What is the motivation behind “Use Extension Methods Sparingly?”

Question

I find them a very natural way to extend existing classes, especially when you just need to \"spot-weld\" some functionality onto an existing class.

Microsoft says, \"In

Answer 1

I have a growing library of extension methods that operate on BCL classes for simplifying common problems, that look confusing at first glance. Frankly, I think Extension Methods make things a lot more readable. For instance, take an if statement testing to see if a number is between two numbers:

if (x <= right && x >= left )
{
   // Do Something
}

What if right is less than left on accident? And exactly what does that do? When the variables are simply like x and left, then its easy to undertand. But what if they are properties on a long class? The if statement can get pretty big, which obfuscates the rather simple thing you are trying to do.

So I wrote this:

public static bool Between(this T test, T minValue, T maxValue)
    where T : IComparable
{
    // If minValue is greater than maxValue, simply reverse the comparision.
    if (minValue.CompareTo(maxValue) == 1)
        return (test.CompareTo(maxValue) >= 0 && test.CompareTo(minValue) <= 0);
    else
        return (test.CompareTo(minValue) >= 0 && test.CompareTo(maxValue) <= 0);
}

Now I can improve my if statement into this:

if (x.Between(left, right))
{
   // Do Something
}

Is this "extraordinary"? I don't think so... but I do think it's a significant improvement on readability, and it allows some extra checking on the test inputs for safety.

The danger I think Microsoft wants to avoid here is people writing a ton of Extension Methods that redefine Equals and ToString.