获取内部对象的属性 [英] get property of inner object

问题描述

我正在尝试使用该代码来获取包含User的客户的User.Email属性。但它的一个对象（User类型的用户），所以它抛出异常。我应该修复什么？

  public static IQueryable< T>如果（！string.IsNullOrWhiteSpace（SortField））
 {
 var param = Expression 。参数（typeof（T），p）; 
 var prop = Expression.Property（param，SortField）; 
 var exp = Expression.Lambda（prop，param）; 
 string method = Ascending？ OrderBy：OrderByDescending; 
 Type [] types = new Type [] {q.ElementType，exp.Body.Type}; 
 var mce = Expression.Call（typeof（Queryable），method，types，q.Expression，exp）; 
 return q.Provider.CreateQuery< T>（mce）; 
} 
 else 
 {
 return q; 
} 
} 
  

解决方案

你需要将属性路径传递给函数（如User.Email），并在其中记录该内容，如此

  public静态IQueryable< T> OrderByField< T>（此IQueryable< T>源，字符串sortField，bool升序）
 {
 if（string.IsNullOrWhiteSpace（sortField））返回源; 
 var item = Expression.Parameter（typeof（T），item）; 
表达式member = null; 
 foreach（var memberName in sortField.Split（'。'））
 member = Expression.PropertyOrField（member ?? item，memberName）; 
 var selector = Expression.Lambda（member，item）; 
 var method = ascending？ OrderBy：OrderByDescending; 
 var types = new [] {source.ElementType，selector.Body.Type}; 
 var expression = Expression.Call（typeof（Queryable），method，types，source.Expression，selector）; 
 return source.Provider.CreateQuery< T>（expression）; 
} 
  

主要部分是

var item = Expression.Parameter（typeof（T），item）;

  
表达式member = null; 
 foreach（var memberName in sortField.Split（'。'））
 member = Expression.PropertyOrField（member ?? item，memberName）; 
  

从表达式参数开始，并为路径中指定的每个成员构建一个访问器。 >

Hi I am trying to use that code to get property User.Email of customer that contains User. but its an object (User of type User) so it throws exception. what should i fix?

public static IQueryable<T> OrderByField<T>(this IQueryable<T> q, string SortField, bool Ascending)
    {
        if (!string.IsNullOrWhiteSpace(SortField))
        {
            var param = Expression.Parameter(typeof (T), "p");
            var prop = Expression.Property(param, SortField);
            var exp = Expression.Lambda(prop, param);
            string method = Ascending ? "OrderBy" : "OrderByDescending";
            Type[] types = new Type[] {q.ElementType, exp.Body.Type};
            var mce = Expression.Call(typeof (Queryable), method, types, q.Expression, exp);
            return q.Provider.CreateQuery<T>(mce);
        }
        else
        {
            return q;
        }
    }

解决方案

You need to pass a property path to the function (like "User.Email") and account for that inside, like this

public static IQueryable<T> OrderByField<T>(this IQueryable<T> source, string sortField, bool ascending)
{
    if (string.IsNullOrWhiteSpace(sortField)) return source;
    var item = Expression.Parameter(typeof(T), "item");
    Expression member = null;
    foreach (var memberName in sortField.Split('.'))
        member = Expression.PropertyOrField(member ?? item, memberName);
    var selector = Expression.Lambda(member, item);
    var method = ascending ? "OrderBy" : "OrderByDescending";
    var types = new [] { source.ElementType, selector.Body.Type };
    var expression = Expression.Call(typeof(Queryable), method, types, source.Expression, selector);
    return source.Provider.CreateQuery<T>(expression);
}

The essential part is

    var item = Expression.Parameter(typeof(T), "item");
    Expression member = null;
    foreach (var memberName in sortField.Split('.'))
        member = Expression.PropertyOrField(member ?? item, memberName);

which starts from the expression parameter and builds an accessor for each member specified in the path.

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