How to store value to bigint from long

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C#

SQL-Server-2008

In database table i have set field as bigint, from c# coding i am sending the value as long datatype (using edmx) , if i use static value 1 then its getting stored, by using the following format unable to store the value such as

long _Storevar= convert.int64(dropdownlist1.SelectedValue);
(this _Storevar passing to the class properties created by edmx from there to database)
Note : In edmx table bigint represents as Int64(checked in properties). where the problem occurs..? (my operating system is in 32 bit).. please give me suggestion about where i have done mistake?