问题描述
本质上我有一个表格，其中包含邮政编码。邮政编码字段定义为char（5）。我正在使用代码，所以我把这些属性放在我的ZipCode属性上：
Essentially I have a table with zip codes in it. The zipcode field is defined as 'char(5)'. I'm using code first, so I've put these attributes on my ZipCode property:
[Key, Column( Order = 0, TypeName = "nchar"), StringLength(5)]
public string ZipCode { get; set; }
现在，如果我在EF中查询这个：
Now if I query against this in EF:
var zc = db.ZipCodes.FirstOrDefault(zip => zip.ZipCode == "12345");
生成的SQL使用nvarchar（4000）注入参数。咦？是因为12345在技术上是长度不明的字符串？请问EF是否足够聪明才能在查询该表时使用正确的nchar（5）？
The generated SQL uses nvarchar(4000) to inject the parameters. Huh? Is it because "12345" is technically a string of unknown length? Shouldn't EF be smart enough to just use the proper "nchar(5)" when querying that table?
我问，因为nvarchar（4000）查询需要一半第二，而正确范围的查询要快得多（读取更少）。
I ask because the nvarchar(4000) query takes half a second whereas the properly-scoped query is much faster (and less reads).
任何帮助/建议将不胜感激。
Any assistance/advice would be appreciated.
推荐答案
这是利用自动参数化。以下文章解释了一般概念以及为什么特别使用nvarchar（4000）。
This is to take advantage of auto parameterization. The following article explains the general concept as well as why specifically nvarchar(4000) is used.
这篇关于为什么代码第一/ EF在原始SQL命令中的字符串使用'nvarchar（4000）'？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！