Dynamic build lambda expressions

Question

So, I started to build a small test application to test lambda expressions. I found several examples here and elsewhere but I just don't get them.

Can anybody explain my how to build an expression by using textboxes or any other variables?

My Test List

        List<People> lPeople = new List<People> 
        {
            new People { Name= "Jean", LastName = "Borrow", Age= 21 } ,
            new People { Name= "Dean", LastName = "Torrow", Age= 20 }
        };

Working lambda Expression

        IEnumerable<People> result = lPeople.Where(p => p.Age < 21);
        dgv_1.DataSource = result.ToList();
        dgv_1.Update();

How can I build the expressions dynamically?

Something like lPeople.Where(p => p.LastName == Textbox.Text); (which of course doesn't work)

Thanks!

Edit: Added some code to the solution below

        Int32 iAge;
        Boolean bSuc = Int32.TryParse(tb_filter_age.Text, out iAge);
        if (!bSuc)
        {
            iAge = 0;
        }

Answer 1

"which of course doesn't work"

What happens when you try it? By the look of it, that's the kind of thing I do all the time.

To switch operations based on a ComboBox specifying the operator:

int age = int.Parse(textBoxAge.Text);

IEnumerable<People> result;
if (comboBoxOperator.Text == "=")
    result = lPeople.Where(p => p.Age == age);
else if (comboBoxOperator.Text == "<")
    result = lPeople.Where(p => p.Age < age);
else
    result = lPeople.Where(p => p.Age > age);

dgv_1.DataSource = result.ToList();
dgv_1.Update();

The code that converts the age string into an int will throw if the user enters something that can't be converted. Look up TryParse to avoid exceptions.

Answer 2

You can use the Linq Dynamic Query Library to accomplish this. See the following blog post from Scott Guthrie for more information:

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

Answer 3

Try the Predicate Builder at http://www.albahari.com/nutshell/predicatebuilder.aspx

I used it to make an advanced search where the user could keep adding optional search criteria.

Answer 4

Your example lambda expression will work. How dynamic do you need it to be? If you have a static UI of 'filters' to apply to a collection, you can create code similar to the following:

IEnumerable<People> result = lPeople;
if (txtLastName.Text.Trim().Length != 0) 
    result = result.Where(p => p.LastName == txtLastName.Text); 
if (chkSeniors.Checked) 
    result = result.Where(p => p.Age >= 65);
dgv_1.DataSource = result.ToList();
dgv_1.Update();

If you want the consumer of your data source to apply truly dynamic expressions (afford them the ability to choose other fields to filter and the expressions to use), that's a more complicated feature to implement using a predicate builder tool or LINQ Expression objects.

Answer 5

There should be nothing wrong with the way you're going about it. I have created a simple Windows Forms Application with a TextBox, a Button, and a DataGridView (with names textBox1, button1, and dgv_1 respectively.)

Here is the code I used for the Form1.cs file that worked as expected:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        List<People> lPeople = new List<People> 
        {
            new People { Name= "Jean", LastName = "Borrow", Age= 21 } ,
            new People { Name= "Dean", LastName = "Torrow", Age= 20 }
        };

        IEnumerable<People> result = lPeople.Where(p => p.Name == textBox1.Text);

        dgv_1.DataSource = result.ToList();
        dgv_1.Update();
    }
}    

public class People
{
    public string Name { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }
}