哦,这看起来是个有趣的问题:)
所以首先,让我们设置我们的人造源,因为我手边没有你的数据库:
// SETUP: fake up a data source
var folks = new[]{"Alex", "James", "Jessica"};
var cats = new[]{"C#", "VB.NET", "LINQ"};
var r = new Random();
var entryCount = 100;
var entries =
from i in Enumerable.Range(0, entryCount)
let id = r.Next(0, 999999)
let person = folks[r.Next(0, folks.Length)]
let category = cats[r.Next(0, cats.Length)]
let date = DateTime.Now.AddDays(r.Next(0, 100) - 50)
select new Journal() {
Id = id,
AuthorName = person,
Category = category,
CreatedAt = date };
好的,现在我们有了一组数据可以使用,让我们看看我们想要什么......我们想要一些“形状”的东西,比如:
public Expression<Func<Journal, ????>> GetThingToGroupByWith(
string[] someMagicStringNames,
????)
这与(在伪代码中)具有大致相同的功能:
GroupBy(x => new { x.magicStringNames })
让我们一次一个地剖析它。首先,我们到底是如何动态地做到这一点的?
x => new { ... }
编译器通常为我们做了魔法——它所做的是定义一个 new Type
,我们也可以这样做:
var sourceType = typeof(Journal);
// define a dynamic type (read: anonymous type) for our needs
var dynAsm = AppDomain
.CurrentDomain
.DefineDynamicAssembly(
new AssemblyName(Guid.NewGuid().ToString()),
AssemblyBuilderAccess.Run);
var dynMod = dynAsm
.DefineDynamicModule(Guid.NewGuid().ToString());
var typeBuilder = dynMod
.DefineType(Guid.NewGuid().ToString());
var properties = groupByNames
.Select(name => sourceType.GetProperty(name))
.Cast<MemberInfo>();
var fields = groupByNames
.Select(name => sourceType.GetField(name))
.Cast<MemberInfo>();
var propFields = properties
.Concat(fields)
.Where(pf => pf != null);
foreach (var propField in propFields)
{
typeBuilder.DefineField(
propField.Name,
propField.MemberType == MemberTypes.Field
? (propField as FieldInfo).FieldType
: (propField as PropertyInfo).PropertyType,
FieldAttributes.Public);
}
var dynamicType = typeBuilder.CreateType();
所以我们在这里所做的是定义一个自定义的一次性类型,它为我们传入的每个名称都有一个字段,它与源类型上的(属性或字段)类型相同。好的!
现在我们如何给 LINQ 它想要的东西?
首先,让我们为我们将返回的函数设置一个“输入”:
// Create and return an expression that maps T => dynamic type
var sourceItem = Expression.Parameter(sourceType, "item");
我们知道我们需要“新建”一种新的动态类型......
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes))
我们需要使用来自该参数的值来初始化它......
Expression.MemberInit(
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
bindings),
但是我们到底要用来做什么bindings
呢?嗯...好吧,我们想要一些绑定到源类型中相应属性/字段的东西,但将它们重新映射到我们的dynamicType
字段...
var bindings = dynamicType
.GetFields()
.Select(p =>
Expression.Bind(
p,
Expression.PropertyOrField(
sourceItem,
p.Name)))
.OfType<MemberBinding>()
.ToArray();
哎呀......看起来很讨厌,但我们还没有完成 - 所以我们需要为Func
我们通过表达式树创建的声明一个返回类型......如果有疑问,请使用object
!
Expression.Convert( expr, typeof(object))
最后,我们将它绑定到我们的“输入参数” via Lambda
,形成整个堆栈:
// Create and return an expression that maps T => dynamic type
var sourceItem = Expression.Parameter(sourceType, "item");
var bindings = dynamicType
.GetFields()
.Select(p => Expression.Bind(p, Expression.PropertyOrField(sourceItem, p.Name)))
.OfType<MemberBinding>()
.ToArray();
var fetcher = Expression.Lambda<Func<T, object>>(
Expression.Convert(
Expression.MemberInit(
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
bindings),
typeof(object)),
sourceItem);
为了便于使用,让我们将整个混乱包装为一个扩展方法,所以现在我们得到了:
public static class Ext
{
// Science Fact: the "Grouper" (as in the Fish) is classified as:
// Perciformes Serranidae Epinephelinae
public static Expression<Func<T, object>> Epinephelinae<T>(
this IEnumerable<T> source,
string [] groupByNames)
{
var sourceType = typeof(T);
// define a dynamic type (read: anonymous type) for our needs
var dynAsm = AppDomain
.CurrentDomain
.DefineDynamicAssembly(
new AssemblyName(Guid.NewGuid().ToString()),
AssemblyBuilderAccess.Run);
var dynMod = dynAsm
.DefineDynamicModule(Guid.NewGuid().ToString());
var typeBuilder = dynMod
.DefineType(Guid.NewGuid().ToString());
var properties = groupByNames
.Select(name => sourceType.GetProperty(name))
.Cast<MemberInfo>();
var fields = groupByNames
.Select(name => sourceType.GetField(name))
.Cast<MemberInfo>();
var propFields = properties
.Concat(fields)
.Where(pf => pf != null);
foreach (var propField in propFields)
{
typeBuilder.DefineField(
propField.Name,
propField.MemberType == MemberTypes.Field
? (propField as FieldInfo).FieldType
: (propField as PropertyInfo).PropertyType,
FieldAttributes.Public);
}
var dynamicType = typeBuilder.CreateType();
// Create and return an expression that maps T => dynamic type
var sourceItem = Expression.Parameter(sourceType, "item");
var bindings = dynamicType
.GetFields()
.Select(p => Expression.Bind(
p,
Expression.PropertyOrField(sourceItem, p.Name)))
.OfType<MemberBinding>()
.ToArray();
var fetcher = Expression.Lambda<Func<T, object>>(
Expression.Convert(
Expression.MemberInit(
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)),
bindings),
typeof(object)),
sourceItem);
return fetcher;
}
}
现在,要使用它:
// What you had originally (hand-tooled query)
var db = entries.AsQueryable();
var query = db.GroupBy(x => new
{
Year = x.CreatedAt.Year,
Month = x.CreatedAt.Month
}, prj => prj.AuthorName)
.Select(data => new {
Key = data.Key.Year * 100 + data.Key.Month, // very ugly code, I know
Details = data.GroupBy(y => y).Select(z => new { z.Key, Count = z.Count() })
});
var func = db.Epinephelinae(new[]{"CreatedAt", "AuthorName"});
var dquery = db.GroupBy(func, prj => prj.AuthorName);
该解决方案缺乏“嵌套语句”的灵活性,例如“CreatedDate.Month”,但只要有一点想象力,您就可以扩展这个想法以处理任何自由形式的查询。