System.InvalidCastException：指定的强制转换无效的C＃

Question

Can you please help with this error? I followed a tutorial to make a forum for my project,however I'm getting the error stated above.Thank you for any suggestions. Error line:

Int64 Forum_Id = (Int64)GridView1.SelectedValue;

my code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Forum : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string course_Id = DropDownList1.Text;
        int ccourse_Id = Convert.ToInt32(course_Id);
        string question = TextBox1.Text;
        string posterName = TextBox2.Text;
        DateTime blog_date = DateTime.Now;
        PostForum.INSERTforum(ccourse_Id, question, posterName, blog_date);

        GridView1.DataBind();
        }



    protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Int64 Forum_Id = (Int64)GridView1.SelectedValue;


        Session["forum_Id"] = Forum_Id;


        Response.Redirect("Thread.aspx");
    }

    protected void Button2_Click(object sender, EventArgs e)
    {

    }
}

In my databse I have a table called Forum with forum_id that is Integer not null

Answer 1

尝试这个：

Int64 Forum_Id = Convert.ToInt64(GridView1.SelectedValue.ToString());

Answer 2

SelectedValue might not be an Int64. Try parsing to get the numeric value.

Int64 Forum_Id;
Int64.TryParse( GridView1.SelectedValue.ToString(), out Forum_Id);

Answer 3

You can use following line if you are sure that the selected value contains only numeric characters.

Int64 value = Int64.Parse(GridView1.SelectedValue.ToString());

Otherwise, it would throw an exception.