Am create code in dblayer then display error: cannot assign to 'autostid' because it is a 'method group' how to solve

Your method name is Autostid, and you're trying to assign something to Autostid. That isn't going to work. Give your variable a different name, and don't forget to either specify the type or put var in front of it:

var result = c.Autonumber("stid", "tblstudent", "");

[Edit]

If Autonumber is a void (i.e. it doesn't return a value), you cannot assign it to a variable, and you have to call it like this:

c.Autonumber("stid", "tblstudent", "");

That is, calling the method without assigning the result to a variable.