Finding the floor value in an array

In terms of speed, your solution is about as optimized as it can get. In terms of readability, your solution has much room for improvement. I prefer readability over speed. You should try to make your code as readable as possible, then, if you have performance issues, search for bottlenecks using a profiler and optimize them. The method body of CalclualteFloorValue can be made into a one-liner, which happens to be slower but is much more readable.

private static int CalclualteFloorValue(int[] sortedArray, int key)
{
    return sortedArray.Last(x => x < key);
}

If you wish to keep the speed of your original solution and increase readability, there are many improvements you can make:

1. Have a consistent parameter naming convention. _sortedarray has an underscore, but the other parameter names don't. I prefer to only use underscores for variables declared within classes but not within methods.

2. _sortedarray should be lowerCamelCase. That is, sortedArray.

3. If l will always be zero and r will always be array length - 1, declare them in the method body. Don't make the method caller do unnecessary work.

4. Use descriptive variable names instead of single letters.

The beginning of the method will now look like this:

private static int CalclualteFloorValue(int[] sortedArray, int key)
{
    int left = 0;
    int right = sortedArray.Length - 1;

You use m, r and l as your variable names.

I suggest spelling the name in full, not just the starting letter, like middle, start, end (or middle, right, left)

This is actually a standard problem. If you're doing CP, heed what others have said, but if you're doing development, there's a function called lower_bound. Use that.

You're using -1 to indicate that the floor of the key is not in the array, but what if -1 is the floor of the key? Conflating indices and array elements in the return value is going to cause trouble.

The floor of x is the greatest value less than or equal to x.

Also, what happens when key < sortedArray[l]?

For the array 1, 2, 3, here are the results I would expect, and the actual results:

key    expected    actual
0      (no floor)  1
1      1           -1
2      2           1
3      3           2
4      3           3