构建具有多重排序的表达式

Question

我正在尝试构建一个用于排序的表达式，并且我编写了使用一个属性对列表进行排序的代码。

但我需要先按一个属性排序，然后再按另一个属性排序，依此类推。

我的意思是我想构建一个表达式来实现类似的东西：students.OrderBy(fistExpression.Compile()).ThenBy(secondImpression.Complie()).ThenBy(thirdExpression.Compile())。

那么如何动态放置ThenBy 方法呢？

这是我的代码：

Type studentType = typeof(Student);
ParameterExpression studentParam = Expression.Parameter(studentType, "x");
MemberInfo ageProperty = studentType.GetProperty("Age");
MemberExpression valueInNameProperty = 
     Expression.MakeMemberAccess(studentParam, ageProperty);
Expression<Func<Student, int>> orderByExpression =
     Expression<Func<Student, int>>.Lambda<Func<Student, int>>(valueInNameProperty, studentParam);
var sortedStudents = students.OrderBy(orderByExpression.Compile());

Answer 1

我的解决方案：

public static Func<Student, object> BuildPredicate(string propertyName)
{
    Type studentType = typeof(Student);
    ParameterExpression studentParam = Expression.Parameter(studentType, "x");
    MemberInfo ageProperty = studentType.GetProperty(propertyName);
    MemberExpression valueInNameProperty = Expression.MakeMemberAccess(studentParam, ageProperty);
    UnaryExpression expression = Expression.Convert(valueInNameProperty, typeof (object));
    Expression<Func<Student, object>> orderByExpression = Expression.Lambda<Func<Student, object>>(expression, studentParam);
    return orderByExpression.Compile();
}

在您的表达式制作代码中添加了转换为object。

这就是你可以创建ThenBy链的方法：

var sortedStudents = students.OrderBy(BuildPredicate("Age"));
foreach (var property in typeof(Student).GetProperties().Where(x => !String.Equals(x.Name, "Age")))
{
    sortedStudents = sortedStudents.ThenBy(BuildPredicate(property.Name));
}

var result = sortedStudents.ToList();

最后，Student 示例类：

public class Student
{
    public int Age { get; set; }
    public string Name { get; set; } 
}

更新：

另一种方法是使用属性来标记Student 中的属性，以便在OrderBy 和ThenBy 中使用它们。喜欢：

public class Student
{
    [UseInOrderBy]
    public int Age { get; set; }

    [UseInOrderBy(Order = 1)]
    public string Name { get; set; } 
}

[AttributeUsage(AttributeTargets.Property)]
internal class UseInOrderByAttribute : Attribute
{
    public int Order { get; set; }
}

这就是你可以使用UseInOrderByAttribute构建排序链的方法：

Type studentType = typeof (Student);
var properties = studentType.GetProperties()
                            .Select(x => new { Property = x, OrderAttribute = x.GetCustomAttribute<UseInOrderByAttribute>() })
                            .Where(x => x.OrderAttribute != null)
                            .OrderBy(x => x.OrderAttribute.Order);

var orderByProperty = properties.FirstOrDefault(x => x.OrderAttribute.Order == 0);
if (orderByProperty  == null)
    throw new Exception("");

var sortedStudents = students.OrderBy(BuildPredicate(orderByProperty.Property.Name));
foreach (var property in properties.Where(x => x.Property.Name != orderByProperty.Property.Name))
{
    sortedStudents = sortedStudents.ThenBy(BuildPredicate(property.Property.Name));
}

var result = sortedStudents.ToList();

修复：BuildPredicate 可以在没有dynamic 的情况下写入。 BuildPredicate 示例代码已更改。

Answer 2

我假设您有想要排序的私有属性。 例如，如果你有这个类：

public class Student 
{
    public Student (int age, string name)
    {
        Age = age;
        Name = name;
    }

    private string Name { get;  set; }

    public int Age { get; set; }

    public override string ToString ()
    {
        return string.Format ("[Student: Age={0}, Name={1}]", Age, Name);
    }
}

您可以使用以下方法构建将获取公共和私有属性的表达式：

public static Func<TType, TResult> CreateExpression<TType, TResult>(string propertyName) 
{
    Type type = typeof(TType);
    ParameterExpression parameterExpression = Expression.Parameter(type, propertyName);
    MemberInfo property = type.GetProperty(propertyName, BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
    MemberExpression valueInProperty =  Expression.MakeMemberAccess(parameterExpression, property);

    return Expression.Lambda<Func<TType,TResult>>(valueInProperty, parameterExpression).Compile();
}

使用示例：

        var students = new [] { 
            new Student(20, "Ben"), 
            new Student(20, "Ceasar"),
            new Student(20, "Adam"),
            new Student(21, "Adam"),
        };

        var sortedStudents = students
            .OrderBy(CreateExpression<Student, string>("Name"))
            .ThenBy(CreateExpression<Student, int>("Age"));

        sortedStudents.ToList().ForEach(student => Console.WriteLine(student)); 
        /*
        Prints:
        [Student: Age=20, Name=Adam]
        [Student: Age=21, Name=Adam]
        [Student: Age=20, Name=Ben]
        [Student: Age=20, Name=Ceasar]
         */