How to pass element of a list as a function parameter

Hello,

Can anybody help me please how to pass an element of a list as a parameter in a function?

I tried like this

huffman(ref x[r, i]);

static void huffman(ref int[,] number)
        {
           
        }

but I get this error

Error 1 The best overloaded method match for 'JPEG_Codecs_Lossless_Technique.Lossless.huffman(ref int[*,*])' has some invalid arguments path\Lossless.cs 178 34 JPEG Codecs–Lossless Technique

Error 2 Argument 1: cannot convert from 'ref int' to 'ref int[*,*]' path\Lossless.cs 178 46 JPEG Codecs–Lossless Technique

CAn anybody help me please what mistake I make?

Thanks