用变量序列化表达式

Question

我编写了一些类来将System.Linq.Expressions 序列化为 DataContracts，以便能够通过 WCF 发送。它工作得很好很好。问题是当我想序列化一个包含变量的表达式时。这是一个解释问题的例子：

public class Foo
{
    public string Name { get; set; }
}

// CASE 1
Expression<Func<Foo, bool>> lambda = foo => foo.Name == "Test";
Console.WriteLine(lambda);
// OUTPUT: foo => (foo.Name == "Test")

// CASE 2
var variable = "Test";
lambda = foo => foo.Name == variable;            
this.AssertExpression(lambda, "Class Lambda expression with variable.");
// OUTPUT: foo => (foo.Name == value(MyTest+<>c__DisplayClass0).variable)

序列化CASE 2表达式没有问题，但是序列化的数据没有用，因为在服务端，没有什么可以解决value(MyTest+<>c__DisplayClass0).variable

所以我需要在序列化该表达式之前解析变量，以便 CASE 2 表达式序列化为与 CASE1 相同的结果

Answer 1

对不起 VB，但是下面的摘录是我在评论中的意思。我不认为它涵盖了所有基础 （即它可能没有深入研究，所以请确保你测试它） 但对于 simple 大多数示例它都有效：

代码基于this MSDN Expression Visitor example:

class CustomExpressionWalker<TSource> : ExpressionVisitor
{
    protected override Expression VisitMemberAccess(MemberExpression m)
    {
        if (m.Member.DeclaringType != typeof(TSource))
        {
            // We are accessing a member/variable on a class
            // We need to descend the tree through the navigation properties and eventually derive a constant expression
            return this.VisitMemberAccess(m, m.Type);
        }
        throw new NotImplementedException();
    }

    protected Expression VisitMemberAccess(MemberExpression m, Type expectedType)
    {
        if (m.Expression.NodeType == ExpressionType.Constant)
        {
            // We are at the end of the member expression 
            // i.e. MyClass.Something.Something.Value <-- we're at the Value part now
            ConstantExpression constant = (ConstantExpression)m.Expression;
            return Expression.Constant(m.Expression.Type.GetFields().Single(n => n.FieldType == expectedType && m.Member.Name.Contains(n.Name)).GetValue(constant.Value));
        }
        else if (m.Member.DeclaringType == typeof(TSource))
        {
            // I'm unsure of your current implementation but the original Member access
            // regarding serializing the expression, but if the code reaches here a nested
            // MemberAccess has landed on a Property/variable of type TSource, so you'll need
            // to decide whether to serialize here or not.  For example, if TSource was of 
            // type "myClass", it could be 
            // (myOtherClass x) => x.myClass
            throw new NotImplementedException();
        }
        else if (m.Member.DeclaringType == typeof(Nullable))
        {
            // never got round to implementing this as we don't need it yet
            // if you want to deal with Nullable<T> you're going to have to 
            // examine the logic here
            throw new NotImplementedException();
        }
        else
        {
            // continue walking the member access until we derive the constant
            return this.VisitMemberAccess((MemberExpression)m.Expression, expectedType);
        }
    }
}   

希望这会有所帮助！

编辑：The original issue I had 是当 MemberAccess 不是 TSource 类时我没有继续走树，上面的逻辑实际上应该递归地根除这些情况，所以忽略我原来的评论。我留在了Nullable<T> 子句（在 else if） 语句中，因为我认为现有的逻辑不会涵盖这些情况，它也可能会遇到困难使用泛型类。

也就是说，这应该让你处于有利地位。如果您没有使用 Expression Visitor，您能否提供更多详细信息/代码？

祝你好运！