Passing the database value by using Session and redirect to another page

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C#

Database as follows;

Bthid  bigint (Datatype in the Database)

  Startexec table (Records as follows)

  Bthid

   718

I am passing the Bthid the database value from Startexec table using session and redirect to another page.

My code as follows;

SQl = "Select Bthid from startexec";
        BthID = TxtBthNo.Text.ToString().Trim();
        Session["Bthid"] = BthID;
        Response.Redirect("Instruction.aspx");

In database Bthid Datatype is bigint

When i run Error as follows;

Cannot implicitly convert type 'string' to 'long'

for that how can i do using csharp?

i tried several times but it is not working? please help me.'

how can i solve?

Regards,
Narasiman P