Transfer duplicate code and encapsulate it as a static method

Author: zqlovejyc

Dapper/SqlMapper.GridReader.Async.cs

@@ -179,7 +179,7 @@ private Task<IEnumerable<T>> ReadAsyncImpl<T>(Type type, bool buffered)
                 else
                 {
                     var result = ReadDeferred<T>(gridIndex, deserializer.Func, type);
-                    if (buffered) result = result.ToList(); // for the "not a DbDataReader" scenario
+                    if (buffered) result = result?.ToList(); // for the "not a DbDataReader" scenario
                     return Task.FromResult(result);
                 }
             }
@@ -212,18 +212,7 @@ private async Task<T> ReadRowAsyncImplViaDbReader<T>(DbDataReader reader, Type t
                         cache.Deserializer = deserializer;
                     }
 
-                    object val = deserializer.Func(reader);
-
-                    if (val != null)
-                    {
-                        if (val is T)
-                            result = (T)val;
-                        else
-                        {
-                            var convertToType = Nullable.GetUnderlyingType(type) ?? type;
-                            result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
-                        }
-                    }
+                    result = ConvertTo<T>(deserializer.Func(reader));
 
                     if ((row & Row.Single) != 0 && await reader.ReadAsync(cancel).ConfigureAwait(false)) ThrowMultipleRows(row);
                     while (await reader.ReadAsync(cancel).ConfigureAwait(false)) { /* ignore subsequent rows */ }

Dapper/SqlMapper.GridReader.cs

@@ -159,7 +159,7 @@ private IEnumerable<T> ReadImpl<T>(Type type, bool buffered)
                 }
                 IsConsumed = true;
                 var result = ReadDeferred<T>(gridIndex, deserializer.Func, type);
-                return buffered ? result.ToList() : result;
+                return buffered ? result?.ToList() : result;
             }
 
             private T ReadRow<T>(Type type, Row row)
@@ -181,18 +181,8 @@ private T ReadRow<T>(Type type, Row row)
                         deserializer = new DeserializerState(hash, GetDeserializer(type, reader, 0, -1, false));
                         cache.Deserializer = deserializer;
                     }
-                    object val = deserializer.Func(reader);
 
-                    if (val != null)
-                    {
-                        if (val is T)
-                            result = (T)val;
-                        else
-                        {
-                            var convertToType = Nullable.GetUnderlyingType(type) ?? type;
-                            result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
-                        }
-                    }
+                    result = ConvertTo<T>(deserializer.Func(reader));
 
                     if ((row & Row.Single) != 0 && reader.Read()) ThrowMultipleRows(row);
                     while (reader.Read()) { /* ignore subsequent rows */ }
@@ -363,22 +353,9 @@ private IEnumerable<T> ReadDeferred<T>(int index, Func<IDataReader, object> dese
             {
                 try
                 {
-                    var convertToType = Nullable.GetUnderlyingType(effectiveType) ?? effectiveType;
                     while (index == gridIndex && reader.Read())
                     {
-                        T result = default;
-                        object val = deserializer(reader);
-                        if (val != null)
-                        {
-                            if (val is T)
-                                result = (T)val;
-                            else
-                            {
-                                result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
-                            }
-                        }
-
-                        yield return result;
+                        yield return ConvertTo<T>(deserializer(reader));
                     }
                 }
                 finally // finally so that First etc progresses things even when multiple rows
@@ -440,6 +417,16 @@ public void Dispose()
                 }
                 GC.SuppressFinalize(this);
             }
+
+            private static T ConvertTo<T>(object value)
+            {
+                if (value is null or DBNull)
+                    return default;
+                else if (value is T t)
+                    return t;
+                else
+                    return (T)Convert.ChangeType(value, Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T), CultureInfo.InvariantCulture);
+            }
         }
     }
 }