Fix/1480 fix ReadFirstOrDefaultAsync throw exception

Author: zqlovejyc

Dapper/SqlMapper.GridReader.Async.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Data;
 using System.Data.Common;
+using System.Globalization;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -210,7 +211,20 @@ private async Task<T> ReadRowAsyncImplViaDbReader<T>(DbDataReader reader, Type t
                         deserializer = new DeserializerState(hash, GetDeserializer(type, reader, 0, -1, false));
                         cache.Deserializer = deserializer;
                     }
-                    result = (T)deserializer.Func(reader);
+
+                    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);
+                        }
+                    }
+
                     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

@@ -182,15 +182,18 @@ private T ReadRow<T>(Type type, Row row)
                         cache.Deserializer = deserializer;
                     }
                     object val = deserializer.Func(reader);
-                    if (val == null || val is T)
-                    {
-                        result = (T)val;
-                    }
-                    else
+
+                    if (val != null)
                     {
-                        var convertToType = Nullable.GetUnderlyingType(type) ?? type;
-                        result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
+                        if (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()) { /* ignore subsequent rows */ }
                 }
@@ -363,15 +366,19 @@ private IEnumerable<T> ReadDeferred<T>(int index, Func<IDataReader, object> dese
                     var convertToType = Nullable.GetUnderlyingType(effectiveType) ?? effectiveType;
                     while (index == gridIndex && reader.Read())
                     {
+                        T result = default;
                         object val = deserializer(reader);
-                        if (val == null || val is T)
+                        if (val != null)
                         {
-                            yield return (T)val;
-                        }
-                        else
-                        {
-                            yield return (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
+                            if (val is T)
+                                result = (T)val;
+                            else
+                            {
+                                result = (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
+                            }
                         }
+
+                        yield return result;
                     }
                 }
                 finally // finally so that First etc progresses things even when multiple rows