[Blazor] Allow null parameter values to be supplied to interactive components via enhanced page update (#53317) (#53342)

# Allow `null` parameter values to be supplied to interactive components via enhanced page update

Backport of #53317

Fixes an issue where a `NullRefrenceException` would be thrown if an enhanced page update supplied a `null` parameter to an interactive root component.

## Description

In .NET 8, SSR'd components can supply updated parameters to existing interactive root components. If one of those updated parameters is `null`, an exception currently gets thrown from within the framework. This causes the circuit to crash when using Server interactivity, and it would causes an error to be logged in the browser console when using WebAssembly interactivity.

This PR fixes the problem by treating `null` as a valid value for a serialized parameter that gets supplied to an interactive root component.

Fixes #52434

## Customer Impact

Without this fix, customers may encounter the unfriendly exception and have a hard time figuring out the underlying cause. We have not yet seen customer reports of the issue, but it's possible that customers have this bug in their apps without knowing it, especially since it only occurs when supplying updated parameters to existing components (not when supplying the initial set of parameters).

One workaround would be to use a different value than `null` to specify an empty parameter value, but this may not be possible in cases where the parameter gets supplied by the framework (e.g., via route value), or if the interactive root component's implementation is not under the developer's control.

## Regression?

- [ ] Yes
- [X] No

Only applicable to new scenarios in .NET 8.

## Risk

- [ ] High
- [ ] Medium
- [X] Low

The fix is straightforward and well-tested.

## Verification

- [x] Manual (required)
- [x] Automated

## Packaging changes reviewed?

- [ ] Yes
- [ ] No
- [X] N/A

Author: MackinnonBuck

src/Components/Endpoints/test/WebRootComponentParametersTest.cs

@@ -100,36 +100,73 @@ public void WebRootComponentParameters_DefinitelyEquals_ReturnsTrue_ForEmptySetO
     }
 
     [Fact]
-    public void WebRootComponentParameters_DefinitelyEquals_Throws_WhenComparingNonJsonElementParameterToJsonElement()
+    public void WebRootComponentParameters_DefinitelyEquals_ReturnsFalse_WhenComparingNonJsonElementParameterToJsonElement()
     {
         // Arrange
         var parameters1 = CreateParametersWithNonJsonElements(new() { ["First"] = 123 });
         var parameters2 = CreateParameters(new() { ["First"] = 456 });
 
-        // Act/assert
-        Assert.Throws<InvalidCastException>(() => parameters1.DefinitelyEquals(parameters2));
+        // Act
+        var result = parameters1.DefinitelyEquals(parameters2);
+
+        // Assert
+        Assert.False(result);
     }
 
     [Fact]
-    public void WebRootComponentParameters_DefinitelyEquals_Throws_WhenComparingJsonElementParameterToNonJsonElement()
+    public void WebRootComponentParameters_DefinitelyEquals_ReturnsFalse_WhenComparingJsonElementParameterToNonJsonElement()
     {
         // Arrange
         var parameters1 = CreateParameters(new() { ["First"] = 123 });
         var parameters2 = CreateParametersWithNonJsonElements(new() { ["First"] = 456 });
 
-        // Act/assert
-        Assert.Throws<InvalidCastException>(() => parameters1.DefinitelyEquals(parameters2));
+        // Act
+        var result = parameters1.DefinitelyEquals(parameters2);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public void WebRootComponentParameters_DefinitelyEquals_ReturnsTrue_WhenComparingEqualNonJsonElementParameters()
+    {
+        // Arrange
+        var parameters1 = CreateParametersWithNonJsonElements(new() { ["First"] = 123 });
+        var parameters2 = CreateParametersWithNonJsonElements(new() { ["First"] = 123 });
+
+        // Act
+        var result = parameters1.DefinitelyEquals(parameters2);
+
+        // Assert
+        Assert.True(result);
     }
 
