MemberExpression:从范围''引用的InvalidOperationExpression变量'x',但未定义

Question

我正在使用System.Linq.Expressions

我试图建立一个简单的LambdaExpression包括MemberExpression.如果我使用System.Linq.Expressions API(例如MakeMemberAccess)显式创建了MemberExpression ,我将在我调用Compile()时得到错误"InvalidOperationExpression变量'x'引用范围'',但它没有定义" LambdaExpression.

例如,这是我的代码

Expression<Func<Customer, string>> expression1, expression2, expression3;
Func<Customer, string> fn;
expression1 = (x) => x.Title;
fn = expression1.Compile();//works
fn(c);
MemberExpression m;
m = Expression.MakeMemberAccess(
Expression.Parameter(typeof(Customer), "x"), typeof(Customer).GetProperty("Title"));
expression2 = Expression.Lambda<Func<Customer, string>>(m,
    Expression.Parameter(typeof(Customer), "x"));

m = Expression.Property(Expression.Parameter(typeof(Customer),"x"), "Title");
expression3 = Expression.Lambda<Func<Customer, string>>(m,
    Expression.Parameter(typeof(Customer), "x"));

fn = expression3.Compile();//InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined
fn = expression2.Compile();//InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined

当调用Compile()方法时,expression2和expression3抛出一个异常,但是expression1没有; expression1有效.为什么是这样？如何在表达式2,3中创建一个MemberExpression,并在调用Compile()时让它们工作(不抛出异常)？

谢谢

Answer 1

您将多次创建名为"x"的不同参数.如果你使用单一ParameterExpression,它应该都可以正常工作.

ParameterExpression p = Expression.Parameter(typeof(Customer), "x");
MemberExpression m = Expression.MakeMemberAccess(p, 
    typeof(Customer).GetProperty("Title"));
expression2 = Expression.Lambda<Func<Customer, string>>(m, p);

m = Expression.Property(p, "Title");
expression3 = Expression.Lambda<Func<Customer, string>>(m, p);

fn = expression3.Compile();
fn = expression2.Compile();

基本上参数表达式不匹配名称 - 您必须在任何地方使用相同的表达式.这有点痛,但我们去......