Fix DataRowExtensions.Field for nullable types

roji · roji · commit ea9be74f37ee · 2019-11-04T17:56:01.000+01:00
Fixes dotnet/corefx#41879 Commit migrated from dotnet/corefx@f664740
diff --git a/src/libraries/System.Data.Common/src/System/Data/DataRowExtensions.cs b/src/libraries/System.Data.Common/src/System/Data/DataRowExtensions.cs
@@ -147,25 +147,30 @@ private static class UnboxT<T>
 
             private static Converter<object, T> Create()
             {
-                if (default(T) == null)
-                    return ReferenceOrNullableField;
-                else
-                    return ValueField;
-            }
+                if (typeof(T).IsValueType)
+                {
+                    return typeof(T).IsGenericType && typeof(T).GetGenericTypeDefinition() == typeof(Nullable<>)
+                        ? (Converter<object, T>)Delegate.CreateDelegate(
+                            typeof(Converter<object, T>),
+                                typeof(UnboxT<T>)
+                                    .GetMethod("NullableField", System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic)
+                                    .MakeGenericMethod(typeof(T).GetGenericArguments()[0]))
+                        : ValueField;
+                }
 
-            private static T ReferenceOrNullableField(object value)
-            {
-                return ((DBNull.Value == value) ? default(T) : (T)value);
+                return ReferenceField;
             }
 
+            private static T ReferenceField(object value)
+                => value == DBNull.Value ? default : (T)value;
+
             private static T ValueField(object value)
-            {
-                if (DBNull.Value == value)
-                {
-                    throw DataSetUtil.InvalidCast(SR.Format(SR.DataSetLinq_NonNullableCast, typeof(T)));
-                }
-                return (T)value;
-            }
+                => value == DBNull.Value
+                    ? throw DataSetUtil.InvalidCast(SR.Format(SR.DataSetLinq_NonNullableCast, typeof(T)))
+                    : (T)value;
+
+            private static Nullable<TElem> NullableField<TElem>(object value) where TElem : struct
+                => value == DBNull.Value ? default : new Nullable<TElem>((TElem)value);
         }
     }
 }
diff --git a/src/libraries/System.Data.DataSetExtensions/tests/System/Data/DataRowExtensionsTests.cs b/src/libraries/System.Data.DataSetExtensions/tests/System/Data/DataRowExtensionsTests.cs
@@ -94,6 +94,23 @@ public void Field_ColumnVersion_NullColumnThrows()
             AssertExtensions.Throws<ArgumentNullException>("column", () => DataRowExtensions.Field<int>(row, column: null, version: DataRowVersion.Default));
         }
 
+        [Fact]
+        public void Field_Nullable_Enum()
+        {
+            DataTable table = new DataTable("test");
+            table.Columns.Add(new DataColumn("col", typeof(int)));
+            DataRow row = table.NewRow();
+            row["col"] = 0;
+            table.Rows.Add(row);
+
+            Assert.Equal(SomeEnum.Foo, table.Rows[0].Field<SomeEnum?>("col"));
+        }
+
+        enum SomeEnum
+        {
+            Foo
+        }
+
         [Fact]
         public void SetField_IndexValue_NullRowThrows()
         {
@@ -170,7 +187,5 @@ public void SetField_ColumnValue_NullValueReplacedByDBNull()
             DataRowExtensions.SetField<string>(row, column: column, value: null);
             Assert.Equal(DBNull.Value, row[column]);
         }
-
-
     }
 }
