11-12. 定义内置函数
问题
想要定义一个在eSQL 和LINQ 查询里使用的内置函数.
解决方案
我们要在数据库中使用IsNull 函数,但是EF没有为eSQL 或LINQ发布这个函数. 假设我们已有一个WebProduct实体模型,如Figure 11-12所示.
Figure 11-12. A WebProduct entity in our model
为我们的查询发布函数:
1. 在解决方案资源管理器里右击.edmx 文件, 打开方式 ➤ XML 编辑器.
2.在.edmx 文件的存储模型(storage models)节里<Schema>标签下,插入如Listing 11-19所示 的代码. 这样我们就在存储层定义好了函数.
Listing 11-19. Defining Our Function in the Storage Layer
<Function Name="ISNULL" ReturnType="varchar" BuiltIn="true" Schema="dbo">
<Parameter Name="expr1" Type="varchar" Mode="In" />
<Parameter Name="expr2" Type="varchar" Mode="In" />
</Function>
3.插入和查询模型的代码如下列的Listing 11-19所示.
Listing 11-20. Using the ISNULL() Function in an eSQL and LINQ Query
class Program
{
static void Main(string[] args)
{
RunExample();
}
static void RunExample()
{
using (var context = new EFRecipesEntities())
{
context.Database.ExecuteSqlCommand("delete from chapter11.webproduct");
var w1 = new WebProduct
{
Name = "Camping Tent",
Description = "Family Camping Tent, Color Green"
};
var w2 = new WebProduct { Name = "Chemical Light" };
var w3 = new WebProduct
{
Name = "Ground Cover",
Description = "Blue ground cover"
};
context.WebProducts.Add(w1);
context.WebProducts.Add(w2);
context.WebProducts.Add(w3);
context.SaveChanges();
}
using (var context = new EFRecipesEntities())
{
Console.WriteLine("Query using eSQL...");
var esql = @"select value
EFRecipesModel.Store.ISNULL(p.Description,p.Name)
from EFRecipesEntities.WebProducts as p";
var objectContext = (context as IObjectContextAdapter).ObjectContext;
var prods = objectContext.CreateQuery<string>(esql);
foreach (var prod in prods)
{
Console.WriteLine("Product Description: {0}", prod);
}
}
using (var context = new EFRecipesEntities())
{
Console.WriteLine();
Console.WriteLine("Query using LINQ...");
var prods = from p in context.WebProducts
select BuiltinFunctions.ISNULL(p.Description, p.Name);
foreach (var prod in prods)
{
Console.WriteLine(prod);
}
}
}
}
public class BuiltinFunctions
{
[EdmFunction("EFRecipesModel.Store", "ISNULL")]
public static string ISNULL(string check_expression, string replacementvalue)
{
throw new NotSupportedException("Direct calls are not supported.");
}
}
上述Listing 11-20 代码输出结果如下:
Query using eSQL...
Product Description: Family Camping Tent, Color Green
Product Description: Chemical Light
Product Description: Blue ground cover
Query using LINQ...
Family Camping Tent, Color Green
Chemical Light
Blue ground cover
它是如何工作的?
如Listing 11-18所示 的 ISNULL() 函数的定义,函数名必须与数据库里的函数名一致(大小写不必一致), 这次不像本章前面小节的函数在概念层定义, 是在数据存储层. 这个函数原本在数据库里就是可用的,我们只是简单的在存储层定义一下.当在eSQL 语句中使用该函数时,必须使用完整的命名空间.( EFRecipesModel.Store.ISNULL()).当在LINQ查询中使用该函数时,需创建引导方法,由于不返回IQueryable<T>类型,所以不必实现方法体.