我有这些简单的界面:
public interface IQuery<TResult> { }
public interface IQueryHandler<in TQuery, out TResult>
where TQuery : IQuery<TResult> {
TResult Handle(TQuery query);
}
并且有一些它们的实现。我正在尝试创建一个表达式树来调用 Handle
指定处理程序上的方法。我的意思是:
var query = new MyQuery(); // which MyQuery implements IQuery<int>
object handler = _someServiceProvider
.Get<IQueryHandler<MyQuery, int>>();
此外,还有一个MyQueryHandler
:
public class MyQueryHandler : IQueryHandler<MyQuery, int> {
public int Handle(MyQuery query) { return 20; }
}
现在,我正在尝试创建一个 Func<object, MyQuery, int>
像这样调用:
var func = GetMethod<MyQuery, int>(handler);
var result = func(handler, query);
这是我的GetMethod
实现:
private Func<object, TQuery, TResult> GetMethod<TQuery, TResult>(object obj)
where TQuery : IQuery<TResult> {
var methodInfo = obj.GetType().GetMethod(nameof(IQueryHandler<TQuery, TResult>.Handle));
var insExp = Expression.Parameter(typeof(object), "ins");
var inputExp = Expression.Parameter(typeof(TQuery), "query");
var instanceExp = Expression.Variable(obj.GetType(), "instance");
var assExp = Expression.Assign(instanceExp, Expression.Convert(insExp, obj.GetType()));
var castExp = Expression.Convert(inputExp, methodInfo.GetParameters()[0].ParameterType);
var callExp = Expression.Call(instanceExp, methodInfo, castExp);
var blockExp = Expression.Block(new Expression[] {
insExp,
inputExp,
instanceExp,
assExp,
castExp,
callExp
});
var func =
Expression.Lambda<Func<object, TQuery, TResult>>(
blockExp,
insExp,
inputExp).Compile();
return func;
}
但是,当我尝试编译 Lambda
时,我收到此错误:
An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code
Additional information: variable 'instance' of type 'Namespace.MyQueryHandler' referenced from scope '', but it is not defined
我哪里错了?我错过了什么?你有什么主意吗?提前致谢。
最佳答案
据我所知,您正在尝试编写此函数:
TResult f(object ins, TQuery query)
{
var instance = (MyQueryHandler)ins;
return instance.Handle(query);
}
要使用表达式树执行此操作,您必须在 Expression.Block
中声明变量,但随后仅指定上面的两个语句,而不是所有子表达式:
var blockExp = Expression.Block(new[] { instanceExp }, new Expression[] {
assExp,
callExp
});
但更简单的选择是编写以下函数:
TResult f(object ins, TQuery query)
{
return ((MyQueryHandler)ins).Handle(query);
}
看起来像这样:
var callExp = Expression.Call(
Expression.Convert(insExp, obj.GetType()), methodInfo, castExp);
var func =
Expression.Lambda<Func<object, TQuery, TResult>>(
callExp,
insExp,
inputExp).Compile();
关于c# - 创建 lambda 表达式以调用泛型类中的方法,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/44098384/