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

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.