From a9dc626cbed549cb80af193551085d4fad44f388 Mon Sep 17 00:00:00 2001
From: Zachary Becknell <zbecknell@users.noreply.github.com>
Date: Mon, 16 Apr 2018 22:28:05 -0400
Subject: [PATCH] SimpleJson: more informative exception when attempting to
 deserialize object with non-empty constructor

---
 .../Json/SimpleJson/SimpleJson.cs             |  5 +++-
 .../JsonUtilTest.cs                           | 26 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/Microsoft.AspNetCore.Blazor/Json/SimpleJson/SimpleJson.cs b/src/Microsoft.AspNetCore.Blazor/Json/SimpleJson/SimpleJson.cs
index da3c82ef22..44eaa1d585 100644
--- a/src/Microsoft.AspNetCore.Blazor/Json/SimpleJson/SimpleJson.cs
+++ b/src/Microsoft.AspNetCore.Blazor/Json/SimpleJson/SimpleJson.cs
@@ -1430,7 +1430,10 @@ namespace SimpleJson
                             obj = value;
                         else
                         {
-                            obj = ConstructorCache[type]();
+                            var constructorDelegate = ConstructorCache[type]
+                                ?? throw new InvalidOperationException($"Cannot deserialize JSON into type '{type.FullName}' because it does not have a public parameterless constructor.");
+                            obj = constructorDelegate();
+
                             foreach (KeyValuePair<string, KeyValuePair<Type, ReflectionUtils.SetDelegate>> setter in SetCache[type])
                             {
                                 object jsonValue;
diff --git a/test/Microsoft.AspNetCore.Blazor.Test/JsonUtilTest.cs b/test/Microsoft.AspNetCore.Blazor.Test/JsonUtilTest.cs
index a0dd701b05..33115400a7 100644
--- a/test/Microsoft.AspNetCore.Blazor.Test/JsonUtilTest.cs
+++ b/test/Microsoft.AspNetCore.Blazor.Test/JsonUtilTest.cs
@@ -113,6 +113,32 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Equal(1, simpleError.NullableIntProperty);
         }
 
+        [Fact]
+        public void NonEmptyConstructorThrowsUsefulException()
+        {
+            // Arrange
+            var json = "{\"Property\":1}";
+            var type = typeof(NonEmptyConstructorPoco);
+
+            // Act
+            var exception = Assert.Throws<InvalidOperationException>(() =>
+            {
+                JsonUtil.Deserialize<NonEmptyConstructorPoco>(json); 
+            });
+
+            // Assert
+            Assert.Equal(
+                $"Cannot deserialize JSON into type '{type.FullName}' because it does not have a public parameterless constructor.", 
+                exception.Message);
+        }
+
+        class NonEmptyConstructorPoco
+        {
+            public NonEmptyConstructorPoco(int parameter) {}
+
+            public int Property { get; set; }
+        }
+
         struct SimpleStruct
         {
             public string StringProperty { get; set; }