如何从（）=> foo.Title表达式获取对象实例 [英] How can I get object instance from ()=>foo.Title expression

问题描述

我有一个简单的类，有一个属性

I have a simple class with a property

class Foo 
{ 
    string Title { get; set; } 
}

我正在尝试通过调用像

BindToText(titleTextBox, ()=>foo.Title );

被声明为

void BindToText<T>(Control control, Expression<Func<T>> property)
{
    var mex = property.Body as MemberExpression;
    string name = mex.Member.Name;

    control.DataBindings.Add("Text", ??? , name);
}

所以我放在 / code>为我的 Foo 类的实例。如何从lambda表达式中调用 foo 实例？

so what do I put in ??? for the instance of my Foo class. How do I get a refernce to the calling foo instance from the lambda expression?

编辑： / strong>
实例应该在某处，因为我可以调用 property.Compile（）并创建一个使用 foo 实例里面我的 BindToText 函数。所以我的问题是如果这可以在没有添加对函数参数中的实例的引用的话。我呼吁 Occum's Razor 产生最简单的解决方案。

edit: The instance should be there somewhere because I can call property.Compile() and create a delegate that uses the foo instance inside my BindToText function. So my question is if this can be done without adding a reference to the instance in the function parameters. I call upon Occum's Razor to yield the simplest solution.

编辑2：
许多人未能注意到访问的实例中存在的封闭 foo 在我的函数内，如果我编译lambda。编译器怎么知道在哪里找到实例，我不知道我坚持要有一个答案，没有必须传递一个额外的参数。

edit 2: What many have failed to notice is the closure that exists in accessing the instance of foo inside my function, if I compile the lambda. How come the compiler knows where to find the instance, and I don't? I insist that there has to be an answer, without having to pass an extra argument.

感谢 VirtualBlackFox ，解决方案如下： / p>

Thanks to VirtualBlackFox the solution is such:

void BindText<T>(TextBoxBase text, Expression<Func<T>> property)
{
    var mex = property.Body as MemberExpression;
    string name = mex.Member.Name;
    var fex = mex.Expression as MemberExpression;
    var cex = fex.Expression as ConstantExpression;            
    var fld = fex.Member as FieldInfo;
    var x = fld.GetValue(cex.Value);
    text.DataBindings.Add("Text", x, name);            
}

这让我简单地键入 BindText（titleText， （）=> foo.Title）; 。

推荐答案

小 LINQPad 您想要的样本：

void Foo<T>(Expression<Func<T>> prop)
{
    var propertyGetExpression = prop.Body as MemberExpression;

    // Display the property you are accessing, here "Height"
    propertyGetExpression.Member.Name.Dump();

    // "s" is replaced by a field access on a compiler-generated class from the closure
    var fieldOnClosureExpression = propertyGetExpression.Expression as MemberExpression;

    // Find the compiler-generated class
    var closureClassExpression = fieldOnClosureExpression.Expression as ConstantExpression;
    var closureClassInstance = closureClassExpression.Value;

    // Find the field value, in this case it's a reference to the "s" variable
    var closureFieldInfo = fieldOnClosureExpression.Member as FieldInfo;
    var closureFieldValue = closureFieldInfo.GetValue(closureClassInstance);

    closureFieldValue.Dump();

    // We know that the Expression is a property access so we get the PropertyInfo instance
    // And even access the value (yes compiling the expression would have been simpler :D)
    var propertyInfo = propertyGetExpression.Member as PropertyInfo;
    var propertyValue = propertyInfo.GetValue(closureFieldValue, null);
    propertyValue.Dump();
}

void Main()
{
    string s = "Hello world";
    Foo(() => s.Length);
}

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