Why ByRef gives exception while passing to unmanaged code?

Question

This is my C++ unmanaged dll code.

void Only(char *a)
{
    *a++ = 'p';
    *a++ = 'r';
    *a++ = 'a';
    *a++ = 'n';
    *a++ = 'i';
    *a++ = 0;
}

.def file is used for exporting symbols.

I have used Only function in VB.Net application as follows,

Declare Sub Only Lib "dllproj2.dll" Alias "Only" (b As StringBuilder)
....
Dim s As StringBuilder = New StringBuilder()
Only(s)

It works fine. But when I try to use b as ByRef as follows,

Declare Sub Only Lib "dllproj2.dll" Alias "Only" (ByRef b As StringBuilder)

It gives exception shown in image.

I do not understand why. Doesn't ByRef is more appropriate here?

Thanks.

Answer 1

It must be ByVal. The function will not overwrite the reference to StringBuilder itself. It will rather update the content of the StringBuilder. (See here for a similar question.)