IsNullOrEmpty vs IsNullOrWhiteSpace

At first sight, both these methods may sound like they are doing two different things. You would think the IsNullOrWhiteSpace function doesn’t check for an empty string but it actually does.

I have seen some blocks of code where they would write something like this:

if (!string.IsNullOrEmpty(someString) && !string.IsNullOrWhiteSpace(someString))
{
...
}

Actually it is quite redundant to use IsNullOrEmpty here, because IsNullOrWhiteSpace already makes an empty string check according to this reference source code at Microsoft.

The code for both are:

[Pure]
public static bool IsNullOrEmpty(String value) {
    return< (value == null || value.Length == 0);
}
 
[Pure]
public static bool IsNullOrWhiteSpace(String value) {
    if (value == null) return true;
 
    for (int i = 0; i < value.Length; i++) {
        if (!Char.IsWhiteSpace(value[i]))
            return false;
    }

    return true;
}

As you can see, if the length of the string is 0 in IsNullOrWhiteSpace, the for loop will be skipped and true will be returned. So doing another check with IsNullOrEmpty is redundant.