TicTacToe Board Check

I have written a code in C# that checks whether the user have won the TicTacToe game. I feel that maybe there is a more elegant way to check it, but I couldn't find one. Would like your help.

public static bool CheckWinnings(string[,] gameboardsquares)
{
bool iscontinue = true;

//rows;
if (gameboardsquares[0,0] == gameboardsquares[0,1] && gameboardsquares[0,1] == gameboardsquares[0,2] && gameboardsquares[0,0] != " ")
{
iscontinue = false;
}
else if (gameboardsquares[1,0] == gameboardsquares[1,1] && gameboardsquares[1,1] == gameboardsquares[1,2] && gameboardsquares[1,0] != " ")
{
iscontinue = false;
}
else if (gameboardsquares[2,0] == gameboardsquares[2,1] && gameboardsquares[2,1] == gameboardsquares[2,2] && gameboardsquares[2,0] != " ")
{
iscontinue = false;
}

//columns;
else if (gameboardsquares[0,0] == gameboardsquares[1,0] && gameboardsquares[1,0] == gameboardsquares[2,0] && gameboardsquares[0,0] != " ")
{
iscontinue = false;
}
else if (gameboardsquares[0,1] == gameboardsquares[1,1] && gameboardsquares[1,1] == gameboardsquares[2,1] && gameboardsquares[0,1] != " ")
{
iscontinue = false;
}
else if (gameboardsquares[0,2] == gameboardsquares[1,2] && gameboardsquares[1,2] == gameboardsquares[1,2] && gameboardsquares[0,2] != " ")
{
iscontinue = false;
}

//diagonals;
else if (gameboardsquares[0,0] == gameboardsquares[1,1] && gameboardsquares[1,1] == gameboardsquares[2,2] && gameboardsquares[0,0] != " ")
{
iscontinue = false;
}
else if (gameboardsquares[0,2] == gameboardsquares[1,1] && gameboardsquares[1,1] == gameboardsquares[2,0] && gameboardsquares[0,2] != " ")
{
iscontinue = false;
}

return iscontinue;
}

• (nested loops & index arithmetic - please to not present cut&dried solutions in answers without spoil retarder (>!)) – greybeard May 4 at 9:02