Make stringBuilder.AppendValueOf safer

...by only showing the first 10 items of enumerables for certain safe
collection types (arrays and `List<T>`). It cannot be generally assum-
ed that `IEnumerable` actually work.

I cannot easily add the test case provided in #741 to the regression
test suite as it relies on EF Core, and the test project already uses
EF, so we'd likely have type conflicts if both of them are referenced.

CHANGELOG.md

@@ -42,6 +42,7 @@ This release contains several **minor breaking changes**. Please review your cod
 * Use of `SetupSet` 'forgets' method setup (@TimothyHayes, #432)
 * Recursive mocks don't work with argument matching (@thalesmello, #142)
 * Recursive property setup overrides previous setups (@jamesfoster, #110)
+* Formatting of enumerable object for error message broke EF Core test case (@MichaelSagalovich, #741)
 
 
 ## 4.10.1 (2018-12-03)

src/Moq/StringBuilderExtensions.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
 using System.Text;
@@ -82,14 +83,15 @@ public static StringBuilder AppendValueOf(this StringBuilder stringBuilder, obje
 			{
 				stringBuilder.Append(d.ToString("G17"));
 			}
-			else if (obj is IEnumerable enumerable && !(obj is IMocked))
-			{                                      // ^^^^^^^^^^^^^^^^^
-			                                       // This second check ensures that we have a usable implementation of IEnumerable.
-			                                       // If value is a mocked object, its IEnumerable implementation might very well
-			                                       // not work correctly.
+			else if (obj.GetType().IsEnum)
+			{
+				stringBuilder.AppendNameOf(obj.GetType()).Append('.').Append(obj);
+			}
+			else if (obj.GetType().IsArray || (obj.GetType().IsConstructedGenericType && obj.GetType().GetGenericTypeDefinition() == typeof(List<>)))
+			{
 				stringBuilder.Append('[');
 				const int maxCount = 10;
-				var enumerator = enumerable.GetEnumerator();
+				var enumerator = ((IEnumerable)obj).GetEnumerator();
 				for (int i = 0; enumerator.MoveNext() && i < maxCount + 1; ++i)
 				{
 					if (i > 0)
@@ -107,17 +109,17 @@ public static StringBuilder AppendValueOf(this StringBuilder stringBuilder, obje
 				}
 				stringBuilder.Append(']');
 			}
-			else if (obj.ToString() == obj.GetType().ToString())
-			{
-				stringBuilder.AppendNameOf(obj.GetType());
-			}
-			else if (obj.GetType().IsEnum)
-			{
-				stringBuilder.AppendNameOf(obj.GetType()).Append('.').Append(obj);
-			}
 			else
 			{
-				stringBuilder.Append(obj.ToString());
+				var formatted = obj.ToString();
+				if (formatted == null || formatted == obj.GetType().ToString())
+				{
+					stringBuilder.AppendNameOf(obj.GetType());
+				}
+				else
+				{
+					stringBuilder.Append(formatted);
+				}
 			}
 
 			return stringBuilder;