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  • Entity SQL rules for Wrapped and Unwrapped Results

    Here are some rules to remember for Entity SQL queries:

    1.Use SELECT VALUE when projecting more than one type;

    2.When querying with SELECT,the ObjectQuery type must be a DbDataRecord ;

    3.You can use SELECT VALUE when projecting a single value or entity;

    4.When querying with SELECT VALUE,the ObjectQuery type must be the same type as the value being returned.

    用 Entity SQL 查询“包装”和“未包装” 结果的规则:

    下面是几条用Entity SQL 查询需要记住的规则:

    1、当投射的类型超过一个时,用 select value ;

    2、当用 select 查询时,ObjectQuery 结果集类型必须为DbDataRecord类型;

    3、当查询单个值或单个实体的时候,推荐使用 select value 查询;

    4、当用 select value 进行查询时,ObjectQuery 类型必须和返回值类型保持一致;

    Examples:

    1、Wrapped Contact needs to be cast:

    private static void WrappedContactInEntitySql()
            {
                using (var context = new PEF())
                {
                    var queryString =
                        "select c from PEF.Contacts as c where c.FirstName ='Robert' ";
    
                    var contacts = context.CreateQuery<DbDataRecord>(queryString);
    
                    foreach (DbDataRecord c in contacts)
                    {
                        var contact = (Contact)c[0];
    
                        Console.WriteLine("{0} {1} {2}",
                            contact.Title.Trim(),
                            contact.FirstName.Trim(),
                            contact.LastName.Trim());
                    }
    
                }
    
                Console.WriteLine("Press Enter...");
                Console.ReadLine();
    
            }

    2、Unwrapped Contact does not need to be cast:

    private static void UnwrappedContactInEntitySql()
            {
                using (var context = new PEF())
                {
                    var queryString =
                        "select value c from PEF.Contacts as c where c.FirstName = 'Robert' ";
    
                    var contacts = new ObjectQuery<Contact>(queryString, context);
    
                    foreach (var contact in contacts)
                    {
                        Console.WriteLine("{0} {1} {2}",
                            contact.Title.Trim(),
                            contact.FirstName.Trim(),
                            contact.LastName.Trim());
                    }
    
                }
    
                Console.WriteLine("Press Enter...");
                Console.ReadLine();
    
            }
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  • 原文地址:https://www.cnblogs.com/ARMdong/p/3516001.html
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