C# rows of multi-dimensional arrays

You don't, with a rectangular array like that. It's a single object.

Instead, you'd need to use a jagged array, like this:

// Note: new int[6][4] will not compile
int[][] foo = new int[6][];
for (int i = 0; i < foo.Length; i++) {
    foo[i] = new int[4];
}

Then you can pass each "sub"-array:

int[] least = new int[foo.Length];
for(int i = 0; i < 6; i++)
{
    least[i] = FindLeast(foo[i]);
}

Note that there's no need to pass foo[i] by reference¹, and also it's a good idea to assign local variables values at the point of declaration, when you can. (It makes your code more compact and simpler to understand.)

¹ If you're not sure about this, you might want to read my article on parameter passing in C#.

Update: As Jon Skeet rightly points out, this does not provide a reference to the row, but rather creates a new copy. If your code needs to change a row, this method doesn't work. I have renamed the method to make this clear.

Update 2: If you want to be able to edit the fields, and have the changes happen to the parent array, too, you can use the wrapper I provide in this library I maed. The resulting row foo.Row(i) is not an array, but instead implements IList, so if you need to pass an array this is not a solution, either.

This extension method will allow you to query a multi-dimensional array for rows. It should be noted that this is computationally heavy (not efficient) and if it is possible you should use a jagged array for these situations. If, however, you find yourself in a situation where you cannot use a jagged array, this might be useful.

public static T[] CopyRow<T>(this T[,] arr, int row)
{
    if (row > arr.GetLength(0))
        throw new ArgumentOutOfRangeException("No such row in array.", "row");

    var result = new T[arr.GetLength(1)];
    for (int i = 0; i < result.Length; i++)
    {
        result[i] = arr[row, i];
    }
    return result;
}

Your code can now be rewritten:

int[,] foo;
foo = new int[6,4];
int[] least;
least = new int[6];

for(int i = 0; i < 6; i++)
{
    least[i] = FindLeast(ref foo.CopyRow(i));
}