从MemberExpression中获取对象?

Expression<Func<string>> expr = () => foo.Bar;
var me = (MemberExpression)((MemberExpression)expr.Body).Expression;
var ce = (ConstantExpression)me.Expression;
var fieldInfo = ce.Value.GetType().GetField(me.Member.Name, BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
var value = (Foo)fieldInfo.GetValue(ce.Value);

我也有同样的问题,但有点复杂,达林·迪米特洛夫的回答给了我一个良好的开端。尽管这是一个“老”问题,我还是会在这里发布我的结果。

案例1:根对象是对象成员

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

…相当于以下表达式树:

.                                    +====================+
.                                    |  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

在这个表达式树中,您实际获得对象引用的唯一位置是 ConstantExpression :它允许您获取 this . 因此,在此树中获取任何对象引用的基本思想如下:

1. 沿着 .Expression 直到你到达 恒压 节点。

2. 抓住那个节点 .Value 财产。这是根对象引用(即。 这 在上面的例子中)。

3. 使用反射和 MemberInfo 从表达式树中获取节点,获取对象引用并以“向上”的方式返回表达式树。

下面的代码演示了这一点:

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);
    }
}

案例2:根对象是静态类成员

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

…结果生成不同的表达式树。请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,你不能通过等待一个 恒压 . 相反,当到达空引用时停止降序。接下来,检索根对象引用,如下所示:

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

从那里开始的“提升”阶段和以前一样。

当然还有更多的情况(例如命名参数作为根对象),但是我希望到现在为止,我已经了解了基本的概念,所以我将在这里中断。

有一个更简单的解决方案:

var pExpression = ((MemberExpression)expr.Body);
var bindingObject = Expression.Lambda(((MemberExpression)pExpression.Expression)).Compile().DynamicInvoke();

谢谢,斯塔克-你的榜样帮了我很多忙!因此,我想在第一个案例的基础上增加一些内容:

要从方法中提取值,应替换代码:

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

附有代码:

    var newExpression = expr as NewExpression;
    if (newExpression != null)
    {                
        return newExpression.Constructor.Invoke(newExpression.Arguments.Select(GetObjectValue).ToArray());
    }

    var methodCallExpr = expr as MethodCallExpression;
    if (methodCallExpr != null)
    {
        var value = methodCallExpr.Method.Invoke(methodCallExpr.Object == null
                                                                 ? null
                                                                 : GetObjectValue(methodCallExpr.Object),
methodCallExpr.Arguments.Select(GetObjectValue).ToArray());
                    return value;
    }

    // fetch the root object reference:
    var constExpr = expr as ConstantExpression;
    if (constExpr == null)
    {
         return null;
    }
    var objReference = constExpr.Value;
    // ... the rest remains unchanged

这样就可以从如下表达式中提取值:

aInstane.MethodCall(anArgument1, anArgument2) or
AType.MethodCall(anArgument1, anArgument2) or
new AType().MethodCall(anArgument1, aInstane.MethodCall(anArgument2, anArgument3))

这就是我在单元测试中使用的:

 internal static INotifyPropertyChanged SubModel < T, TProperty > (T model, Expression < Func < T, TProperty >> pickProperty) where T: INotifyPropertyChanged {
   MemberExpression memberExpression = (MemberExpression) pickProperty.Body;
   ParameterExpression parameterExpression = pickProperty.Parameters[0];
   Expression mem = memberExpression.Expression;
   var delegateType = typeof(Func < , > ).MakeGenericType(typeof(T), mem.Type);
   LambdaExpression lambdaExpression = Expression.Lambda(delegateType, mem, parameterExpression);
   object subModel = lambdaExpression.Compile().DynamicInvoke(model);
   return subModel as INotifyPropertyChanged ? ? model;
  }