使用 Expression 为对象赋值

Question

我有这个使用表达式注入数据的注入器

    /// <summary>
    /// Inject a data to an instance of T
    /// </summary>
    /// <typeparam name="T">The type of object as parameter</typeparam>
    /// <param name="data">Object instance to where the data is injected</param>
    /// <param name="pairData">Set of data info</param>
    public static void InjectData<T>(this T data, Dictionary<Expression<Func<T, object>>, object> pairData) where T : IAuditable
    {
        // posible content of T data
        // Employee
        //      : Code
        //      : Name
        //      : ID

        // possible content of pairData
        // Key: a => a.Code
        // Value: "12345"

        // Key: a => a.Name
        // Value: "Vincent"

        // after the injection, the data (Employee) will now have value on each properties: Code, Name

        foreach (var _o in pairData)
        {

            var _value = Expression.Constant(_o.Value);
            var _assign = Expression.Assign(_o.Key, _value);

            //_lambda.Compile()(data);
            //var _lambda = Expression.Lambda<Action<object, T>>(_assign, _value, _o.Key);

            //_lambda.Compile()(_o.Value, data);

        }
    }

我只传了一个表达式和值的集合。

Expression on which where to assign the value (eg. a => a.Code) and value (eg. "123456")

我想将它分配给一个对象 T，但在我当前的尝试中，我得到了一个 System.ArgumentException：为 lambda 声明提供的参数数量不正确。我被困住了，不知道下一步该怎么做。 :) 任何人都可以为我指出正确的方向来完成这项工作吗？

这是示例用法

Employee _employee = new Employee();

Dictionary<Expression<Func<Employee, object>>, object> _x = new Dictionary
                <Expression<Func<Employee, object>>, object>
            {
                {a => a.Code, "12345"}
            };

_employee.InjectData(_x);

Assert.AreEqual("12345", _employee.Code);

我也试过

 foreach (var _o in pairData)
        {
           var _mem = _o.Key.Body as MemberExpression;
           var _prop = _mem.Member as PropertyInfo;
           var _setMethod = _prop.GetSetMethod();

           _prop.SetValue(data, _o.Value);

我可以在这里看到进展，因为私有属性已经分配，但 getter 为 NULL

提前致谢

如有任何帮助，我们将不胜感激。

Answer 1

您可以使用以下代码：

public static void InjectData<T>(this T data, Dictionary<Expression<Func<T, object>>, object> pairData)
{
    foreach (var pair in pairData)
    {
        data.SetPropertyValue(pair.Key, pair.Value);
    }
}

public static T SetPropertyValue<T>(this T target, Expression<Func<T, object>> memberLamda, object value)
{
    var memberSelectorExpression = memberLamda.Body as MemberExpression;
    if (memberSelectorExpression == null)
    {
        return target;
    }

    var property = memberSelectorExpression.Member as PropertyInfo;
    if (property == null)
    {
        return target;
    }

    property.SetValue(target, value, null);

    return target;
}

基本上，这添加了另一种扩展方法来根据表达式设置属性。

如果您只想在值实际不同时进行更改（这在使用 INotifyPropertyChanged 时很方便），您可以使用以下代码，它需要一个额外的比较器来检查值是否更改：

public static T SetPropertyValue<T, R>(this T target, Expression<Func<T, R>> memberLamda, R value)
{
    return target.SetPropertyValue(memberLamda, value, EqualityComparer<R>.Default);
}

public static T SetPropertyValue<T, R>(this T target, Expression<Func<T, R>> memberLamda, R value, IEqualityComparer<R> comparer)
{
    var memberSelectorExpression = memberLamda.Body as MemberExpression;
    if (memberSelectorExpression == null)
    {
        return target;
    }

    var property = memberSelectorExpression.Member as PropertyInfo;
    if (property == null)
    {
        return target;
    }

    var currentValue = (R) property.GetValue(target, null);

    if (comparer.Equals(currentValue, value))
    {
        return target;
    }

    property.SetValue(target, value, null);

    return target;
}

注意，这里我们使用比较器来检查当前值是否等于新值。如果是这样，我们只是 return（不需要更新），如果不是我们设置值。

Answer 2

对我来说意义不大，但它是：

public static void InjectData<T>(this T data, Dictionary<Expression<Func<T, object>>, object> pairData) where T : IAuditable
{
    foreach (var item in pairData)
    {
        var member = item.Key;
        // If member type is a reference type, then member.Body is the property accessor.
        // For value types it is Convert(property accessor)
        var memberBody = member.Body as MemberExpression ?? (MemberExpression)((UnaryExpression)member.Body).Operand;
        var lambda = Expression.Lambda<Action<T>>(Expression.Assign(memberBody, Expression.Constant(item.Value)), member.Parameters);
        var action = lambda.Compile();
        action(data);
    }
}

UPDATE 根据评论中的要求，支持嵌套 class 属性初始化的版本：

public static void InjectData<T>(this T data, Dictionary<Expression<Func<T, object>>, object> pairData) //where T : IAuditable
{
    var assignments = new List<Expression>();
    foreach (var item in pairData)
    {
        var member = item.Key;
        // If member type is a reference type, then member.Body is the property accessor.
        // For value types it is Convert(property accessor)
        var memberBody = member.Body as MemberExpression ?? (MemberExpression)((UnaryExpression)member.Body).Operand;
        assignments.Clear();
        assignments.Add(Expression.Assign(memberBody, Expression.Constant(item.Value)));
        var target = member.Parameters[0];
        while (memberBody.Expression != target)
        {
            var childMember = (MemberExpression)memberBody.Expression;
            assignments.Add(Expression.IfThen(Expression.ReferenceEqual(childMember, Expression.Constant(null)),
                Expression.Assign(childMember, Expression.New(childMember.Type))));
            memberBody = childMember;
        }
        assignments.Reverse();
        var body = assignments.Count > 1 ? Expression.Block(assignments) : assignments[0];
        var lambda = Expression.Lambda<Action<T>>(body, target);
        var action = lambda.Compile();
        action(data);
    }
}

使用 Expression 为对象赋值

Assign value to object using Expression

c#

lambda

expression