Question

I am trying to build an expression for sorting, and i wrote code that sorts my list using one property.

But I need to sort it firstly by one property, secondly by another property and so on.

I mean I want to build an expression that will implement something like that: students.OrderBy(fistExpression.Compile()).ThenBy(secondImpression.Complie()).ThenBy(thirdExpression.Compile()).

So how to dynamically put that ThenBy methods?

Here is my code:

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

My solution:

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();
}

in your expression making code is added casting to object.

That is how you can create a chain of ThenBy:

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();

Finally, Student sample class:

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

Update:

Another approach is using attributes to mark properies from your Student to use them in OrderBy and ThenBy. Like:

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; }
}

That is how you can build sorting chain using UseInOrderByAttribute:

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();

Fix: BuildPredicate can be writen without dynamic. BuildPredicate sample code is changed.