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.
但我需要首先将其分类为一个属性,其次是另一个属性,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()).
那么如何动态地放置那个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());
解决方法
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();
}
这就是你如何创建一个ThenBy链：
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,学生样本班：
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学生的属性,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可以在没有动态的情况下写入. BuildPredicate示例代码已更改.
