Simplify IsEnumerableType implementation

Author: Michael Croes

src/AutoMapper.Extensions.ExpressionMapping/ExpressionMapper.cs

@@ -173,7 +173,7 @@ private Expression VisitAllParametersExpression<T>(Expression<T> expression)
                     from t in expression.Parameters
                     let sourceParamType = t.Type
                     from destParamType in _destSubTypes.Where(dt => dt != sourceParamType)
-                    let a = destParamType.IsEnumerableType(out var itemType) ? itemType : destParamType
+                    let a = destParamType.IsEnumerableType() ? destParamType.GetGenericElementType() : destParamType
                     let typeMap = _configurationProvider.ResolveTypeMap(a, sourceParamType)
                     where typeMap != null
                     let oldParam = t

src/AutoMapper.Extensions.ExpressionMapping/TypeExtensions.cs

@@ -172,13 +172,7 @@ public static bool IsQueryableType(this Type type)
         public static Type GetGenericElementType(this Type type)
             => type.HasElementType ? type.GetElementType() : type.GetTypeInfo().GenericTypeArguments[0];
 
-        public static bool IsEnumerableType(this Type type, out Type itemType)
-        {
-            itemType = type.GetInterfaces()
-                .FirstOrDefault(t => t.GetGenericTypeDefinitionIfGeneric() == typeof(IEnumerable<>))
-                ?.GetGenericArguments()[0];
-
-            return itemType != null;
-        }
+        public static bool IsEnumerableType(this Type type) =>
+            type.IsGenericType && typeof(System.Collections.IEnumerable).IsAssignableFrom(type);
     }
 }