我想从数据库中检索十进制值,并且想知道检查空值的推荐方法。
我见过MSDN - DBNull.Value 字段 http://msdn.microsoft.com/en-us/library/system.dbnull.value.aspx该检查很少使用。
因此,是reader.IsDBNull
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];
}
大多数时候这些字段将为空。
提前致谢!
我不会太纠结于哪种方法更好,因为这两种方法都有效,而且我以前在代码中使用过这两种方法。
例如,这是我从我的一个旧项目中挖掘出来的实用函数:
/// <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);
}
}
Usage:
yourSqlReaderObject.GetValue<int?>("SOME_ID_COLUMN");
yourSqlReaderObject.GetValue<string>("SOME_VALUE_COLUMN");
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