c# – SqlDataReader检查空值的最佳方法-sqlDataReader.IsDBNull对DBNull.Value

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");