     [Fact]
-    public void WebRootComponentParameters_DefinitelyEquals_Throws_WhenComparingNonJsonElementParameters()
+    public void WebRootComponentParameters_DefinitelyEquals_ReturnsFalse_WhenComparingInequalNonJsonElementParameters()
     {
         // Arrange
         var parameters1 = CreateParametersWithNonJsonElements(new() { ["First"] = 123 });
         var parameters2 = CreateParametersWithNonJsonElements(new() { ["First"] = 456 });
 
-        // Act/assert
-        Assert.Throws<InvalidCastException>(() => parameters1.DefinitelyEquals(parameters2));
+        // Act
+        var result = parameters1.DefinitelyEquals(parameters2);
+
+        // Assert
+        Assert.False(result);
+    }
+
+    [Fact]
+    public void WebRootComponentParameters_DefinitelyEquals_ReturnsTrue_WhenComparingNullParameters()
+    {
+        // Arrange
+        var parameters1 = CreateParametersWithNonJsonElements(new() { ["First"] = null });
+        var parameters2 = CreateParametersWithNonJsonElements(new() { ["First"] = null });
+
+        // Act
+        var result = parameters1.DefinitelyEquals(parameters2);
+
+        // Assert
+        Assert.True(result);
     }
 
     private static WebRootComponentParameters CreateParameters(Dictionary<string, object> parameters)

src/Components/Shared/src/WebRootComponentParameters.cs

@@ -45,9 +45,17 @@ public bool DefinitelyEquals(in WebRootComponentParameters other)
                 return false;
             }
 
-            var value = ((JsonElement)_serializedParameterValues[i]).GetRawText();
-            var otherValue = ((JsonElement)other._serializedParameterValues[i]).GetRawText();
-            if (!string.Equals(value, otherValue, StringComparison.Ordinal))
+            // We expect each serialized parameter value to be either a 'JsonElement' or 'null'.
+            var value = _serializedParameterValues[i];
+            var otherValue = other._serializedParameterValues[i];
+            if (value is JsonElement jsonValue && otherValue is JsonElement otherJsonValue)
+            {
+                if (!string.Equals(jsonValue.GetRawText(), otherJsonValue.GetRawText(), StringComparison.Ordinal))
+                {
+                    return false;
+                }
+            }
+            else if (!Equals(value, otherValue))
             {
                 return false;
             }

src/Components/test/E2ETest/ServerRenderingTests/EnhancedNavigationTest.cs

@@ -554,6 +554,38 @@ public void LocationChangingEventGetsInvokedOnEnhancedNavigationOnlyForRuntimeTh
         Browser.Equal("0", () => Browser.Exists(By.Id($"location-changing-count-{anotherRuntime}")).Text);
     }
 
+    [Theory]
+    [InlineData("server")]
+    [InlineData("wasm")]
+    public void CanReceiveNullParameterValueOnEnhancedNavigation(string renderMode)
+    {
+        // See: https://github.com/dotnet/aspnetcore/issues/52434 
+        Navigate($"{ServerPathBase}/nav");
+        Browser.Equal("Hello", () => Browser.Exists(By.TagName("h1")).Text);
+
+        Browser.Exists(By.TagName("nav")).FindElement(By.LinkText($"Null component parameter ({renderMode})")).Click();
+        Browser.Equal("Page rendering component with null parameter", () => Browser.Exists(By.TagName("h1")).Text);
+        Browser.Equal("0", () => Browser.Exists(By.Id("current-count")).Text);
+
+        Browser.Exists(By.Id("button-increment")).Click();
+        Browser.Equal("0", () => Browser.Exists(By.Id("location-changed-count")).Text);
+        Browser.Equal("1", () => Browser.Exists(By.Id("current-count")).Text);
+
+        // This refresh causes the interactive component to receive a 'null' parameter value
+        Browser.Exists(By.Id("button-refresh")).Click();
+        Browser.Equal("1", () => Browser.Exists(By.Id("location-changed-count")).Text);
+        Browser.Equal("1", () => Browser.Exists(By.Id("current-count")).Text);
+
+        // Increment the count again to ensure that interactivity still works
+        Browser.Exists(By.Id("button-increment")).Click();
+        Browser.Equal("2", () => Browser.Exists(By.Id("current-count")).Text);
+
+        // Even if the interactive runtime continues to function (as the WebAssembly runtime might),
+        // fail the test if any errors were logged to the browser console
+        var logs = Browser.GetBrowserLogs(LogLevel.Warning);
+        Assert.DoesNotContain(logs, log => log.Message.Contains("Error"));
+    }
+
     private void AssertEnhancedUpdateCountEquals(long count)
         => Browser.Equal(count, () => ((IJavaScriptExecutor)Browser).ExecuteScript("return window.enhancedPageUpdateCount;"));
 

