[英]How do I merge (or dynamically build and combine) lambda expressions in C#?
I'm trying to dynamically combin lambda expressions. 我正在尝试动态组合lambda表达式。 The code below will explain what I want. 下面的代码将解释我想要什么。 This is NOT a case of combining a=>b and b=>c to a=>c. 这不是将a => b和b => c组合为a => c的情况。 Instead, I want to prevent code duplication by reusing conversion-expressions: 相反,我想通过重用conversion-expressions来防止代码重复:
class User // convert from...
{
public string FirstName {get;set;}
public string LastName {get;set;}
}
class Person // convert to...
{
public string Name
}
public class UserTransaction
{
User FromUser {get;set;}
User ToUser {get;set;}
decimal Amount {get;set;}
}
public class PersonTransaction
{
Person FromPerson {get;set;}
Person ToPerson {get;set;}
bool IsPositive;
}
Expression<Func<User, Person>> ToPerson = u => new Person {Name = u.FirstName + " " + u.LastName};
Expression<Func<UserTransaction, PersonTransaction>> PersonTransaction = ut => new PersonTransaction {
FromPerson = FromUser.Compile()(ut.FromUser), // Actually, I do not want to compile
ToPerson = ToUser.Compile()(ut.FromUser), // (or double code)
IsPositive = ut.Amount > 0
}
In the example above, I already have an expression to convert a user to a person. 在上面的示例中,我已经有了一个将用户转换为人的表达式。 I do not want to duplicate this code or compile it. 我不想重复此代码或对其进行编译。 I've tried using stripping out the "compile"-calls by manually editing the expression tree. 我尝试通过手动编辑表达式树来剥离“编译”调用。 I did not succeed. 我没有成功。 Has anybody tried something similar and succeed? 有人尝试过类似的方法并且成功了吗?
You can do some voodoo with ExpressionVisitor
to rewrite your existing code to be fully inline; 您可以使用ExpressionVisitor
进行一些巫术改写以完全内联; this: 这个:
Have fun! 玩得开心!
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq.Expressions;
using System.Reflection;
public class User // convert from...
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class Person // convert to...
{
public string Name { get; set; }
}
public class UserTransaction
{
public User FromUser { get; set; }
public User ToUser { get; set; }
public decimal Amount { get; set; }
}
public class PersonTransaction
{
public Person FromPerson { get; set; }
public Person ToPerson { get; set; }
public bool IsPositive { get; set; }
}
static class Program
{
static void Main()
{
Expression<Func<User, Person>> ToPerson = u => new Person { Name = u.FirstName + " " + u.LastName };
Expression<Func<UserTransaction, PersonTransaction>> PersonTransaction = ut => new PersonTransaction
{
FromPerson = ToPerson.Compile()(ut.FromUser), // Actually, I do not want to compile
ToPerson = ToPerson.Compile()(ut.ToUser), // (or double code)
IsPositive = ut.Amount > 0
};
var visitor = new RemoveCompilationsExpressionVisitor();
var inlined = (Expression<Func<UserTransaction, PersonTransaction>>)visitor.Visit(PersonTransaction);
}
class ParameterSwapExpressionVisitor :ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, Expression> swaps;
public ParameterSwapExpressionVisitor(Dictionary<ParameterExpression, Expression> swaps)
{
this.swaps = swaps;
}
protected override Expression VisitParameter(ParameterExpression node)
{
Expression result;
return swaps.TryGetValue(node, out result) ? result : base.VisitParameter(node);
}
}
class RemoveCompilationsExpressionVisitor : ExpressionVisitor
{
protected override Expression VisitInvocation(InvocationExpression node)
{
var lambda = TryGetInnerLambda(node.Expression);
if(lambda != null)
{
// this would be a partial solution, but we want to go further!
// return Expression.Invoke(lambda, node.Arguments);
var swaps = new Dictionary<ParameterExpression, Expression>();
for(int i = 0; i < lambda.Parameters.Count; i++)
{
swaps.Add(lambda.Parameters[i], node.Arguments[i]);
}
var visitor = new ParameterSwapExpressionVisitor(swaps);
return visitor.Visit(lambda.Body);
}
return base.VisitInvocation(node);
}
LambdaExpression TryGetInnerLambda(Expression node)
{
try
{
if(node.NodeType == ExpressionType.Call)
{
var mce = (MethodCallExpression)node;
var method = mce.Method;
if (method.Name == "Compile" && method.DeclaringType.IsGenericType && method.DeclaringType.GetGenericTypeDefinition()
== typeof(Expression<>))
{
object target;
if (TryGetLiteral(mce.Object, out target))
{
return (LambdaExpression)target;
}
}
}
}
catch (Exception ex)
{
/* best effort only */
Debug.WriteLine(ex);
}
return null;
}
static bool TryGetLiteral(Expression node, out object value)
{
value = null;
if (node == null) return false;
switch(node.NodeType)
{
case ExpressionType.Constant:
value = ((ConstantExpression)node).Value;
return true;
case ExpressionType.MemberAccess:
var me = (MemberExpression)node;
object target;
if (TryGetLiteral(me.Expression, out target))
{
switch (me.Member.MemberType)
{
case System.Reflection.MemberTypes.Field:
value = ((FieldInfo)me.Member).GetValue(target);
return true;
case MemberTypes.Property:
value = ((PropertyInfo)me.Member).GetValue(target, null);
return true;
}
}
break;
}
return false;
}
}
}
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