c# - 在 C# 中使用表达式访问结构属性

Question

我一直在使用以下代码来缓存属性 getter/setter 委托(delegate)，以便快速访问该功能:

class PropertyHelper
{
    public static Func<object, object> BuildGetter(PropertyInfo propertyInfo)
    {
        var method = propertyInfo.GetGetMethod(true);

        var obj = Expression.Parameter(typeof(object), "o");
        Expression<Func<object, object>> expr =
                Expression.Lambda<Func<object, object>>(
                        Expression.Convert(
                                Expression.Call(
                                        Expression.Convert(obj, method.DeclaringType),
                                        method),
                                typeof(object)),
                        obj);
        return expr.Compile();
    }

    public static Action<object, object> BuildSetter(PropertyInfo propertyInfo)
    {
        var method = propertyInfo.GetSetMethod(true);

        var obj = Expression.Parameter(typeof(object), "o");
        var value = Expression.Parameter(typeof(object));

        Expression<Action<object, object>> expr =
            Expression.Lambda<Action<object, object>>(
                Expression.Call(
                    Expression.Convert(obj, method.DeclaringType),
                    method,
                    Expression.Convert(value, method.GetParameters()[0].ParameterType)),
                obj,
                value);

        Action<object, object> action = expr.Compile();
        return action;
    }
}

这在访问类对象的属性时效果很好，但当我将它用于结构对象时却失败了。例如，考虑以下代码:

public struct LocationStruct
{
    public double X { get; set; }
    public double Y { get; set; }
}

public class LocationClass
{
    public double X { get; set; }
    public double Y { get; set; }
}

public class Tester
{
    public static void TestSetX()
    {
        Type locationClassType = typeof(LocationClass);
        PropertyInfo xProperty = locationClassType.GetProperty("X");
        Action<object, object> setter = PropertyHelper.BuildSetter(xProperty);

        LocationStruct testLocationClass = new LocationClass();
        setter(testLocationClass, 10.0);
        if (testLocationClass.X == 10.0)
        {
            MessageBox.Show("Worked for the class!");
        }


        Type locationStructType = typeof(LocationStruct);
        xProperty = locationStructType.GetProperty("X");
        setter = PropertyHelper.BuildSetter(xProperty);

        LocationStruct testLocationStruct = new LocationStruct();
        setter(testLocationStruct, 10.0);
        if (testLocationStruct.X != 10.0)
        {
            MessageBox.Show("Didn't work for the struct!");
        }
    }
}

第一部分有效，将 testLocationClass 的 X 值设置为 10。但是，因为 LocationStruct 是一个结构，所以 testLocationStruct 是按值传入的，该值(在委托(delegate)调用的方法内部)将其 X 设置为10，但上面代码块中的testLocationStruct对象保持不变。

因此，我需要一种方法来访问类似于上述方法的结构对象的属性(仅适用于类对象的属性)。我尝试使用“按引用传递”模式来完成此操作，但我就是无法让它工作。

谁能提供类似的 BuildGetter 和 BuildSetter 方法来缓存结构属性值的 getter/setter 委托(delegate)？

Answer 1

您需要注意两件事才能使其正常工作:

1. 创建setter表达式树时，需要对值类型使用Expression.Unbox，对引用类型使用Expression.Convert。
2. 使用值类型调用 setter 时，您需要确保它被装箱以使用指向结构的指针设置值(而不是处理结构的副本)。

新的实现看起来像这样(只显示新的 setter 和测试方法，因为其余的都是一样的):

public static Action<object, object> BuildSetter(PropertyInfo propertyInfo)
{
    // Note that we are testing whether this is a value type
    bool isValueType = propertyInfo.DeclaringType.IsValueType;
    var method = propertyInfo.GetSetMethod(true);
    var obj = Expression.Parameter(typeof (object), "o");
    var value = Expression.Parameter(typeof (object));

    // Note that we are using Expression.Unbox for value types
    // and Expression.Convert for reference types
    Expression<Action<object, object>> expr = 
        Expression.Lambda<Action<object, object>>(
            Expression.Call(
                isValueType ? 
                    Expression.Unbox(obj, method.DeclaringType) :
                    Expression.Convert(obj, method.DeclaringType), 
                method, 
                Expression.Convert(value, method.GetParameters()[0].ParameterType)), 
                obj, value);
    Action<object, object> action = expr.Compile();
    return action;
}

调用编译后的setter的代码:

...
Type locationStructType = typeof (LocationStruct);
xProperty = locationStructType.GetProperty("X");
setter = PropertyHelper.BuildSetter(xProperty);
LocationStruct testLocationStruct = new LocationStruct();

// Note the boxing of the struct before calling the setter
object boxedStruct = testLocationStruct;
setter(boxedStruct, 10.0);
testLocationStruct = (LocationStruct)boxedStruct;
...

这打印:

Worked for the class!
Worked for the struct!

我还准备了一个 .Net fiddle，它显示了这里的工作实现:https://dotnetfiddle.net/E6WZmK

有关 Expression.Unbox 步骤的说明，请参阅此答案:https://stackoverflow.com/a/32158735/521773