Building LINQ query from parameters - asp.net

Question

Consider a simple Employee class.
Class Employee
{
public String FirstName {get; set;}
public int Id {get; set;}
public int Marks {get; set;}
public String LastName {get; set;}
}
So for selecting by LINQ query we can write.
var query = Employee.Where(i => i.Id > 2).OrderBy(i => i.Marks);
So can we create a function which takes 2 parameter where we can send Id and Marks.
So that i can make a function call and pass parameter what ever i need like.
var query = Employee.Where(i => i.Marks > 2).OrderBy(i => i.FirstName);
Sample looking function where we can pass any parameter
Public String GetQuery(String para1,String para2,......)
{
var query = Employee.Where(i => i.para1 > 2).OrderBy(i => i.para2);
return query;
}
or
Public String GetQuery(String para1,String para2,......)
{
String str1=para1...... // with some format included
String str2=para2...... // with some format included
var query = Employee.Where(str1).OrderBy(str2);
return query;
}
The concept is that i want to create a common(generic) query in which i can select values
of any type of parameter passed.

Answer 1

A type safe solution could use delegates instead of strings:
IEnumerable<Employee> employees = getExampleData();
IEnumerable<Employee> example1 = Query.Employees(employees, Query.UseID, Query.UseMarks);
IEnumerable<Employee> example2 = Query.Employees(employees, Query.UseMarks, Query.UseFirstName);
I made a helper class Query to wrap the functionality:
static class Query
{
public static int UseID(Employee employee, int i) { return employee.Id; }
public static int UseMarks(Employee employee, int i) { return employee.Marks; }
public static string UseFirstName(Employee employee, string s) { return employee.FirstName; }
public static string UseLastName(Employee employee, string s) { return employee.LastName; }
static public IEnumerable<Employee> Employees(IEnumerable<Employee> employees, func_returnInt where, func_returnInt orderby)
{
return employees.Where(i => where(i, 0) > 2).OrderBy(i => orderby(i, 0));
}
static public IEnumerable<Employee> Employees(IEnumerable<Employee> employees, func_returnInt where, func_returnString orderby)
{
return employees.Where(i => where(i, 0) > 2).OrderBy(i => orderby(i, ""));
}
}
public delegate int func_returnInt(Employee employee, int i);
public delegate string func_returnString(Employee employee, string s);

Answer 2

You have to use Expression : LambdaExpression.
http://msdn.microsoft.com/en-us/library/bb335710.aspx.
In my code i have used something similar:
private User GetUser(Expression<Func<User, bool>> query)
{
User user = context.Users.Where(query).FirstOrDefault();
if (user == null)
{
throw new ProviderException("The supplied user name could not be found.");
}
return user;
}
User user = GetUser(u => u.Marks == 123);
User user = GetUser(u => u.FirstName== "abc");