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

Question

我想从数据库中检索十进制值，我想知道哪种方法是检查空值的推荐方法。

我在MSDN - DBNull.Value Field 上看到很少使用此检查。

因此，reader.IsDBNull 是检查空值的最佳/最有效方法吗？

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，这些字段将为空。

提前致谢！

Answer 1

我不会太纠结于哪种方法更好，因为这两种方法都有效，而且我之前在代码中都使用过。

例如，这是我从一个旧项目中挖掘出来的一个实用函数：

/// <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);
    }
}

用法：

yourSqlReaderObject.GetValue<int?>("SOME_ID_COLUMN");
yourSqlReaderObject.GetValue<string>("SOME_VALUE_COLUMN");

Answer 2

如果您想检查 null 并处理它（而不是检查 null 并提醒程序它为 null），您可以使用 as 运算符和 null-coalescing 运算符 ??。 所以在我的程序中

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 答案的更简单版本：

public static class DbHelper
{
    public static T GetValue<T>(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<T>(this SqlDataReader sqlDataReader, string columnName)
    {
        var value = sqlDataReader[columnName];

        return value == DBNull.Value ? default(T) : (T) value;
    }
}

对于可空或不可空类型，直接强制转换似乎都可以正常工作。