Extension methods vs Static utilities class

Both extension methods and static utility classes serve a similar goal of providing utility functions that perform specific tasks on a type. The choice between using an extension method or a static utility class depends on the context and the requirements of your task. Here are some comparison points:
Extension Methods
Usage: Extension methods allow you to "extend" a class with new methods, without modifying it or creating a new derived type. They can be called as if they were instance methods on the class.
public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }
}
// Usage
string input = "jetbrains";
bool result = input.IsNullOrEmpty();
Cohesion: They allow better cohesion, as you can create sets of methods that are logically related to the type they extend.
Limitation: Extension methods can only be static methods, you cannot use instance method or property.

Static Utility Class
Usage: A static utility class is a "container" for methods that operate on a certain type, requiring you to specify the type each time you call the method.
public static class StringUtils
{
    public static bool IsNullOrEmpty(string str)
    {
        return string.IsNullOrEmpty(str);
    }
}
// Usage
string input = "jetbrains";
bool result = StringUtils.IsNullOrEmpty(input);
Versatility: They do not extend a specific type, providing more flexibility for methods that may operate on multiple types.
Clarity: It is clear that we are not operating on an instance, but using a utility function.
In conclusion, neither approach is inherently better than the other, it depends on your exact needs. Extension methods can lead to more readable code, especially if your methods logically belong to the type they operate on. Static utility classes might be easier to understand and handle, since they function much like traditional classes. Consider the nature and requirements of your task when deciding whether to use extension methods or a static utility class.

Note: Another useful resource
https://stackoverflow.com/questions/4646328/extension-methods-vs-static-utility-class