cannot implicitly convert type string to int

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C#

private void button1_Click(object sender, EventArgs e)
{
    Staff s = new Staff();
    staffAG j = new staffAG();
    j.id = comboBox2.Text; error in this row "cannot implicitly convert type string to int" 
    j.name = textBox2.Text;
    j.age = textBox3.Text;
    j.phone = textBox4.Text;
    j.address = textBox5.Text;
    j.post = comboBox1.Text;
    s.updatestaff(j);
    MessageBox.Show("Record updated");
}

coding of my class staff is

public void updatestaff(staffAG j)
{
    int z = 0;
    cn.Open();
    cmd = new SqlCommand("update staff set name = @name,age = @age,phoneno = @phoneno,address = @address,post = @post where id = @id", cn);
    cmd.Parameters.AddWithValue("@id",Convert. ToString (j.id));
    cmd.Parameters.AddWithValue("@name",j.name);
    cmd.Parameters.AddWithValue("@age",j.age);
    cmd.Parameters.AddWithValue("@phoneno",j.phone);
    cmd.Parameters.AddWithValue("@address",j.address);
    cmd.Parameters.AddWithValue("@post",j.post);
    cmd.ExecuteNonQuery();
    cn.Close();
    {
        MessageBox.Show("Added");
    }
}

coding of my class staffAG is

class staffAG
{
int _id;
string _name;

string _age;
string _phone;
string _address;
string _post;
string _gender;
public string name
{
    get
    {
        return _name;
    }
    set
    {
        _name = value;
    }
}
public int id
{
    get
    {
        return _id;
    }
    set
    {
        _id = value;
    }
}
public string age
{
    get
    {
        return _age;
    }
    set
    {
        _age = value;
    }
}
public string phone
{
    get
    {
        return _phone;
    }
    set
    {
        _phone = value;
    }
}
public string address
{
    get
    {
        return _address;
    }
    set
    {
        _address = value;
    }
}
public string post
{
    get
    {
        return _post;
    }
    set
    {
        _post = value;
    }
}
public string gender
{
    get
    {
        return _gender;
    }
    set
    {
        _gender = value;
    }
}

why error is shown in my button coding

j.id=combobox1.text;  "cannot implicitly convert type string to int" 

please help me if an other problem with my coding for update staff also tell me...sorry for my English....