improve error message when multi mapper is misused

Dapper/SqlMapper.cs

@@ -804,9 +804,15 @@ IEnumerator IEnumerable.GetEnumerator()
 
         private static Func<IDataReader, T> GetDynamicDeserializer<T>(IDataRecord reader, int startBound, int length, bool returnNullIfFirstMissing)
         {
+            var fieldCount = reader.FieldCount;
             if (length == -1)
             {
-                length = reader.FieldCount - startBound;
+                length = fieldCount - startBound;
+            }
+
+            if (fieldCount <= startBound)
+            {
+                throw new ArgumentException("When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id", "splitOn");
             }
 
             return
@@ -1143,7 +1149,13 @@ static readonly MethodInfo
                 length = reader.FieldCount - startBound;
             }
 
+            if (reader.FieldCount <= startBound)
+            {
+                throw new ArgumentException("When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id", "splitOn");
+            }
+
             var names = new List<string>();
+
             for (int i = startBound; i < startBound + length; i++)
             {
                 names.Add(reader.GetName(i));

Tests/Tests.cs

@@ -958,5 +958,25 @@ public void TestNullableCharInputAndOutputNull()
 
             obj.ValueNullable.IsEqualTo(test);
         }
+        public void TestInvalidSplitCausesNiceError()
+        {
+            try
+            {
+                connection.Query<User, User, User>("select 1 A, 2 B, 3 C", (x, y) => x);
+            }
+            catch (ArgumentException)
+            { 
+                // expecting an app exception due to multi mapping being bodged 
+            }
+
+            try
+            {
+                connection.Query<dynamic, dynamic, dynamic>("select 1 A, 2 B, 3 C", (x, y) => x);
+            }
+            catch (ArgumentException)
+            {
+                // expecting an app exception due to multi mapping being bodged 
+            }
+        }
     }
 }