Creating lambda expressions from the selected field

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I'm trying to filter a object list (List<item>) bounded to a grid. Now I want to filter this list according to the user required.
I have combo box to select the field to filter and text box to enter the value. From there user can select ItemCode or Cost or any property relevant to Item class. Then how can I create the lambda expression according to the selected field and entered value.

Posted 9-May-11 17:29pm

Sanyon_d

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Pong D. Panda · Answer 1 · 2011-05-09T20:02:00

Do a switch case and define your lambda expression based on your selected value.

C#

//note: update the type (sample format you want to get)
var result = new List<classnamehere>();
var filter = txtbox.Text;

switch (ddl.SelectedValue) {
   case "cost":
      //do filtering
      result = listSource.Find(s=>{s.cost == filter});
      break;
   case "itemcode":
      result = listSource.Find(s=>{s.itemcode == filter});
      break;
   default:
      result = listSource;
      break;
}</classnamehere>

Correct me if I'm wrong but I think you CANT dynamically construct lambda exp on your scenario.

Good luck!

nit_singh · Answer 2 · 2011-05-09T20:38:00

Solution 2

Try like this

This is the class on which you want to apply the filtering

public class ClassName
{
public string ItemCode { get; set; }
public string Cost { get; set; }
public override string ToString()
{
return "ItemCode " + ItemCode + ", Cost " + Cost;
}
}

This is the method to apply filtering

List<ClassName> getFiltered(List<ClassName> className, Func<ClassName, bool> func)
{
return className.Where(c => func(c)).ToList();
}

If you notice that we are passing a predicate named "func" which takes a parameter "ClassName" as input and return type is bool.

List<ClassName> i = new List<ClassName>();
i.Add(new ClassName { ItemCode = "1", Cost = "1" });
i.Add(new ClassName { ItemCode = "2", Cost = "2" });
i.Add(new ClassName { ItemCode = "3", Cost = "3" });
i.Add(new ClassName { ItemCode = "4", Cost = "4" });

List<ClassName> ClassName = getFiltered(i, t => t.Cost == txtFilterBox.Text);

Now see the second parameter "t => t.Cost == txtFilterBox.Text" which is accepting a parameter of type "ClassName" and return type is bool.

foreach (ClassName im in ClassName)
Response.Write(im.ToString());

I Think try with this, it should work for you

Posted 9-May-11 20:38pm

nit_singh

Updated 10-May-11 8:19am

v5

Comments

Wayne Gaylard 10-May-11 3:25am

There seems to be problem with the formatting of your code in your answer. There are portions missing from the Func declaration and extra tags floating all over. I would also suggest you explain how this will work as I tried to replicate your code but could not get it to compile, or even to make sense of it, to be honest.

Pong D. Panda 10-May-11 4:08am

He uses Delegates on his filtering, but its more complicated this way. It can be done by a simple .Find(lambda exp)

Wayne Gaylard 10-May-11 4:18am

It might be more complicated, but I was interested in a way of of doing this without the switch code, but this poster didn't reply or fix his answer.

Pong D. Panda 10-May-11 4:32am

His code only filters the Cost. I almost got my cpu burned trying to figure out how to do it without using switch or using a lot of OR statements inside the lambda condition based on the combobox selected.

Wayne Gaylard 10-May-11 4:40am

Yeah - I'm almost burning out my brain trying to come up with a solution. I tried using reflection, but it get's really complicated, and so the switch code ends up being the shortest route. But I don't give up - this should in theory be possible.

Pong D. Panda 10-May-11 4:44am

Good luck and be sure to attach a heatsink on your head!

Wayne Gaylard 10-May-11 4:59am

Thanks

nit_singh 10-May-11 14:17pm

sorry for the delay as i was out of the reach of internet. Now check the updated code. Still there are some problem like "Func<classname, bool> func". I improved the answer after adding explanations...But I have to remove formatting ... Thanks

yesotaso · Answer 3 · 2011-05-10T06:28:00

Reference Expression Trees[^]

C#

string filterText = "Hello";
string fieldName = "FieldName";
var type = typeof(YourClass);
ParameterExpression param = Expression.Parameter(type, "s");
var field = type.GetProperty(fieldName);
MemberExpression member = Expression.MakeMemberAccess(param,field);
ConstantExpression filter = Expression.Constant(filterText);
BinaryExpression compare = Expression.Equal(member, filter);
Expression<Func<YourClass, bool>> lambdaExpression =
    Expression.Lambda<Func<YourClass, bool>>(compare,param);

This should do it.

Creating lambda expressions from the selected field

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