我怎样才能对象实例从（）=> foo.Title表达 [英] How can I get object instance from ()=>foo.Title expression

问题描述

我有一个简单的类属性。

I have a simple class with a property

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

我试图简化调用一个函数，像

I am trying to simplify data binding by calling a function like

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

该声明像

which is declared like

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);
}

让我怎么放在 ??? 我的富类的实例。我如何获得refernce给调用从lambda表达式富实例

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>
上的实例应该存在的地方，因为我可以叫 property.Compile（），并创建一个使用富<委托/ code>例如我的 BindToText 函数内。所以我的问题是，如果这样可以不会在功能参数，增加了实例的引用来完成。我呼吁 Occum剃刀来产生简单的解决方案。

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：
众多什么都没有注意到的是关闭的存在于访问的实例富我的函数里，如果我编译的lambda。编译器怎么会知道哪里可以找到实例，我不？我坚持认为，必须有一个答案，的没有的有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 解决方案是这样的：

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alows我只需键入 BindText（titleText， （）=> foo.Title）;

推荐答案

小的你想要什么LINQPad 示例：

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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