Add support for types with constructors matching the types pulled by data reader.

Dapper/SqlMapper.cs

@@ -1497,12 +1497,31 @@ static List<FieldInfo> GetSettableFields(Type t)
 
             int index = startBound;
 
+            ConstructorInfo specializedConstructor = null;
             if (type.IsValueType)
             {
                 il.Emit(OpCodes.Ldloca_S, (byte)1);
                 il.Emit(OpCodes.Initobj, type);
             }
             else
+            {
+                var types = new Type[length - startBound];
+                for (int i = startBound; i < startBound + length; i++)
+                {
+                    types[i - startBound] = reader.GetFieldType(i);
+                }
+                var ctorWithParameters = type.GetConstructor(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null, types, null);
+                if (ctorWithParameters != null)
+                {
+                    var ctorParams = ctorWithParameters.GetParameters();
+                    for(int i =0; i< ctorParams.Length; i++)
+                    {
+                        if (!String.Equals(ctorParams[i].Name, names[i], StringComparison.OrdinalIgnoreCase))
+                            break;
+                        specializedConstructor = ctorWithParameters;
+                    }
+                }
+                if(specializedConstructor == null)
                 {
                     var ctor = type.GetConstructor(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null, Type.EmptyTypes, null);
                     if (ctor == null)
@@ -1512,11 +1531,13 @@ static List<FieldInfo> GetSettableFields(Type t)
                     il.Emit(OpCodes.Newobj, ctor);
                     il.Emit(OpCodes.Stloc_1);
                 }
+            }
             il.BeginExceptionBlock();
             if(type.IsValueType)
             {
                 il.Emit(OpCodes.Ldloca_S, (byte)1);// [target]
-            } else
+            }
+            else if(specializedConstructor == null)
             {
                 il.Emit(OpCodes.Ldloc_1);// [target]
             }
@@ -1529,6 +1550,7 @@ static List<FieldInfo> GetSettableFields(Type t)
             {
                 if (item.Property != null || item.Field != null)
                 {
+                    if(specializedConstructor == null)
                         il.Emit(OpCodes.Dup); // stack is now [target][target]
                     Label isDbNullLabel = il.DefineLabel();
                     Label finishLabel = il.DefineLabel();
@@ -1587,6 +1609,8 @@ static List<FieldInfo> GetSettableFields(Type t)
                             {
                                 il.Emit(OpCodes.Newobj, memberType.GetConstructor(new[] { nullUnderlyingType }));
                             }
+                            if (specializedConstructor == null)
+                            {
                                 if (item.Property != null)
                                 {
                                     il.Emit(OpCodes.Callvirt, item.Property.Setter); // stack is now [target]
@@ -1595,6 +1619,7 @@ static List<FieldInfo> GetSettableFields(Type t)
                                 {
                                     il.Emit(OpCodes.Stfld, item.Field); // stack is now [target]
                                 }
+                            }
                             il.Emit(OpCodes.Br_S, finishLabel);
 
 
@@ -1614,6 +1639,8 @@ static List<FieldInfo> GetSettableFields(Type t)
                             il.Emit(OpCodes.Newobj, memberType.GetConstructor(new[] { nullUnderlyingType }));
                         }
                     }
+                    if (specializedConstructor == null)
+                    {
                         if (item.Property != null)
                         {
                             if (type.IsValueType)
@@ -1629,13 +1656,24 @@ static List<FieldInfo> GetSettableFields(Type t)
                         {
                             il.Emit(OpCodes.Stfld, item.Field); // stack is now [target]
                         }
+                    }
                     
                     il.Emit(OpCodes.Br_S, finishLabel); // stack is now [target]
 
                     il.MarkLabel(isDbNullLabel); // incoming stack: [target][target][value]
-
+                    if (specializedConstructor != null)
+                    {
+                        Type itemType = item.Property != null ? item.Property.Type : item.Field.FieldType;
+                        if (itemType.IsValueType)
+                            il.Emit(OpCodes.Initobj, itemType);
+                        else
+                            il.Emit(OpCodes.Ldnull);
+                    }
+                    else
+                    {
                         il.Emit(OpCodes.Pop); // stack is now [target][target]
                         il.Emit(OpCodes.Pop); // stack is now [target]
+                    }
 
                     if (first && returnNullIfFirstMissing)
                     {
@@ -1656,6 +1694,10 @@ static List<FieldInfo> GetSettableFields(Type t)
             }
             else
             {
+                if (specializedConstructor != null)
+                {
+                    il.Emit(OpCodes.Newobj, specializedConstructor);
+                }
                 il.Emit(OpCodes.Stloc_1); // stack is empty
             }
             il.MarkLabel(allDone);

Tests/Tests.cs

@@ -33,29 +33,21 @@ public class ConcreteOrder : Order
             }
         }
 
-        class NoDefualtConstructor 
+        class NoDefaultConstructor 
         {
-            public NoDefualtConstructor(int a)
+            public NoDefaultConstructor(int a)
             {
                 A = a;
             }
             public int A { get; set; }
         }
 
-        public void EnsureNoConstructorGivesNiceError()
+        public void TestNoDefaultConstructor()
         {
-            try
-            {
-                connection.Query<NoDefualtConstructor>("select 1 A").First();
-            }
-            catch(InvalidOperationException e)
-            {
-                e.Message.IsEqualTo("A parameterless default constructor is required to allow for dapper materialization");
+            NoDefaultConstructor nodef = connection.Query<NoDefaultConstructor>("select 1 A").First();
+            nodef.A.IsEqualTo(1);
         }
 
-        }
-
-
         // http://stackoverflow.com/q/8593871
         public void TestAbstractInheritance() 
         {