未能启用约束.一个或多个行包含违反非null、唯一或外键约束的值.asp.net c#

我必须从两个文本框中获取开始日期和结束日期，根据该日期，我必须从数据库中获取数据并显示在网格视图中。

protected void Button1_Click(object sender, EventArgs e)
{    
    String str = DateTime.Now.ToString(TextBox1.Text);
    DateTime tm = Convert.ToDateTime(str);
    String str2 = DateTime.Now.ToString(TextBox2.Text);
    DateTime tm2 = Convert.ToDateTime(str2);
    solarDataSet2TableAdapters.tbl_energy_reportTableAdapter state;
    state = new solarDataSet2TableAdapters.tbl_energy_reportTableAdapter();
    DataTable dt = new DataTable();  
    dt = state.GetGridDataBy(DropDownList1.SelectedValue,DropDownList2.SelectedValue,  
    str, str2);  // Failed to enable constraints. One or more rows contain values  
    //violating non-null, unique, or foreign-key constraints. asp.net c#  
    GridView1.DataSource = dt;
    GridView1.DataBind();
}