Fix #524 - `GridReader` should use the same conversion code as the regular reader

Dapper.Tests/Tests.QueryMultiple.cs

@@ -141,7 +141,7 @@ public void Issue524_QueryMultiple_Cast()
         { // aka: Read<int> should work even if the data is a <long>
             
             // using regular API
-            connection.Query<int>("select cast(42 as bigint)").IsEqualTo(42);
+            connection.Query<int>("select cast(42 as bigint)").Single().IsEqualTo(42);
             connection.QuerySingle<int>("select cast(42 as bigint)").IsEqualTo(42);
             
             // using multi-reader API

Dapper/SqlMapper.GridReader.Async.cs

@@ -181,7 +181,7 @@ private Task<IEnumerable<T>> ReadAsyncImpl<T>(Type type, bool buffered)
                 }
                 else
                 {
-                    var result = ReadDeferred<T>(gridIndex, deserializer.Func, typedIdentity);
+                    var result = ReadDeferred<T>(gridIndex, deserializer.Func, typedIdentity, type);
                     if (buffered) result = result.ToList(); // for the "not a DbDataReader" scenario
                     return Task.FromResult(result);
                 }

Dapper/SqlMapper.GridReader.cs

@@ -2,7 +2,7 @@
 using System.Collections.Generic;
 using System.Data;
 using System.Linq;
-
+using System.Globalization;
 namespace Dapper
 {
     partial class SqlMapper
@@ -161,7 +161,7 @@ private IEnumerable<T> ReadImpl<T>(Type type, bool buffered)
                     cache.Deserializer = deserializer;
                 }
                 IsConsumed = true;
-                var result = ReadDeferred<T>(gridIndex, deserializer.Func, typedIdentity);
+                var result = ReadDeferred<T>(gridIndex, deserializer.Func, typedIdentity, type);
                 return buffered ? result.ToList() : result;
             }
 
@@ -184,8 +184,16 @@ private T ReadRow<T>(Type type, Row row)
                         deserializer = new DeserializerState(hash, GetDeserializer(type, reader, 0, -1, false));
                         cache.Deserializer = deserializer;
                     }
-                    result = (T) deserializer.Func(reader);
-                    if ((row & Row.Single) != 0 && reader.Read()) ThrowMultipleRows(row);
                    while (reader.Read()) { }
+                    object val = deserializer.Func(reader);
+                    if(val == null || val is T)
+                    {
+                        result = (T)val;
+                    } else {
+                        var convertToType = Nullable.GetUnderlyingType(type) ?? type;
+                        result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
+                    }
+                    if ((row & Row.Single) != 0 && reader.Read()) ThrowMultipleRows(row);
+                    while (reader.Read()) { }
                 }
                 else if((row & Row.FirstOrDefault) == 0) // demanding a row, and don't have one
                 {
@@ -297,13 +305,19 @@ public IEnumerable<TReturn> Read<TReturn>(Type[] types, Func<object[], TReturn>
                 return buffered ? result.ToList() : result;
             }
 
-            private IEnumerable<T> ReadDeferred<T>(int index, Func<IDataReader, object> deserializer, Identity typedIdentity)
+            private IEnumerable<T> ReadDeferred<T>(int index, Func<IDataReader, object> deserializer, Identity typedIdentity, Type effectiveType)
             {
                 try
                 {
+                    var convertToType = Nullable.GetUnderlyingType(effectiveType) ?? effectiveType;
                     while (index == gridIndex && reader.Read())
                     {
-                        yield return (T)deserializer(reader);
+                        object val = deserializer(reader);
+                        if (val == null || val is T) {
+                            yield return (T)val;
+                        } else {
+                            yield return (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
+                        }
                     }
                 }
                 finally // finally so that First etc progresses things even when multiple rows