如何使用带有表达式参数的扩展名？

Question

How can I use this extension?

public static IQueryable<TSource> MyOrderByType<TSource>(this IQueryable<TSource> source, bool condition, Expression<Func<TSource, string>> predicate)
{
    return condition ? source.OrderBy(predicate) : source.OrderByDescending(predicate);
}

I used it this way:

using (var ctx = new MyEntities())
{
    result = ctx.Users
       .MyOrderByType(sortType,p=>sortKey)
        .ToList();                
}

search method:

 public List<User> Search(string name, string sortKey, bool sortType)
{
    List<User> result;
     using (var ctx = new MyEntities())
            {
                result = ctx.Users
.where(u => u.name.contains(name))
.MyOrderByType(sortType, p => sortKey)
}
}
.ToList();

            }
            return result;
}

in code behind:

 bool sortType=ddlSortType.SelectedValue=="0"?true:false;
string sortKey=ddlSortKey.SelectedValue;
var lstUser = userService.Search(txtName.Text,sortType,sortKey);

but it doesn't sort the result.

What is wrong?

Answer 1

Because you didn't pass the predicate correctly. When calling your MyOrderByType extensions, you typically choose which field of User you want to sort by. Use your sortKey string to create a proper predicate. I handled cases to resolve an expression for name and email, you can easily add another fields to sort by.

var sortKey = "name"; // use your sortKey
Expression<Func<User, string>> sortExpression;
switch (sortKey)
{
    case "email":
        sortExpression = x => x.Email;
        break;
    case "name":
        sortExpression = x => x.Name;
        break;
    default:
        sortExpression = x => x.Name;
        break;
}

using (var ctx = new MyEntities())
{
    result = ctx.Users
       .MyOrderByType(sortType, sortExpression)
       .ToList();                
}

// defined below
public class User
{
    public string Name {get;set;}

    public string Email {get;set;}

    // other properties
}

public class MyEntities : DbContext 
{ 
    public DbSet<User> Users { get; set; }
}