What is the equivalent of bigint in C#?

That corresponds to the long (or Int64), a 64-bit integer.

Although if the number from the database happens to be small enough, and you accidentally use an Int32, etc., you'll be fine. But the Int64 will definitely hold it.

And the error you get if you use something smaller and the full size is needed? A stack overflow! Yay!

Int64 maps directly to BigInt.

Source

I just had a script that returned the primary key of an insert and used a

SELECT @@identity

on my bigint primary key, and I get a cast error using long - that was why I started this search. The correct answer, at least in my case, is that the type returned by that select is NUMERIC which equates to a decimal type. Using a long will cause a cast exception.

This is one reason to check your answers in more than one Google search (or even on Stack Overflow!).

To quote a database administrator who helped me out:

... BigInt is not the same as INT64 no matter how much they look alike. Part of the reason is that SQL will frequently convert Int/BigInt to Numeric as part of the normal processing. So when it goes to OLE or .NET the required conversion is NUMERIC to INT.

We don't often notice since the printed value looks the same.