Detect all object are touching as one cluster

I am lost. Few days of thinking how to solve it the right way. C#. the problem is next: I have 10 blocks (as example, could be more or less) I need to know/detect, once each block touch other with its side. Bad - https://imgur.com/jrgaEUM in that case there is 3 groups of block, 4,1 and 5 block in each group. there is a connection but 1 block doesn't have any. and 2 different groups are not touching.

Good - https://imgur.com/IulkujR in this example i have only 1 group and all blocks are touching.

any idea what is the most efficient way to solve it?