Question

For example this encryption function could be called a Utility ?

public static string HashText(string text)
{
    byte[] encodedBytes;
    using (var md5 = new MD5CryptoServiceProvider())
    {
        var originalBytes = Encoding.UTF8.GetBytes(text);
        encodedBytes = md5.ComputeHash(originalBytes);
    }
    return Encode(encodedBytes);
}

While this other function would be a Helper ?

public static string Encode(byte[] byteArray)
{
    if (byteArray == null || byteArray.Length == 0)
        return "";

    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < byteArray.Length; i++)
    {
        sb.Append(byteArray[i].ToString("x2"));
    }
    return sb.ToString();
}

How about a Extension could you provide any simple examples of when I should turn something into a extension of a method ?

Answer 1

When to make a method a static helper method or an extension method is a subjective decision. If you need help deciding whether or not to make a method an extension method, then ask yourself whether it makes sense to include that method in the other class as a normal instance method.

For example, you might have a method like this:

public class Customers
{
    public static Customer GetCustomer(string name)
    {
        // implementation
        return customer;
    }
}

Does this make sense as an instance method of the String class?

public class String
{
    public Customer GetCustomer()
    {
        // implementation
        return customer;
    }

    // other methods
}

Design-wise this polutes the String class with a completely unrelated class. It makes the String class a “kitchen sink” that collects anything and everything remotely related to strings.

On the other hand, suppose you have method like this:

public class StringHelper
{
    public static string[] Split(string s, string separator)
    {
            // implementation
            return array;
    }
}

Does this make sense as an instance method of the String class?

public class String
{
    public string[] Split(string separator)
    {
            // implementation
            return array;
    }

    // other methods
}

Absolutely! This an natural extension to the existing Split functionality. It could have been included in the original implementation of String.

Everything in between can be judged on a case-by-case basis.