More Fun With Linq

Say you have a class named BannedProgram and it has a collection of DayOfWeek and a string ProcessName. Now the collection of DayOfWeek is basically a way to set the days of the week it's banned. With this you want to create a collection of these BannedPrograms, each with their own names and days they are banned. Simple, I know. Next you have a list of processes that are currently running and you want to get all the processes that match the names in the BannedPrograms list AND if the current day is a banned day. First you need the day checked function:

private static Func<DayOfWeek, Boolean> dayIsToday = 
  currentDay => currentDay == DateTime.Now.DayOfWeek;

Then you need the method to get the banned processes that are currently running:

private static Process[] GetBannedProcesses(BannedProgram[] programs, Process[] processes)
{
  var processList = from process in processes
  where
  (
    from program in programs
    where program.DaysBanned.Any(dayIsToday)
    select program.ProcessName
  ).Contains(process.ProcessName)
  select process;

  return processList.ToArray();
}

What this is doing: Well if you look at this:

from program in programs
where program.DaysBanned.Any(dayIsToday)
select program.ProcessName

This is going to grab any BannedProgram that has a DayOfWeek that matches today and it will select only it's name. This will give you a list of names of the BannedProcesses that can not be played today.

var processList = from process in processes
where
(
  from program in programs
  where program.DaysBanned.Any(dayIsToday)
  select program.ProcessName
).Contains(process.ProcessName)

This checks to see if any of the currently running processes have a name that matches a name in the banned program list. And now you have a list of processes to kill. Yay. Not sure this is a big deal, just thought it was a fun example of using linq and subselects. USING???

using System;
using System.Diagnostics;
using System.Linq;
using System.ServiceProcess;
using System.Windows.Forms;

Combining Lambda Expressions

Found this post here but wanted to make a really simple example to demonstrate this. The idea is simple, take something like this:

currentItem => currentItem.BooleanMethodOne() && currentItem.BooleanMethodTwo()

but say you only want to have one clause or both. Well you could make three separate expressions, but what if you wanted to add even more later? What if you wanted to mix and match? What if you're reading thing because you watched to see what page could possibly be on the last page of a google search? Well I have answers... stolen answers. First the needed And and Or methods:

public static Expression<Func<T, Boolean>> And<T>(
   Expression<Func<T, Boolean>> expressionOne,
    Expression<Func<T, Boolean>> expressionTwo
)
{
  //Basically this is like a bridge between the two expressions.  It will take the T
  //parameter from expressionOne and apply it to expression two. So if 
  // oneItem => oneItem.OneMethod() is expressionOne
  // twoItem => twoItem.TwoMethod() is expressionTwo
  //it would be like replacing the twoItem with the oneItem so that they now point
  //to the same thing.
  var invokedSecond = Expression.Invoke(expressionTwo, expressionOne.Parameters.Cast<Expression>());
  
  //Now this is to create the needed expresions to return.  It will take both early expressions
  //and use the item from the first expression in both.
                    
  //It will look something like this:
  //currentItem => (currentItem.OneMethod And Invoke(currentItem => currentItem.TwoMethod()))
  //As you can see, it looks to be running expressionOne and then a new method that basically
  //calls expressionTwo with the same value (currentItem)
  return Expression.Lambda<Func<T, Boolean>>(
   Expression.And(expressionOne.Body, invokedSecond), expressionOne.Parameters
  );
}

public static Expression<Func<T, Boolean>> Or<T>(
Expression<Func<T, Boolean>> expressionOne, 
Expression<Func<T, Boolean>> expressionTwo
)
{
  var invokedSecond = Expression.Invoke(expressionTwo, expressionOne.Parameters.Cast<Expression>());

  return Expression.Lambda<Func<T, Boolean>>(
   Expression.Or(expressionOne.Body, invokedSecond), expressionOne.Parameters
  );
}

And here's a test for it:

String[] list;
  
list = new String[] { "a", "b", "c", "ac", "ab", "cc", "d", "dd", "dc" };

Expression<Func<String, Boolean>> stringLikeA = currentString => currentString.Contains("a");
Expression<Func<String, Boolean>> stringLikeB = currentString => currentString.Contains("b");
Expression<Func<String, Boolean>> stringLikeC = currentString => currentString.Contains("c");

Expression<Func<String, Boolean>> neededUser = And<String>(stringLikeA, stringLikeB);
list.Where(neededUser.Compile());

//a
Assert.IsTrue(list.Where(neededUser.Compile()).Count() == 1);  //ab

//a, c, ac, ab, cc, dc
neededUser = Or<String>(stringLikeA, stringLikeC);

Assert.IsTrue(list.Where(neededUser.Compile()).Count() == 6);

//ab, c, ac, cc, dc
neededUser = And<String>(stringLikeA, stringLikeB);
neededUser = Or<String>(neededUser, stringLikeC);
Assert.IsTrue(list.Where(neededUser.Compile()).Count() == 5);

USINGS!!ONEONE

using System;
using System.Linq;
using System.Linq.Expressions;
using Microsoft.VisualStudio.TestTools.UnitTesting;

Speaking of Select

So like Where and other fine Extension methods, Select allows you to either give it a lambda expression like so:

  List<String> newList = userList.Select(user => user.Name);

Or

  List<Int32> newList = userList.Select(user => user.ID);

So either you get a list of Names or IDs. What the hell do you care? Well if you are capable of breathing, you should also notice that one list is a list of strings the other a list of integers. What if you wanted a generic select method? Well for starters you would do something like this:

public List<String> SelectFromUserList(Func<User, String> selectMethod, List<User> userList)
{
  return userList.Select(selectMethod).ToList();
}

Where the select method for a user name would be something like this:

  List<String> nameList = SelectFromUserList(currentUser => currentUser.UserName, userList);

Sweet... oh wait, that only works if I want a list strings. Great if I wanted LastName, FirstName, ect but sucks if I wanted an integer ID.

