c# - 使用变量序列化表达式

Question

我写了一些类来序列化System.Linq.Expressions到 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其中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)

我在序列化 CASE 2 表达式时没有遇到问题，但是我序列化的数据是无用的，因为在服务端，没有什么可以解决的 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 VB 感到抱歉，但是 以下摘录是我在评论中的部分代码。我不认为它涵盖了所有的基础 (即它可能没有深入到足够深的地方，所以一定要测试它) 但是对于 简单 的大多数例子来说它是有效的:

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);
        }
    }
}   

希望这对您有所帮助!

编辑:The original issue I had是当 MemberAccess 不是 TSource 时我没有继续遍历树类，上面的逻辑实际上应该递归地根除这些案例，所以请忽略我原来的评论。我留在了 Nullable<T> clause (on the else if ) 语句，因为我认为现有的逻辑不会涵盖这些情况，它也可能与通用类斗争。

也就是说，这应该对您有利。如果您不使用 Expression Visitor，能否提供更多详细信息/代码？

祝你好运!