[XamlC] better error reporting on missing Add()er (#1982)

- fixes #1978

Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs

@@ -184,7 +184,9 @@ namespace Xamarin.Forms.Build.Tasks
 					Context.IL.Append(AddToResourceDictionary(node, node, Context));
 					return;
 				} 
-				var adderTuple = propertyType.GetMethods(md => md.Name == "Add" && md.Parameters.Count == 1, Module).First();
+				var adderTuple = propertyType.GetMethods(md => md.Name == "Add" && md.Parameters.Count == 1, Module).FirstOrDefault();
+				if (adderTuple == null)
+					throw new XamlParseException($"Can not Add() elements to {parent.VariableType}.{localname}", node);
 				var adderRef = Module.ImportReference(adderTuple.Item1);
 				adderRef = Module.ImportReference(adderRef.ResolveGenericParameters(adderTuple.Item2, Module));
 

Xamarin.Forms.Xaml.UnitTests/Issues/Gh1978.xaml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Xamarin.Forms.Xaml.UnitTests.Gh1978">
+    <ContentPage.ToolbarItems>
+        <ToolbarItem>
+            <ToolbarItem.Text>
+                <!--Forgot the OnPlatform tag-->
+                    <On Platform="Android,iOS" Value="+" />
+                    <On Platform="UWP" Value="Add" />
+            </ToolbarItem.Text>
+        </ToolbarItem>
+    </ContentPage.ToolbarItems>
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh1978.xaml.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using NUnit.Framework;
+using Xamarin.Forms;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	[XamlCompilation(XamlCompilationOptions.Skip)]
+	public partial class Gh1978 : ContentPage
+	{
+		public Gh1978()
+		{
+			InitializeComponent();
+		}
+
+		public Gh1978(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture]
+		class Tests
+		{
+			[TestCase(true)]
+			public void ReportError(bool useCompiledXaml)
+			{
+				if (!useCompiledXaml)
+					return;
+				Assert.Throws<XamlParseException>(() => MockCompiler.Compile(typeof(Gh1978)));
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml.UnitTests/Xamarin.Forms.Xaml.UnitTests.csproj

@@ -576,6 +576,9 @@
     <Compile Include="Issues\Gh1766.xaml.cs">
       <DependentUpon>Gh1766.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Issues\Gh1978.xaml.cs">
+      <DependentUpon>Gh1978.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="..\.nuspec\Xamarin.Forms.Debug.targets" />
@@ -1029,6 +1032,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Include="Issues\Gh1978.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />