Implement nested Label for DisplayName

src/Components/Web/src/Forms/Label.cs

@@ -0,0 +1,92 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Linq.Expressions;
+using Microsoft.AspNetCore.Components.Rendering;
+
+namespace Microsoft.AspNetCore.Components.Forms;
+
+/// <summary>
+/// Renders a <c>&lt;label&gt;</c> element for a specified field, reading the display name from
+/// <see cref="System.ComponentModel.DataAnnotations.DisplayAttribute"/> or
+/// <see cref="System.ComponentModel.DisplayNameAttribute"/> if present, or falling back to the property name.
+/// The label wraps its child content (typically an input component), providing implicit association
+/// without requiring matching for/id attributes.
+/// </summary>
+/// <typeparam name="TValue">The type of the field.</typeparam>
+public class Label<TValue> : IComponent
+{
+    private RenderHandle _renderHandle;
+    private Expression<Func<TValue>>? _previousFieldAccessor;
+    private string? _displayName;
+
+    /// <summary>
+    /// Specifies the field for which the label should be rendered.
+    /// </summary>
+    [Parameter, EditorRequired]
+    public Expression<Func<TValue>>? For { get; set; }
+
+    /// <summary>
+    /// Gets or sets the child content to be rendered inside the label element.
+    /// Typically this contains an input component that will be implicitly associated with the label.
+    /// </summary>
+    [Parameter]
+    public RenderFragment? ChildContent { get; set; }
+
+    /// <summary>
+    /// Gets or sets a collection of additional attributes that will be applied to the label element.
+    /// </summary>
+    [Parameter(CaptureUnmatchedValues = true)]
+    public IReadOnlyDictionary<string, object>? AdditionalAttributes { get; set; }
+
+    /// <inheritdoc />
+    void IComponent.Attach(RenderHandle renderHandle)
+    {
+        _renderHandle = renderHandle;
+    }
+
+    /// <inheritdoc />
+    Task IComponent.SetParametersAsync(ParameterView parameters)
+    {
+        var previousChildContent = ChildContent;
+        var previousAdditionalAttributes = AdditionalAttributes;
+
+        parameters.SetParameterProperties(this);
+
+        if (For is null)
+        {
+            throw new InvalidOperationException($"{GetType()} requires a value for the " +
+                $"{nameof(For)} parameter.");
+        }
+
+        // Only recalculate display name if the expression changed
+        if (For != _previousFieldAccessor)
+        {
+            var newDisplayName = ExpressionMemberAccessor.GetDisplayName(For);
+
+            if (newDisplayName != _displayName)
+            {
+                _displayName = newDisplayName;
+                _renderHandle.Render(BuildRenderTree);
+            }
+
+            _previousFieldAccessor = For;
+        }
+        else if (ChildContent != previousChildContent || AdditionalAttributes != previousAdditionalAttributes)
+        {
+            // Re-render if other parameters changed
+            _renderHandle.Render(BuildRenderTree);
+        }
+
+        return Task.CompletedTask;
+    }
+
+    private void BuildRenderTree(RenderTreeBuilder builder)
+    {
+        builder.OpenElement(0, "label");
+        builder.AddMultipleAttributes(1, AdditionalAttributes);
+        builder.AddContent(2, _displayName);
+        builder.AddContent(3, ChildContent);
+        builder.CloseElement();
+    }
+}

src/Components/Web/src/PublicAPI.Unshipped.txt

@@ -74,3 +74,11 @@ Microsoft.AspNetCore.Components.Forms.DisplayName<TValue>
 Microsoft.AspNetCore.Components.Forms.DisplayName<TValue>.DisplayName() -> void
 Microsoft.AspNetCore.Components.Forms.DisplayName<TValue>.For.get -> System.Linq.Expressions.Expression<System.Func<TValue>!>?
 Microsoft.AspNetCore.Components.Forms.DisplayName<TValue>.For.set -> void
+Microsoft.AspNetCore.Components.Forms.Label<TValue>
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.AdditionalAttributes.get -> System.Collections.Generic.IReadOnlyDictionary<string!, object!>?
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.AdditionalAttributes.set -> void
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.ChildContent.get -> Microsoft.AspNetCore.Components.RenderFragment?
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.ChildContent.set -> void
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.For.get -> System.Linq.Expressions.Expression<System.Func<TValue>!>?
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.For.set -> void
+Microsoft.AspNetCore.Components.Forms.Label<TValue>.Label() -> void