c# - 从 MemberExpression 中获取对象？

Question

所以，假设我在 C# 中有以下表达式:

Expression<Func<string>> expr = () => foo.Bar;

如何提取对 foo 的引用？

Answer 1

I had the same problem, but somewhat more complex, and Darin Dimitrov's answer gave me a good start. I'll post my results here, despite the fact that this is an "old" question .

---

情况一:根对象是实例成员

    this.textBox.Text    // where 'this' has type 'Form'

...等价于下面的表达式树:

.                                    +====================+
.                                    |  MemberExpression  |
.                                    +====================+
#                                      |                |
#                          .Expression |                | .Member
#                                      v                v
.                    +------------------+              +------------+
.                    | MemberExpression |              | MemberInfo |
.                    +------------------+              +------------+
#                      |              |                 .Name = "Text"
#          .Expression |              | .Member         .MemberType = Property
#                      v              v
.   +--------------------+          +------------+
.   | ConstantExpression |          | MemberInfo |
.   +--------------------+          +------------+
#    .Value = this                   .Name = "textBox"
#    .Type  = typeof(Form)           .MemberType = Field

在此表达式树中，您实际获得对象引用的唯一位置是来自 ConstantExpression:它允许您获得对 this 的引用。因此，在这棵树中获取任何对象引用的基本思路如下:

1. 沿着 .Expression 轴进入表达式树，直到到达 ConstantExpression 节点。

2. 获取该节点的 .Value 属性。这是根对象引用(即上例中的 this)。

3. 使用表达式树中的反射和 MemberInfo 节点，获取对象引用并按照您的方式返回“向上”表达式树。

下面是一些演示这一点的代码:

Expression expr = ...;   // <-- initially set to the expression tree's root

var memberInfos = new Stack<MemberInfo>();

// "descend" toward's the root object reference:
while (expr is MemberExpression)
{
    var memberExpr = expr as MemberExpression;
    memberInfos.Push(memberExpr.Member);
    expr = memberExpr.Expression
}

// fetch the root object reference:
var constExpr = expr as ConstantExpression;
var objReference = constExpr.Value;

// "ascend" back whence we came from and resolve object references along the way:
while (memberInfos.Count > 0)  // or some other break condition
{
    var mi = memberInfos.Pop();
    if (mi.MemberType == MemberTypes.Property)
    {
        objReference = objReference.GetType()
                                   .GetProperty(mi.Name)
                                   .GetValue(objReference, null);
    }
    else if (mi.MemberType == MemberTypes.Field)
    {
        objReference = objReference.GetType()
                                   .GetField(mi.Name)
                                   .GetValue(objReference);
    }
}

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情况2:根对象是静态类成员

    Form.textBox.Text    // where 'textBox' is a static member of type 'Form'

... 导致不同的表达式树。注意左下角的空引用:

.                                    +====================+
.                                    |  MemberExpression  |
.                                    +====================+
#                                      |                |
#                          .Expression |                | .Member
#                                      v                v
.                    +------------------+              +------------+
.                    | MemberExpression |              | MemberInfo |
.                    +------------------+              +------------+
#                      |              |                 .Name = "Text"
#          .Expression |              | .Member         .MemberType = Property
#                      v              v
.                     null          +------------+
.                                   | MemberInfo |
.                                   +------------+
#                                   .Name = "textBox"
#                                   .MemberType = Field
#                                   .DeclaringType = typeof(Form)

在这里，您不能通过等待 ConstantExpression 来停止“下降”阶段。相反，当您到达空引用时，您将停止下降。接下来，您按如下方式检索根对象引用:

var mi = memberInfos.Pop();
objReference = mi.DeclaringType
                 .GetField(member.Name, BindingFlags.Static)  // or .GetProperty!
                 .GetValue(null);

从那里开始的“上升”阶段与以前相同。

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当然还有更多的情况(例如命名参数作为根对象)，但我希望到现在为止，我已经了解了基本的想法，所以我会在这里打断。