用Linq表达式和反射获取属性值的通用方法

Question

亲爱的反思之神

我想有一个通用的GetValue<TEntity, T>方法，可以返回以下属性值给定以下User类:

public class User  
{
   public int Id { get; set; }
   public int ClientId { get; set; }
   public string UserName { get; set; }
   public string FirstName { get; set; }
   public string LastName { get; set; }
   public string MobileNumber { get; set; }   
   public bool IsActive { get; set; }
   public Client Client { get; set; }
   public List<Package> Packages { get; set; }
 }

GetValue<TEntity, T>应该能够做的示例用法:

  var firstName = dataCollector.GetValue<User, string>(x => x.FirstName);
  var client = dataCollector.GetValue<User, Client>(x => x.Client);
  var packages = dataCollector.GetValue<User, List<Package>>(x => x.Packages);
  var packageFirst = dataCollector.GetValue<User, Package>(x => x.Packages[0]);
  var packageName = dataCollector.GetValue<User, string>(x => x.Packages[0].Name);
  var clientName = dataCollector.GetValue<User, string>(x => x.Client.Name);

到目前为止，我有以下方法适用于前3种情况:

 public T GetValue<TEntity, T>(Expression<Func<TEntity, T>> propertyExpression) where TEntity : class
 {
    var response = _responses.FirstOrDefault(p => p.GetType() == typeof(TEntity)) as TEntity;
    if (response != null)
    {
       var expr = (MemberExpression)propertyExpression.Body;
       var prop = (PropertyInfo)expr.Member;
       return (T)prop.GetValue(response);
    }
    return default(T);
  }

但它不工作的最后三个场景:

  var packageFirst = dataCollector.GetValue<User, Package>(x => x.Packages[0]);

抛出:Unable to cast object of type 'System.Linq.Expressions.InstanceMethodCallExpressionN' to type 'System.Linq.Expressions.MemberExpression'.

  var packageName = dataCollector.GetValue<User, string>(x => x.Packages[0].Name);

抛出:Object does not match target type.

  var clientName = dataCollector.GetValue<User, string>(x => x.Client.Name);

抛出:Object does not match target type.

我需要对方法做什么更改?

我现在将牺牲一个u盘等待你的答案:)

Answer 1

问题就在这里:

if (response != null)
{
   var expr = (MemberExpression)propertyExpression.Body;
   var prop = (PropertyInfo)expr.Member;
   return (T)prop.GetValue(response);
}

如果表达式直接引用属性，则仅有效，否则propertyExpression.Body将不是MemberExpression，并且您将得到运行时强制转换错误。不工作的三个不直接引用属性——前两个引用属性顶部的方法(索引器)，最后一个引用嵌套属性。

因为你想要的只是表达式的值，所以我认为你可以这样做:

if (response != null)
{
   Func<TEntity, T> func = propertyExpression.Compile();  
   return func(response);
}

如果你打算用表达式做其他事情(比如获取属性的名称)，那么你需要决定是否要支持不直接引用属性的表达式，并为此添加处理程序。

您可以通过执行Lambda表达式来实现:

        if (response != null)
        {
            return propertyExpression.Compile().Invoke(response);
        }