Add the key in the message on throwing a KeyNotFoundException (#282)

* Add the key in the message on throwing a KeyNotFoundException for trying to access an invalid key in the ResourceDictionary. This helps a lot in tracking down what resource is actually missing.

* Fix test build, use C# 6 string interpolation

2 files changed, 11 insertions, 2 deletions

diff --git a/Xamarin.Forms.Core.UnitTests/ResourceDictionaryTests.cs b/Xamarin.Forms.Core.UnitTests/ResourceDictionaryTests.cs
index d4229221..c4e6bbf3 100644
--- a/Xamarin.Forms.Core.UnitTests/ResourceDictionaryTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/ResourceDictionaryTests.cs
@@ -252,5 +252,14 @@ namespace Xamarin.Forms.Core.UnitTests
 			elt.Parent = parent;
 			Assert.Fail ();
 		}
-	}
+
+        [Test]
+        public void ShowKeyInExceptionIfNotFound()
+        {
+            var rd = new ResourceDictionary();
+            rd.Add("foo", "bar");
+            var ex = Assert.Throws<KeyNotFoundException>(() => { var foo = rd["test_invalid_key"]; });
+            Assert.That(ex.Message, Is.StringContaining("test_invalid_key"));
+        }
+    }
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/ResourceDictionary.cs b/Xamarin.Forms.Core/ResourceDictionary.cs
index 791c6e5d..0747eaac 100644
--- a/Xamarin.Forms.Core/ResourceDictionary.cs
+++ b/Xamarin.Forms.Core/ResourceDictionary.cs
@@ -98,7 +98,7 @@ namespace Xamarin.Forms
 					return _innerDictionary[index];
 				if (_mergedInstance != null && _mergedInstance.ContainsKey(index))
 					return _mergedInstance[index];
-				throw new KeyNotFoundException();
+				throw new KeyNotFoundException($"The resource '{index}' is not present in the dictionary.");
 			}
 			set
 			{