ListView should convert item to string when ItemTemplate is not specified (#4016)

* Convert item to string, when ListView has no ItemTemplate

* ToStringValueConverter tests

* Format value using parameter in ToStringValueConverter
- fixes #3715

Xamarin.Forms.Core.UnitTests/ListViewTests.cs

@@ -45,6 +45,17 @@ namespace Xamarin.Forms.Core.UnitTests
 		{
 			public string Name { get; set; }
 			public string Description { get; set; }
+
+			public override string ToString() => Name ?? base.ToString();
+		}
+
+		static ListItem[] CreateListItemCollection()
+		{
+			return new[]
+			{
+				new ListItem { Name = "Foo", Description = "Bar" },
+				new ListItem { Name = "Baz", Description = "Raz" }
+			};
 		}
 
 		[Test]
@@ -55,10 +66,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			cellTemplate.SetBinding (TextCell.DetailProperty, new Binding ("Description"));
 
 			var listView = new ListView {
-				ItemsSource = new[] {
-					new ListItem {Name = "Foo", Description = "Bar"},
-					new ListItem {Name = "Baz", Description = "Raz"}
-				},
+				ItemsSource = CreateListItemCollection(),
 				ItemTemplate = cellTemplate
 			};
 
@@ -87,6 +95,27 @@ namespace Xamarin.Forms.Core.UnitTests
 			Assert.That (((TextCell) cell).Text, Is.Null);
 		}
 
+		[Test]
+		public void ItemTemplateIsNullObjectExecutesToString()
+		{
+			var listView = new ListView
+			{
+				ItemsSource = CreateListItemCollection()
+			};
+
+			Assert.AreEqual(2, listView.TemplatedItems.Count);
+
+			Cell cell = listView.TemplatedItems[0];
+			Assert.That(cell, Is.Not.Null);
+			Assert.That(cell, Is.InstanceOf<TextCell>());
+			Assert.That(((TextCell)cell).Text, Is.EqualTo("Foo"));
+
+			cell = listView.TemplatedItems[1];
+			Assert.That(cell, Is.Not.Null);
+			Assert.That(cell, Is.InstanceOf<TextCell>());
+			Assert.That(((TextCell)cell).Text, Is.EqualTo("Baz"));
+		}
+
 		[Test]
 		[Description("Setting BindingContext should trickle down to Header and Footer.")]
 		public void SettingBindingContextPassesToHeaderAndFooter()

Xamarin.Forms.Core.UnitTests/ToStringValueConverterTests.cs

@@ -0,0 +1,243 @@
+using System;
+using System.Globalization;
+using NUnit.Framework;
+
+namespace Xamarin.Forms.Core.UnitTests
+{
+	[TestFixture]
+	public class ToStringValueConverterTests : BaseTestFixture
+	{
+		static readonly CultureInfo _enUsCulture = CultureInfo.GetCultureInfo("en-US");
+		static readonly CultureInfo _skSkCulture = CultureInfo.GetCultureInfo("sk-SK");
+
+		[SetUp]
+		public override void Setup()
+		{
+			base.Setup();
+			System.Threading.Thread.CurrentThread.CurrentCulture = _enUsCulture;
+		}
+
+		[Test]
+		public void NullObjectConvertsToNull()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(null, null, null, null);
+
+			Assert.That(result, Is.Null);
+		}
+
+		[Test]
+		public void NullObjectWithTargetTypeConvertsToNull()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(null, typeof(string), null, null);
+
+			Assert.That(result, Is.Null);
+		}
+
+		[Test]
+		public void ObjectConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(new object(), typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo("System.Object"));
+		}
+
+		[Test]
+		public void EmptyStringConvertsToEmptyString()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(string.Empty, typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo(string.Empty));
+		}
+
+		[Test]
+		public void StringConvertsToString()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert("Hello, World!", typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo("Hello, World!"));
+		}
+
+		private class ToStringObject
+		{
+			public ToStringObject(string value)
+			{
+				Value = value;
+			}
+
+			public string Value { get; set; }
+
+			public int ToStringCounter { get; private set; }
+
+			public override string ToString()
+			{
+				ToStringCounter++;
+				return Value;
+			}
+		}
+
+		[Test]
+		public void CustomObjectWithNullValueConvertsToNull()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var value = new ToStringObject(null);
+			object result = toStringValueConverter.Convert(value, typeof(string), null, null);
+
+			Assert.That(result, Is.Null);
+			Assert.That(value.ToStringCounter, Is.EqualTo(1));
+		}
+
+		[Test]
+		public void CustomObjectWithEmptyStringConvertsToEmptyString()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var value = new ToStringObject(string.Empty);
+			object result = toStringValueConverter.Convert(value, typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo(string.Empty));
+			Assert.That(value.ToStringCounter, Is.EqualTo(1));
+		}
+
+		[Test]
+		public void CustomObjectWithStringConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var value = new ToStringObject("Test string");
+			object result = toStringValueConverter.Convert(value, typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo("Test string"));
+			Assert.That(value.ToStringCounter, Is.EqualTo(1));
+		}
+
+		[Test]
+		public void ExecutesToStringTwice()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var value = new ToStringObject("Test string");
+			object result = toStringValueConverter.Convert(value, typeof(string), null, _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("Test string"));
+
+			value.Value = "Hello, World!";
+			result = toStringValueConverter.Convert(value, typeof(string), null, _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("Hello, World!"));
+			Assert.That(value.ToStringCounter, Is.EqualTo(2));
+		}
+
+		[Test]
+		public void DoubleValueConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), null, null);
+
+			Assert.That(result, Is.EqualTo("99123.567"));
+		}
+
+		[Test]
+		public void DoubleValueWithSkCultureConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), null, _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("99123,567"));
+		}
+
+		[Test]
+		public void DoubleValueWithEmptyParameterConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), string.Empty, null);
+
+			Assert.That(result, Is.EqualTo("99123.567"));
+		}
+
+		[Test]
+		public void DoubleValueWithEmptyParameterAndSkCultureConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), string.Empty, _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("99123,567"));
+		}
+
+		[Test]
+		public void DoubleValueWithNumberFormatConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), "N2", null);
+
+			Assert.That(result, Is.EqualTo("99,123.57"));
+		}
+
+		[Test]
+		public void DoubleValueWithNumberFormatAndSkCultureConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			object result = toStringValueConverter.Convert(99123.567, typeof(string), "N2", _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("99 123,57"));
+		}
+
+		[Test]
+		public void DoubleValueWithSpecificFormatConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var format = new ToStringObject("#,##0.000");
+			object result = toStringValueConverter.Convert(99123.56, typeof(string), format, null);
+
+			Assert.That(result, Is.EqualTo("99,123.560"));
+		}
+
+		[Test]
+		public void DoubleValueWithSpecificFormatAndSkCultureConvertsToStringValue()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			var format = new ToStringObject("#,##0.000");
+			object result = toStringValueConverter.Convert(99123.56, typeof(string), format, _skSkCulture);
+
+			Assert.That(result, Is.EqualTo("99 123,560"));
+		}
+
+		[Test]
+		public void NullObjectConvertsBackThrowsException()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			TestDelegate action = () => toStringValueConverter.ConvertBack(null, null, null, null);
+
+			Assert.That(action, Throws.InstanceOf<NotSupportedException>());
+		}
+
+		[Test]
+		public void ObjectConvertsBackThrowsException()
+		{
+			var toStringValueConverter = new ToStringValueConverter();
+
+			TestDelegate action = () => toStringValueConverter.ConvertBack(new object(), typeof(string), null, null);
+
+			Assert.That(action, Throws.InstanceOf<NotSupportedException>());
+		}
+	}
+}

Xamarin.Forms.Core.UnitTests/Xamarin.Forms.Core.UnitTests.csproj

@@ -156,6 +156,7 @@
     <Compile Include="TimePickerUnitTest.cs" />
     <Compile Include="ToolbarItemTests.cs" />
     <Compile Include="ToolbarTrackerTests.cs" />
+    <Compile Include="ToStringValueConverterTests.cs" />
     <Compile Include="TypeUnitTests.cs" />
     <Compile Include="ViewCellTests.cs" />
     <Compile Include="ViewUnitTests.cs" />

Xamarin.Forms.Core/ListView.cs

@@ -50,6 +50,8 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty VerticalScrollBarVisibilityProperty = BindableProperty.Create(nameof(VerticalScrollBarVisibility), typeof(ScrollBarVisibility), typeof(ListView), ScrollBarVisibility.Default);
 
+		static readonly ToStringValueConverter _toStringValueConverter = new ToStringValueConverter();
+
 		readonly Lazy<PlatformConfigurationRegistry<ListView>> _platformConfigurationRegistry;
 
 		BindingBase _groupDisplayBinding;
@@ -337,12 +339,8 @@ namespace Xamarin.Forms
 
 		protected override Cell CreateDefault(object item)
 		{
-			string text = null;
-			if (item != null)
-				text = item.ToString();
-
 			TextCell textCell = new TextCell();
-			textCell.SetBinding(TextCell.TextProperty, ".");
+			textCell.SetBinding(TextCell.TextProperty, ".", converter: _toStringValueConverter);
 			return textCell;
 		}
 

Xamarin.Forms.Core/ToStringValueConverter.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Globalization;
+
+namespace Xamarin.Forms
+{
+	public class ToStringValueConverter : IValueConverter
+	{
+		public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+		{
+			if (value == null)
+			{
+				return null;
+			}
+
+			if (value is IFormattable formattable)
+			{
+				return formattable.ToString(parameter?.ToString(), culture);
+			}
+
+			return value.ToString();
+		}
+
+		public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+		{
+			throw new NotSupportedException();
+		}
+	}
+}