Started on unit tests.

Author: MackinnonBuck

src/Components/Web/test/Virtualization/VirtualizeTest.cs

@@ -1,10 +1,69 @@
 // 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.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Components.Rendering;
+using Microsoft.AspNetCore.Components.Test.Helpers;
+using Microsoft.AspNetCore.Components.Web;
+using Xunit;
+
 namespace Microsoft.AspNetCore.Components.Virtualization
 {
     public class VirtualizeTest
     {
         // TODO: Functional tests.
+        [Fact]
+        public void Virtualize_ThrowsWhenGivenNonPositiveItemSize()
+        {
+            var rootComponent = new VirtualizeTestHostcomponent
+            {
+                InnerContent = BuildVirtualize(
+                    i => builder => { },
+                    null,
+                    context => builder => { },
+                    0f,
+                    null,
+                    new List<int>())
+            };
+
+            var testRenderer = new TestRenderer();
+            var componentId = testRenderer.AssignRootComponentId(rootComponent);
+
+            var ex = Assert.Throws<InvalidOperationException>(() => testRenderer.RenderRootComponent(componentId));
+        }
+
+        public RenderFragment BuildVirtualize<TItem>(
+            RenderFragment<TItem> childContent,
+            RenderFragment<TItem> item,
+            RenderFragment<PlaceholderContext> placeholder,
+            float itemSize,
+            ItemsProviderDelegate<TItem> itemsProvider,
+            ICollection<TItem> items)
+            => builder =>
+        {
+            builder.OpenComponent<Virtualize<TItem>>(0);
+            builder.AddAttribute(1, "ChildContent", childContent);
+            builder.AddAttribute(2, "Item", item);
+            builder.AddAttribute(3, "Placeholder", placeholder);
+            builder.AddAttribute(4, "ItemSize", itemSize);
+            builder.AddAttribute(5, "ItemsProvider", itemsProvider);
+            builder.AddAttribute(6, "Items", items);
+            builder.CloseComponent();
+        };
+
+        private class VirtualizeTestHostcomponent : AutoRenderComponent
+        {
+            public RenderFragment InnerContent { get; set; }
+
+            protected override void BuildRenderTree(RenderTreeBuilder builder)
+            {
+                builder.OpenElement(0, "div");
+                builder.AddAttribute(1, "style", "overflow: auto; height: 800px;");
+                builder.AddAttribute(2, "ChildContent", InnerContent);
+                builder.CloseElement();
+            }
+        }
     }
 }