Select statement: Invalid column name, in C# using Dapper [closed]

Here is the model class for my database

public class Person
{
    public int id { get; set; }

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string PhoneNumber { get; set; }
    public string Fullinfo 
    { 
        get
        {
            return FirstName + " " + LastName + " " + Email;

        }
    }
}

And here is the main class

public partial class DashBoard : Form
{
    List<Person> persons =new List<Person>();
    public DashBoard()
    {
        InitializeComponent();
        Updated();
    }

    public void Updated()
    {
        PeopleFoundListbox.DataSource = persons;
        PeopleFoundListbox.DisplayMember = "Fullinfo";
    }

    public void Searchpeoplebutton_Click(object sender, EventArgs e)
    {
        persons = Getpeople(Lastnametextbox.Text);
        Updated();

    }

    public List<Person> Getpeople(string searchname)
    {        
        using (IDbConnection connection = new System.Data.SqlClient.SqlConnection(Helper.Cnn("peopleDb"))) 
        {
            var output = connection.Query<Person>("select * from People where Lastname= " + searchname).ToList();
            return output;
        }         
    }
}

I want to select records based on last name I have a row having david as last name but when ever I type david in textbox and search I get this error

Invalid column name david

Answer

deleted the answer since it creates a risk of sql injection ; see the other answer instead.