我想从数据库检索十进制值,我想知道哪个是建议的方式来检查空值.
我在MSDN – DBNull.Value Field 年看到这个支票很少使用.
因此,是读者.IsDB是否是检查null的最佳/最有效的方法？
我已经创建了2个样本方法：
public static decimal? GetNullableDecimal(sqlDataReader reader,string fieldName) { if (reader[fieldName] != DBNull.Value) { return (decimal)reader[fieldName]; } return null; } public static decimal? GetNullableDecimal_2(sqlDataReader reader,string fieldName) { if (!reader.IsDBNull(reader[fieldName])) { return (decimal)reader[fieldName]; } return null; }
大多数情况下,这些字段将为空.
提前致谢！
解决方法
我不会太高兴在哪个方法是更好的,因为这两个工作和我以前都使用过代码.
例如,这是一个从我的一个旧项目挖掘出来的效用函数：
/// <summary> /// Helper class for sqlDataReader,which allows for the calling code to retrieve a value in a generic fashion. /// </summary> public static class sqlReaderHelper { private static bool IsNullableType(Type theValueType) { return (theValueType.IsGenericType && theValueType.GetGenericTypeDefinition().Equals(typeof(Nullable<>))); } /// <summary> /// Returns the value,of type T,from the sqlDataReader,accounting for both generic and non-generic types. /// </summary> /// <typeparam name="T">T,type applied</typeparam> /// <param name="theReader">The sqlDataReader object that queried the database</param> /// <param name="theColumnName">The column of data to retrieve a value from</param> /// <returns>T,type applied; default value of type if database value is null</returns> public static T GetValue<T>(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 (i.e. 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 (i.e. int has a default of 0) return default(T); } }
用法：
yoursqlReaderObject.GetValue<int?>("SOME_ID_COLUMN"); yoursqlReaderObject.GetValue<string>("SOME_VALUE_COLUMN");