src/Components/test/testassets/Components.TestServer/RazorComponents/Pages/EnhancedNav/PageRenderingComponentWithNullParameter.razor

@@ -0,0 +1,24 @@
+@page "/nav/null-parameter/{mode}"
+@using TestContentPackage
+
+@* https://github.com/dotnet/aspnetcore/issues/52434 *@
+
+<h1>Page rendering component with null parameter</h1>
+
+@if (Mode == "server")
+{
+    <ComponentAcceptingNullParameter @rendermode="RenderMode.InteractiveServer" Value="@null" />
+}
+else if (Mode == "wasm")
+{
+    <ComponentAcceptingNullParameter @rendermode="RenderMode.InteractiveWebAssembly" Value="@null" />
+}
+else
+{
+    <p>Expected a render mode of 'server' or 'wasm', but got '@Mode'.</p>
+}
+
+@code {
+    [Parameter]
+    public string? Mode { get; set; }
+}

src/Components/test/testassets/Components.TestServer/RazorComponents/Shared/EnhancedNavLayout.razor

@@ -23,6 +23,8 @@
     <NavLink href="nav/location-changed/server">LocationChanged/LocationChanging event (server)</NavLink>
     <NavLink href="nav/location-changed/wasm">LocationChanged/LocationChanging event (wasm)</NavLink>
     <NavLink href="nav/location-changed/server-and-wasm">LocationChanged/LocationChanging event (server-and-wasm)</NavLink>
+    <NavLink href="nav/null-parameter/server">Null component parameter (server)</NavLink>
+    <NavLink href="nav/null-parameter/wasm">Null component parameter (wasm)</NavLink>
 </nav>
 <hr/>
 <main>

src/Components/test/testassets/TestContentPackage/ComponentAcceptingNullParameter.razor

@@ -0,0 +1,52 @@
+@implements IDisposable
+@inject NavigationManager NavigationManager
+@using Microsoft.AspNetCore.Components.Routing
+
+<p>Value: @(Value ?? "(null)")</p>
+
+@if (_interactive)
+{
+    <button id="button-increment" @onclick="Increment">Count: <span id="current-count">@_count</span></button>
+    <button id="button-refresh" @onclick="Refresh">Refresh</button>
+    <p>Location changed count: <span id="location-changed-count">@_locationChangedCount</span></p>
+}
+
+@code {
+    private bool _interactive;
+    private int _count;
+    private int _locationChangedCount;
+
+    [Parameter]
+    public string Value { get; set; }
+
+    protected override void OnAfterRender(bool firstRender)
+    {
+        if (firstRender)
+        {
+            NavigationManager.LocationChanged += OnLocationChanged;
+            _interactive = true;
+            StateHasChanged();
+        }
+    }
+
+    private void OnLocationChanged(object sender, LocationChangedEventArgs e)
+    {
+        _locationChangedCount++;
+        StateHasChanged();
+    }
+
+    private void Increment()
+    {
+        _count++;
+    }
+
+    private void Refresh()
+    {
+        NavigationManager.Refresh();
+    }
+
+    public void Dispose()
+    {
+        NavigationManager.LocationChanged -= OnLocationChanged;
+    }
+}