使用表达式访问C#中的结构属性

本文关键字：结构 属性 表达式 访问 | 更新日期: 2023-09-27 18:28:32

我一直在使用以下代码缓存属性getter/setter委托，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;
    }
}

当访问类对象的属性时，这非常有效，但当我将其用于结构对象时，它会失败。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!");
        }
    }
}

第一部分工作，将testLocationClass的X值设置为10。但是，因为LocationStruct是一个结构，所以testLocationStruct是通过值传入的，该值（委托调用的方法内部）的X设置为10，但上面代码块中的testLocationStruct对象保持不变。

因此，我需要一种访问结构对象属性的方法，类似于上面的方法（它只适用于类对象的属性）。我曾尝试使用"引用传递"模式来实现这一点，但我就是无法实现。

任何人都能提供类似的BuildGetter和BuildSetter方法，用于缓存结构属性值的getter/setter委托吗？

使用表达式访问C#中的结构属性

要使其工作，您需要注意两件事：

  1. 创建setter表达式树时，需要将Expression.Unbox用于值类型，将Expression.Convert用于引用类型
  2. 使用值类型调用setter时，需要确保它已装箱，以便使用指向结构的指针来设置值（而不是处理结构的副本）

新的实现看起来是这样的（只显示了新的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;
}

以及调用编译后的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;
...

此打印：

Worked for the class!
Worked for the struct!

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

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

结构作为参数是通过值传递的，而ref/out似乎不能很好地与表达式一起使用，您可以考虑使用返回结构实例的新函数签名：

static Func<MethodInfo, object, object, object> s1 = (MethodInfo set, object instance, object val) =>
{
    set.Invoke(instance, new object[] { val });
    return instance;
};
// Non-Generic approach
static Func<object, object, object> BuildSetter5(PropertyInfo propertyInfo)
{
    var method = propertyInfo.GetSetMethod(true);
    var obj = Expression.Parameter(typeof(object), "o");
    var value = Expression.Parameter(typeof(object));
    Expression<Func<object, object, object>> expr =
        Expression.Lambda<Func<object, object, object>>(
            Expression.Call(
                s1.Method,
                Expression.Constant(method),
                obj,
                Expression.Convert(value, method.GetParameters()[0].ParameterType)),
            obj,
            value);
    Func<object, object, object> action = expr.Compile();
    return action;
}
// Generic approach
static Func<T, object, T> BuildSetter6<T>(PropertyInfo propertyInfo) where T : struct
{
    var method = propertyInfo.GetSetMethod(true);
    var obj = Expression.Parameter(typeof(T), "o");
    var value = Expression.Parameter(typeof(object));
    Expression<Func<T, object, T>> expr =
        Expression.Lambda<Func<T, object, T>>(
            Expression.Convert(
                Expression.Call(
                    s1.Method,
                    Expression.Constant(method),
                    Expression.Convert(obj, typeof(object)),
                    Expression.Convert(value, method.GetParameters()[0].ParameterType)),
                typeof(T)),
            obj,
            value);
    Func<T, object, T> action = expr.Compile();
    return action;
}
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