Token concatenation using ## for Array value error

I am just using the "Token concatenation with ## " for getting each array.
Some reference i have taken from https://www.ibm.com/docs/ja/rdfi/9.6.0?topic=definitions-multiple-function-declarations-only

Below is the snippet from my code

macro:

#define Columnsize(x) x##[0]
#define get_arrsize(x) sizeof(x)/sizeof(Columnsize(x))

Code:

uint8 test_array[4][7]= {"ok","me","hello","hi"};
get_arrsize(test_array)

After compilations, I am getting below error

error: pasting ")" and "[" does not give a valid preprocessing token
    | #define Columnsize(x) (x)##[0]

Any suggestion how to solve this error? Why this is failed?

> Any suggestion how to solve this error

Remove ##

> Why this is failed?

Because x[ is not a (single) valid token.

The issue is with the way tokenization works. The gcc documentation has a page about tokenization and concatenation from which I glean the following:

The preprocessor separates the source code into tokens which are roughly equivalent to the C compiler tokens (e.g. literals, identifiers or operators). These tokens can be combined with the ## operator. The result of the concatenation must in turn be a token (for example, an identifier-shaped thing, a number, an operator), which is not the case here, which is why the compiler complains.

The key is that the character sequence [0] is separated into the three-token sequence [, 0, ]. The concatenation operator uses only [ as its right hand side operator; the result x[ is not a token.

As a counter-example, the following works:

#define MAKE_ASSIGN_OP(x) x##=
int main() { int x=1; return x MAKE_ASSIGN_OP(+) 1; }

Both + and = as well as their concatenation += are valid tokens.

As an example