问题描述
我正在尝试创建任意类型的运行时lambda函数,该函数在对象列表中收集提供给它的参数,并将它们传递给void Method(List<object> list)
类型的另一种方法来处理它们。我写了这段代码,但对结果感到很困惑:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
namespace LambdaTest
{
class LambdaCreator
{
ParameterExpression[] Parameters;
int Index = 0;
public ParameterExpression Next()
{
return Parameters[Index++];
}
public void ResetIndex()
{
Index = 0;
}
public void Bar(List<object> parameters)
{
foreach (var p in parameters)
{
PrintType(p);
}
}
public void PrintType(object arg)
{
Console.WriteLine(arg.GetType().Name);
}
public T CreateLambda<T>() where T : class
{
var barMethod = GetType().GetMethod("Bar");
Parameters = typeof(T).GetMethod("Invoke")
.GetParameters()
.Select(x => Expression.Parameter(x.ParameterType))
.ToArray();
var parametersCount = Expression.Constant(Parameters.Length);
var listType = typeof(List<object>);
var list = Expression.Variable(listType);
var index = Expression.Variable(typeof(int));
var thisObject = Expression.Constant(this);
var resetIndex = GetType().GetMethod("ResetIndex");
var next = GetType().GetMethod("Next");
var printType = GetType().GetMethod("PrintType");
var add = listType.GetMethod("Add");
var breakLabel = Expression.Label();
var block = Expression.Block(
new ParameterExpression[] { list,index },Expression.Call(thisObject,printType,Parameters.FirstOrDefault()),resetIndex),Expression.Assign(list,Expression.New(listType)),Expression.Loop(
Expression.Block(
Expression.IfThen(Expression.GreaterThanOrEqual(index,parametersCount),Expression.Break(breakLabel)),Expression.Call(list,add,next)),Expression.AddAssign(index,Expression.Constant(1))
),breakLabel
),barMethod,list)
);
var lambda = Expression.Lambda(typeof(T),block,Parameters);
var compiledLambda = lambda.Compile() as T;
return compiledLambda;
}
}
class Program
{
delegate void Foo(string a,int b);
static void Main(string[] args)
{
var test = new LambdaCreator();
var l = test.CreateLambda<Foo>();
l("one",2);
}
}
}
程序的输出为:
String
PrimitiveParameterExpression`1
PrimitiveParameterExpression`1
我期望得到:
String
String
Int32
以某种方式,当我将参数的值放入列表并将其传递给Bar
方法时,它们会丢失。
有人可以告诉我问题出在哪里,我该如何解决。还是有另一种收集参数并将其传递的方法?我真的是这个表达树的新手。预先感谢!
解决方法
您可以在构建NewArrayExpression
块之前,在构造lambda函数时使用Parameters
数组创建Expression.Loop
,并修改调用代码以访问该数组,如下所示: / p>
// Declare a paramArray parameter to use inside the Expression.Block
var paramArray = Expression.Parameter(typeof(object[]),"paramArray");
var block = Expression.Block(
new ParameterExpression[] { list,index,paramArray },// pass in paramArray here
Expression.Call(thisObject,printType,Parameters.FirstOrDefault()),Expression.Call(thisObject,resetIndex),Expression.Assign(list,Expression.New(listType)),/* Assign the array - make sure to box value types using Expression.Convert */
Expression.Assign(
paramArray,Expression.NewArrayInit(
typeof(object),Parameters.Select(p => Expression.Convert(p,typeof(object))))),Expression.Loop(
Expression.Block(
Expression.IfThen(Expression.GreaterThanOrEqual(index,parametersCount),Expression.Break(breakLabel)),//Expression.Call(list,add,next)),Expression.Call(list,Expression.ArrayIndex(paramArray,index)),// use the paramArray here
Expression.AddAssign(index,Expression.Constant(1))
),breakLabel
),barMethod,list)
);
其余部分未更改-此代码完全替换了var block = ...
语句。按您指定的方式工作。
发生这种情况是因为此调用:
Expression.Call(thisObject,Parameters.FirstOrDefault())
实际上已编译为类似的内容:
this.PrintType(a)
a
是您的委托参数,而这是
Expression.Call(list,next))
已编译为类似内容:
this.PrintType(this.Next())
其中一种选择是修改打印方法:
public void PrintType(object arg)
{
if(arg is ParameterExpression expr)
{
Console.WriteLine(expr.Type.Name);
}
else
{
Console.WriteLine(arg.GetType().Name);
}
}
要填充列表,您只需创建一个相应的表达式即可:
var list = Expression.Variable(listType);
var exprs = new List<Expression>
{
Expression.Call(thisObject,};
for (int i = 0; i < @params.Length; i++)
{
var ex = Expression.Call(list,Expression.Convert(@params[i],typeof(object)));
exprs.Add(ex);
}
exprs.Add(Expression.Call(thisObject,list));
var block = Expression.Block(new[] {list},exprs);
或者使用var property = Expression.PropertyOrField(thisObject,nameof(Parameters));
(将Parameters
更改为List<object>
,为其分配新列表并删除块参数)来代替list
。