如何从lambda表达式获取引用实例的实例

Question

我有这个lambda表达式 Expression<Func<bool>> commandToExecute

然后我用一个方法传递一个类的实例:

_commandExecuter.ProcessCommand (() => aClass.Method())

我如何获得实例aClass的内部ProcessCommand方法？

我想执行这个类的一些addiontal方法或获取一些属性值.

这可能吗？

编辑: 我现在已经编写了一个简单的静态帮助方法来获取实例:

private static object GetReferredProviderInstance(Expression body)
{
    var methodCallExpression = body as MethodCallExpression;
    if (methodCallExpression != null)
    {
        var constantExpression = methodCallExpression.Object as ConstantExpression;
        if (constantExpression != null) return constantExpression.Value;
    }
    return null;
}

方法调用看起来像这样......

Expression body = commandToExecute.Body; // this is the method parameter Expression<Func<bool>> commandToExecute
var referredProviderInstance = GetReferredProviderInstance(body);

这里的问题是,ConstantExpression的转换导致了Null.所以constantExpression总是空的.

有任何想法吗？

编辑2 我修复了问题...

private static object GetReferredProviderInstance(Expression body)
{
    var methodCallExpression = body as MethodCallExpression;
    if (methodCallExpression != null)
    {
        var memberExpression = methodCallExpression.Object as MemberExpression;
        if (memberExpression != null)
        {
            var constantExpression = memberExpression.Expression as ConstantExpression;
            if (constantExpression != null) return constantExpression.Value;
        }
    }
    return null;
}

但是这里出现了一个新问题.我只获得了我的提供者的参考实例所在的Windows窗体的实例.

我如何获得lambda表达式的真实对象(aClass)？

Answer 1

这实际上是可能的,但这取决于您传递给此方法的内容.假设您有一个场景,您传递您所在的类的实例方法ProcessCommand:

public class TestClass
{
    public void TestMethod()
    {
        ProcessCommand(() => MethodToCall());
    }
    public bool MethodToCall() { return true; }
    void ProcessCommand(Expression<Func<bool>> expression) { ... }
}

然后您可以使用以下ProcessCommand方法.这只能起作用,因为MethodToCall在这个实例上调用了它.

void ProcessCommand(Expression<Func<bool>> expression)
{
    var lambda = (LambdaExpression) expression;
    var methodCall = (MethodCallExpression) lambda.Body;
    var constant = (ConstantExpression) methodCall.Object;
    var myObject = constant.Value;
}

更复杂的情况如下:

public class CallingClass
{
    public void TestMethod()
    {
        var calledClass = new CalledClass();
        ProcessCommand(() => calledClass.MethodToCall());
    }
    void ProcessCommand(Expression<Func<bool>> expression) { ... }
}
public class CalledClass
{
    public bool MethodToCall() { return true; }
}

我们调用的方法现在在另一个类中,并且不在此实例上调用,而是在CalledClass被调用的实例上调用calledClass.但是编译器如何将calledClass变量传递给lambda表达式？没有任何东西可以定义可以调用calledClass该方法的字段MethodToCall.

编译器通过生成一个带有名称的一个字段的内部类来解决这个问题calledClass.结果这个ProcessCommand方法现在变成了这个:

public void ProcessCommand(Expression<Func<bool>> expression)
{
    // The expression is a lambda expression with a method call body.
    var lambda = (LambdaExpression) expression;
    var methodCall = (MethodCallExpression) lambda.Body;
    // The method is called on a member of some instance.
    var member = (MemberExpression) methodCall.Object;
    // The member expression contains an instance of the anonymous class that
    // defines the member...
    var constant = (ConstantExpression) member.Expression;
    var anonymousClassInstance = constant.Value;
    // ...and the member itself.
    var calledClassField = (FieldInfo) member.Member;
    // With an instance of the anonymous class and the field, we can get its value.
    var calledClass =
        (CalledClass) calledClassField.GetValue(anonymousClassInstance);
}

稍微复杂一点,因为编译器必须生成匿名内部类.