What is the correct way to set StringBuilder.Capacity when using P/Invoke?

Question

Should StringBuilder.Capacity be set to the maximum number of .NET characters, without regards to null termination, or must it be set one higher to reserve space for a null terminator when using P/Invoke.

The natural reaction is that it should be set one higher, but it seems like P/Invoke is supposed to automatically compensate. In fact this is actually documented right here: http://msdn.microsoft.com/en-US/library/s9ts558h(v=VS.100).aspx

The reason for this question is that most examples are not strictly consistent with the above documentation. Almost always they are coded:

StringBuilder sb = new StringBuilder(dotNetChars + 1);
SomeWindowsAPI(sb, sb.Capacity);

Instead of:

StringBuilder sb = new StringBuilder(dotNetChars);
SomeWindowsAPI(sb, sb.Capacity + 1);

(I realize that some APIs handle the buffer size parameter differently. Assume that the API handles this the must common way, like GetFullPathName: http://msdn.microsoft.com/en-us/library/aa364963(v=VS.85).aspx)

Using an expression with sb.Capacity directly in the API call seems to be a best practice to avoid a mismatch. The issue is whether or not adding the +1 is correct.

Look around. You'll probably find that the only place showing sb.Capacity + 1 is the MSDN documentation.

Of course, one can allocate on the side of caution with a larger buffer than is strictly necessary, but I would like to know the consensus on how to do this.