Input string was not in a correct format.

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C#

ASP.NET

Ajax

when I debug this then there is the error "INPUT STRING WAS NOT IN CORRECT FORMAT"
CODE PATH IS --
protected void ddl_GroupName_SelectedIndexChanged(object sender, EventArgs e)
{
int aa = Convert.ToInt32(ddl_GroupName.SelectedValue);
bind_linkedVideo(aa);
}

BELOW IS THE ALL CODE-
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protected void Page_Load(object sender, EventArgs e)
{
if (Session["userid"] != null && Session["userid"].ToString() != string.Empty)
{
if (!IsPostBack)
{
opencms();
bind_audioFile();
bind_linkedVideo(0);
}
}
else
{
Response.Redirect("login.aspx");
}
}

protected void btnUploadCancel_Click(object sender, EventArgs e)
{
ddl_GroupName.SelectedIndex = 0;
ddl_Video.SelectedIndex = 0;
}

public void bind_group()
{
SqlDataAdapter da = new SqlDataAdapter("select group_id, group_title from VideoCategory", con);
DataSet ds = new DataSet();
da.Fill(ds);
if (ds != null && ds.Tables[0].Rows.Count > 0)
{
ddl_GroupName.DataSource = ds.Tables[0];
ddl_GroupName.DataTextField = "group_title";
ddl_GroupName.DataValueField = "group_id";
ddl_GroupName.DataBind();
ddl_GroupName.Items.Insert(0, "Please select");
}
}

public void bind_linkedVideo(int group_id)
{
SqlDataAdapter da = new SqlDataAdapter("select video_id, Video_name from UploadVideo where group_id = " + group_id + " and type = 1", con);
DataSet ds = new DataSet();
da.Fill(ds);
if (ds != null && ds.Tables[0].Rows.Count > 0)
{
ddl_Video.DataSource = ds.Tables[0];
ddl_Video.DataTextField = "Video_name";
ddl_Video.DataValueField = "video_id";
ddl_Video.DataBind();
ddl_Video.Items.Insert(0, "Please select");
}
}

protected void ddl_audiofile_SelectedIndexChanged(object sender, EventArgs e)
{
bind_group();
}

protected void ddl_GroupName_SelectedIndexChanged(object sender, EventArgs e)
{
int aa = Convert.ToInt32(ddl_GroupName.SelectedValue);
bind_linkedVideo(aa);
}