我的主要目标是创建一个动态组,并在NHibernate中使用它。
考虑这个有效的非动态示例:
_repository.Collection<User>().GroupBy(u => new { u.Active }).Select(s => s.Key, Count = s.Count())
现在,我创建一个动态对象来动态生成new { u.Active }
部分:
private Expression<Func<T, object>> CreateGrouping<T>(IEnumerable<string> by)
{
var dynamicTypeForGroup = GetDynamicTypeForGroup<T>(by);
var sourceItem = Expression.Parameter(typeof(T));
var bindings = dynamicTypeForGroup
.GetFields()
.Select(p => Expression.Bind(p, Expression.PropertyOrField(sourceItem, p.Name)))
.Cast<MemberBinding>()
.ToArray();
return Expression.Lambda<Func<T, object>>(Expression.Convert(
Expression.MemberInit(
Expression.New(dynamicTypeForGroup.GetConstructor(Type.EmptyTypes)),
bindings),
dynamicTypeForGroup),
sourceItem);
}
类型是在方法GetDynamicTypeForGroup
中生成的,然后用Expression.MemberInit(Expression.New(dynamicTypeForGroup.GetConstructor(Type.EmptyTypes)), bindings)
实例化
这是类型的生成方式:
private Type GetDynamicTypeForGroup<T>(IEnumerable<string> members)
{
var type = typeof(T);
var dynamicAssembly = AppDomain.CurrentDomain.DefineDynamicAssembly(
new AssemblyName(Guid.NewGuid().ToString()),
AssemblyBuilderAccess.RunAndSave
);
var dynamicModule = dynamicAssembly.DefineDynamicModule(Guid.NewGuid().ToString());
var typeBuilder = dynamicModule.DefineType(Guid.NewGuid().ToString());
var properties = members.Select(prop => type.GetProperty(ObjectExtensions.NormilizePropertyName(prop)))
.Where(prop => prop != null)
.Cast<MemberInfo>();
var fields = properties
.Select(property => typeBuilder.DefineField(
property.Name,
((PropertyInfo)property).PropertyType,
FieldAttributes.Public
)).Cast<FieldInfo>()
.ToArray();
GenerateEquals(typeBuilder, fields);
GenerateGetHashCode(typeBuilder, fields);
return typeBuilder.CreateType();
}
如果我使用_repository.Collection<User>().GroupBy(u => new { u.Active })
可以工作,但是如果我添加Select部分-.Select(s => s.Key, Count = s.Count())
(或任何选择语句),则会得到以下 NotSupportedException:MemberInit
System.NotSupportedException:MemberInit em NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.VisitExpression(表达式)(省略)
我的疑问是:
答案 0 :(得分:4)
显然,NHibernate LINQ查询翻译器在MemberInitExpression
选择器中不支持GroupBy
。
但是为什么匿名类型有效?因为尽管表达式new { Active = u.Active }
在语法上看起来像 MemberInitExpression
(类初始值设定项),但实际上不是!
C#编译器生成(并且NHibernate支持)的是通过参数NewExpression映射到类成员的带有参数的构造函数(Members)的调用-属性的第三个参数以下Expression.New重载:
public static NewExpression New (
ConstructorInfo constructor,
IEnumerable<Expression> arguments,
IEnumerable<MemberInfo> members
)
这是问题的解决方案。在动态类型生成器中,生成一个参数与字段匹配的构造函数(并在体内分配相应的字段):
var fields = properties
.Select(property => typeBuilder.DefineField(
property.Name,
((PropertyInfo)property).PropertyType,
FieldAttributes.Public
)).Cast<FieldInfo>()
.ToArray();
GenerateConstructor(typeBuilder, fields); // <--
GenerateEquals(typeBuilder, fields);
GenerateGetHashCode(typeBuilder, fields);
return typeBuilder.CreateType();
,然后使用类似这样的内容:
private Expression<Func<T, object>> CreateGrouping<T>(IEnumerable<string> by)
{
var keyType = GetDynamicTypeForGroup<T>(by);
var sourceItem = Expression.Parameter(typeof(T));
var members = keyType.GetFields();
var arguments = members.Select(m => Expression.PropertyOrField(sourceItem, m.Name));
var constructor = keyType.GetConstructor(members.Select(m => m.FieldType).ToArray());
var newKey = Expression.New(constructor, arguments, members);
return Expression.Lambda<Func<T, object>>(newKey, sourceItem);
}