  List<String> nameList = SelectFromUserList(currentUser => currentUser.UserID, userList); //BOOM

But wait, you can do that!

public List<K> SelectUserNameList<K>(Func<User, K> selectMethod, List<User> userList)
{
  return userList.Select(selectMethod).ToList();
}

Aw snap, now I can get a list of anything I want, well at least a one type, one dimensional array. By the way, this is the first step in probably a series of posts that will end up with a much cleaner way of doing this.

And some days I love programming

This is the newest edition of Linq madness. So I added an OrderBy to my lame dynamic query thing.

private static IQueryable<User> GetUserList(Expression<Func<User, Boolean>> whereClause, Expression<Func<User, String>> orderBy)
{
    var query = (from user in GetDataContext().Users
             select user).Where(whereClause).OrderBy(orderBy);

    return query;
}

Now you will notice there is a new Expression in town and it's name isn't Reggie Hammond. It is my order by Expression which uses a Func<User, String>. It took me a minute to figure out how this works. I thought it was somehow in need of the property name to order by, but in reality it is looking for a list of strings to order by. Simple. I could do something like: (Ignore the WhereLikeFirstNameLastNameUserName for now)

    return GetUserList
           (
            WhereLikeFirstNameLastNameUserName(name),
            currentUser => currentUser.UserName
           ).ToList();

But that's boring. I want to be able to pass a method that would give the correct string to orderby somewhat cleaner. In comes a method that returns an Expression. OooOoOOo.

private static Expression<Func<User, String>> SortOnUserName()
{
    return currentUser => currentUser.UserName;
}

So now it looks like:

public static IList<User> GetUserListByLikeName(String name)
{
   return GetUserList
          (
           WhereLikeFirstNameLastNameUserName(name),
           SortOnUserName()
          ).ToList();
}

Cleaner... Now what is WhereLikeFirstNameLastNameUserName? Simple, that is my where Expression just being returned in this method:

private static Expression<Func<User, Boolean>> WhereLikeFirstNameLastNameUserName(String name)
{
   return currentUser => SqlMethods.Like(currentUser.UserName, "%" + name + "%")
   || SqlMethods.Like(currentUser.FirstName, "%" + name + "%")
   || SqlMethods.Like(currentUser.LastName, "%" + name + "%");
}

By the way, SqlMethods.Like is just a built in method used only with Linq to Sql. Probably shouldn't have used it in this example but oh well. Live with it. OH NO NOT THE USINGS!!

using System;
using System.Collections.Generic;
using System.Data.Linq.SqlClient;
using System.Linq;
using System.Linq.Expressions;

I likz demz linq

So today I had this weird need to try creating a sort of dynamic linq thing in which I could query a User list but not have to write a Linq query for every method. I just wanted to make a method that would return the correct query that I wanted based on a simple "dynamic" where clause. Well part of the reason this should be easy is if you look at the Where extension method on an IEnumerable collection, you will see it needs a Func<T, K> where T is the type of the items in the list. So ding!, create a method that takes in a Func<User, Boolean> and puts it in them thar where clause. Then you could easily use a lambda expression to create the Func. At first you might thing something like this:

private static IQueryable<User> GetUserList(Func<User, Boolean> whereClause)
{
  var query = from user in Users
            where whereClause(user)
            select user;

  return query;
}

public static IList<User> GetUserListByUserName(String userName)
{
  return GetUserList(currentUser => currentUser.UserName == userName).ToList();
}

AND YOU WOULD BE WRONG! I only know this because... eh... I saw a friend of mine try this and it failed. Fact is, the Linq where doesn't take in a Func, but rather it expects and Expression. So no big deal right? Slap on an expression and go... except you can't keep the same syntax. (Trust me, my... friend tried) Being the absolute genius that I am, I offered my friend a solution, and it's slightly ugly:

private static IQueryable<User> GetUserList(Expression<Func<User, Boolean>> whereClause)
{
  var query = (from user in Users
            select user).Where(whereClause);

  return query;
}

Could be used like:

public static IList<User> GetUserList()
{
  return GetUserList(currentUser => true).ToList();
}

Or

public static IList<User> GetUserListByFirstName(String firstName)
{
  return GetUserList(currentUser => currentUser.FirstName == firstName).ToList();
}

Yeah, not the prettiest of things, but it is what it is. I'm not even sure this is useful yet. Still screwing around with it. Does cut down on code for easy queries. AND NOW FOR THE ASSEMBLIES!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;