You should change your code like this (make no sense to execute non query an given SELECT query) to manage also the case where nothing is selected in your combo box:
private void comboBox2_SelectedIndexChanged_1(object sender, EventArgs e)
{
SqlCommand cmd = new SqlCommand();
cmd.Connection = con;
if(comboBox1.SelectedValue == null)
{
cmd.CommandText = "select * from tbmobile";
}
else
{
cmd.CommandText = "select * from tbmobile where uid=@id";
cmd.Parameters.Add("@id",SqlDbType.VarChar, 50);
cmd.Parameters["@id"]=comboBox1.SelectedValue;
}
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
dataGridView1.DataSource = dt;
dataGridView1.Refresh();
}