtml

@@ -16,6 +16,7 @@
 
     <!-- Used for specific test cases -->
     <script src="js/jsinteroptests.js"></script>
+    <script src="js/renderattributestest.js"></script>
     <script src="js/webComponentPerformingJsInterop.js"></script>
 
     <div id="blazor-error-ui">