我有两个不相关的课程。一个是作为API公开的,另一个是由第三方API在内部使用。
实体从我们的API公开,而EntityProvider来自第三方组装。
class Entity
{
public A { get; set; }
}
class EntityProvider
{
public A { get; set; }
}
我们API的消费者将提供Expression <Func<Entity, bool>>
形式的谓词,我需要将其修改为Expression <Func<EntityProvider, bool>>
,以便我可以将其传递给内部第三方程序集。
请帮助进行此转换。
答案 0 :(得分:3)
由于.NET中的表达式是不可变的,因此唯一的方法是重建整个表达式。要做到这一点,通常需要继承ExpressionVisitor类。根据您必须转换的表达式的复杂性,这可能非常复杂。
这是一个使用简单表达式的访问者的简单示例(例如x =&gt; x.Someproperty == somevalue)。这只是一个让你入门的例子,它没有完成或测试(例如它不会处理表达式中的方法调用)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
//Type from which to convert
public class A
{
public int Property1 { get; set; }
public int Property2 { get; set; }
}
//Type to which we want the Expression converted
public class B
{
public int Property1 { get; set; }
public int Property2 { get; set; }
}
class Program
{
static void Main(string[] args)
{
//the expression we want to convert expresion
Expression<Func<A, bool>> expA = x => x.Property1 == 6 && x.Property2 == 3;
var visitor = new ParameterTypeVisitor<A,B>(expA);
var expB = visitor.Convert();
var b = new B() { Property1 = 6, Property2 = 3 };
//try the converted expression
var result = expB.Compile().Invoke(b);
}
}
public class ParameterTypeVisitor<TFrom,TTo> : ExpressionVisitor
{
private Dictionary<string, ParameterExpression> convertedParameters;
private Expression<Func<TFrom, bool>> expression;
public ParameterTypeVisitor(Expression<Func<TFrom,bool>> expresionToConvert )
{
//for each parameter in the original expression creates a new parameter with the same name but with changed type
convertedParameters = expresionToConvert.Parameters
.ToDictionary(
x => x.Name,
x => Expression.Parameter(typeof (TTo), x.Name)
);
expression = expresionToConvert;
}
public Expression<Func<TTo,bool>> Convert()
{
return (Expression<Func<TTo, bool>>)Visit(expression);
}
//handles Properties and Fields accessors
protected override Expression VisitMember(MemberExpression node)
{
//we want to replace only the nodes of type TFrom
//so we can handle expressions of the form x=> x.Property.SubProperty
//in the expression x=> x.Property1 == 6 && x.Property2 == 3
//this replaces ^^^^^^^^^^^ ^^^^^^^^^^^
if (node.Member.DeclaringType == typeof(TFrom))
{
//gets the memberinfo from type TTo that matches the member of type TFrom
var memeberInfo = typeof (TTo).GetMember(node.Member.Name).First();
//this will actually call the VisitParameter method in this class
var newExp = Visit(node.Expression);
return Expression.MakeMemberAccess(newExp, memeberInfo);
}
else
{
return base.VisitMember(node);
}
}
// this will be called where ever we have a reference to a parameter in the expression
// for ex. in the expression x=> x.Property1 == 6 && x.Property2 == 3
// this will be called twice ^ ^
protected override Expression VisitParameter(ParameterExpression node)
{
var newParameter = convertedParameters[node.Name];
return newParameter;
}
//this will be the first Visit method to be called
//since we're converting LamdaExpressions
protected override Expression VisitLambda<T>(Expression<T> node)
{
//visit the body of the lambda, this will Traverse the ExpressionTree
//and recursively replace parts of the expression we for which we have matching Visit methods
var newExp = Visit(node.Body);
//this will create the new expression
return Expression.Lambda(newExp,convertedParameters.Select(x=>x.Value));
}
}