SqlDataReader检查空值的最佳方法-sqlDataReader.IsDBNull vs DBNull.Value

我想从数据库中检索十进制值，我想知道检查空值的推荐方法。

我在MSDN上看到过- DBNull.Value字段很少使用此检查。

因此，是reader.IsDBNull是否是检查空值的最佳/最有效方法？

我创建了2个示例方法：

 public static decimal? GetNullableDecimal(SqlDataReader reader, string fieldName) { if (reader[fieldName] == DBNull.Value) { return null; } return (decimal)reader[fieldName]; } public static decimal? GetNullableDecimal_2(SqlDataReader reader, string fieldName) { if (reader.IsDBNull(reader[fieldName])) { return null; } return (decimal)reader[fieldName]; } 

大多数情况下，字段将为空。

提前致谢！

Answer 1

我不会太了解哪种方法更好，因为我和之前的代码都使用过。

例如，这是我从我的一个旧项目挖出的实用函数：

 /// 
 /// Helper class for SqlDataReader, which allows for the calling code to retrieve a value in a generic fashion. /// 
 public static class SqlReaderHelper { private static bool IsNullableType(Type theValueType) { return (theValueType.IsGenericType && theValueType.GetGenericTypeDefinition().Equals(typeof(Nullable<>))); } /// 
 /// Returns the value, of type T, from the SqlDataReader, accounting for both generic and non-generic types. /// 
 /// T, type applied /// The SqlDataReader object that queried the database /// The column of data to retrieve a value from /// T, type applied; default value of type if database value is null public static T GetValue(this SqlDataReader theReader, string theColumnName) { // Read the value out of the reader by string (column name); returns object object theValue = theReader[theColumnName]; // Cast to the generic type applied to this method (ie int?) Type theValueType = typeof(T); // Check for null value from the database if (DBNull.Value != theValue) { // We have a null, do we have a nullable type for T? if (!IsNullableType(theValueType)) { // No, this is not a nullable type so just change the value's type from object to T return (T)Convert.ChangeType(theValue, theValueType); } else { // Yes, this is a nullable type so change the value's type from object to the underlying type of T NullableConverter theNullableConverter = new NullableConverter(theValueType); return (T)Convert.ChangeType(theValue, theNullableConverter.UnderlyingType); } } // The value was null in the database, so return the default value for T; this will vary based on what T is (ie int has a default of 0) return default(T); } } 

用法：

 yourSqlReaderObject.GetValue("SOME_ID_COLUMN"); yourSqlReaderObject.GetValue("SOME_VALUE_COLUMN"); 

Answer 2

如果你想检查null并处理它（而不是检查null并警告程序它是null）你可以使用as运算符和null-coalescing运算符?? 。 所以在我的计划中

 SqlDataReader dr = cmd.ExecuteReader(); while (dr.Read()) { response.Employees.Add(new Employee() { Id = dr["id"] as int? ?? default(int), ImageUrl = dr["Photo"] as string, JobTitle = dr["JobTitle"] as string }); } 

Answer 3

这是@Karl Anderson答案的简单版本：

 public static class DbHelper { public static T GetValue(this SqlDataReader sqlDataReader, string columnName) { var value = sqlDataReader[columnName]; if (value != DBNull.Value) { return (T)value; } return default(T); } } 

甚至：

 public static class DbHelper { public static T GetValue(this SqlDataReader sqlDataReader, string columnName) { var value = sqlDataReader[columnName]; return value == DBNull.Value ? default(T) : (T) value; } } 

对于可空或非可空类型，直接投射似乎都可以正常工作。