Prepare to rename ElementRef

Part of: #11610

This change just renames the type, because it's used in the compiler.
We will need to react in the compiler, and then update all of the usage
here.

src/Components/Components/ref/Microsoft.AspNetCore.Components.netstandard2.0.cs

@@ -151,6 +151,13 @@ namespace Microsoft.AspNetCore.Components
         public string __internalId { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
+    public readonly partial struct ElementReference
+    {
+        private readonly object _dummy;
+        [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]
+        public string __internalId { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } }
+    }
+    [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
     public readonly partial struct EventCallback
     {
         private readonly object _dummy;

src/Components/Components/src/ElementReference.cs

@@ -0,0 +1,58 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.ComponentModel;
+using System.Threading;
+
+namespace Microsoft.AspNetCore.Components
+{
+    /// <summary>
+    /// Represents a reference to a rendered element.
+    /// </summary>
+    public readonly struct ElementReference
+    {
+        private static long _nextIdForWebAssemblyOnly = 1;
+
+        /// <summary>
+        /// Gets a unique identifier for <see cref="ElementRef" />.
+        /// </summary>
+        /// <remarks>
+        /// The Id is unique at least within the scope of a given user/circuit.
+        /// This property is public to support Json serialization and should not be used by user code.
+        /// </remarks>
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        public string __internalId { get; }
+
+        internal string Id => __internalId;
+
+        private ElementReference(string id)
+        {
+            __internalId = id;
+        }
+
+        internal static ElementReference CreateWithUniqueId()
+            => new ElementReference(CreateUniqueId());
+
+        private static string CreateUniqueId()
+        {
+            if (PlatformInfo.IsWebAssembly)
+            {
+                // On WebAssembly there's only one user, so it's fine to expose the number
+                // of IDs that have been assigned, and this is cheaper than creating a GUID.
+                // It's unfortunate that this still involves a heap allocation. If that becomes
+                // a problem we could extend RenderTreeFrame to have both "string" and "long"
+                // fields for ElementRefCaptureId, of which only one would be in use depending
+                // on the platform.
+                var id = Interlocked.Increment(ref _nextIdForWebAssemblyOnly);
+                return id.ToString();
+            }
+            else
+            {
+                // For remote rendering, it's important not to disclose any cross-user state,
+                // such as the number of IDs that have been assigned.
+                return Guid.NewGuid().ToString("D");
+            }
+        }
+    }
+}