.NETの式は不変であるため、これを行う唯一の方法は、式全体を再構築することです。これを行うには、通常、ExpressionVisitorクラスから継承する必要があります。変換する必要のある式の複雑さによっては、これは非常に複雑になる可能性があります。
これは、単純な式(x => x.Someproperty == somevalueなど)を使用する訪問者の単純な例です。これは開始するための単なる例であり、終了またはテストされているわけではありません(たとえば、式のメソッド呼び出しは処理されません)。
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));
}
}