实体框架 - 将 LINQ 表达式谓词从一种类型更改为另一种类型

Question

我有两个不相关的类。一个作为 API 公开，另一个由第三方 API 在内部使用。

实体从我们的 API 公开，而实体提供程序来自第三方程序集。

class Entity
{
  public A { get; set; }
}
class EntityProvider
{
  public A { get; set; }
}

我们 API 的使用者将提供Expression <Func<Entity, bool>>形式的谓词，我需要对其进行修改以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。

Answer 1

由于 .NET 中的表达式是不可变的，因此执行此操作的唯一方法是重新生成整个表达式。为此，通常涉及从 ExpressionVisitor 类继承。根据您必须转换的表达式的复杂性，这可能非常复杂。

这是一个简单的访问者示例，它将使用简单的表达式（如 x=>x.Someproperty == somevalue ）。这只是一个让你入门的例子，它根本没有完成或测试（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));
    }        
}