How not to use Static classes in C#

Source: Dev.to

The common trait of static classes and methods

Static classes and methods are often used as utility classes, providing globally accessible functions throughout an application. For instance, instead of repeatedly writing code to convert ‘Y’ and ‘N’ to true or false, you can place this logic in a static utility class and reuse it wherever needed, following the Don’t Repeat Yourself principle.

Extension methods, which are also static, offer similar benefits by allowing you to add reusable functionality to existing types.

Most developers think of static classes mainly for these utility scenarios and don’t explore their full potential. Below are some examples and best‑practice guidelines.

Example: Utility static class

public static class ConverterUtils
{
    public static bool CharToBool(char c)
    {
        return c == 'Y' || c == 'y';
    }
}

bool result = ConverterUtils.CharToBool('Y');

Example: Extension method

public static class CharExtensions
{
    public static bool ToBool(this char c)
    {
        return c == 'Y' || c == 'y';
    }
}

bool result = 'N'.ToBool();

Static class: a general overview

A static class is simply a class that contains only static members and has a private constructor to prevent instantiation. Declaring a class as static signals to the compiler and other developers that the class is not meant to be instantiated—its members should be accessed directly through the class name.

• No instance required – you do not need to create an object to use static members, making them ideal for global utility functions.
• Shared state (or lack thereof) – static methods cannot access instance data (non‑static fields/properties), only other static members. This makes them stateless unless static fields are used, which can introduce shared state across the application.

Best practices for using static classes and methods

• Keep static classes stateless – avoid storing data or tracking state within them. Stateless static classes are efficient and safe because they operate solely on input parameters and return results without side effects.
  Examples in the .NET Framework include System.Console and System.Math; methods like Console.WriteLine or Math.Sqrt take inputs and produce outputs without internal state.
• Ideal scenarios – formatting dates, converting images, performing calculations, or any situation where you need quick, reusable logic without maintaining global or shared data.

In summary, to use static classes effectively, ensure all inputs are provided as method arguments and avoid storing any state within the class.

Watch out for static data

Static variables are shared across all threads and instances, which makes predicting their values difficult—any change affects the single shared memory location. If static methods depend on or modify static data, you might get inconsistent results, especially in multithreaded environments.

public static class Calculation
{
    private static int totalCount;
    public static void GetCount()
    {
        totalCount = 0;
        for (int i = 0; i < 10; i++)
        {
            totalCount += i;
            Thread.Sleep(100);
        }
        Console.WriteLine("TOTAL COUNT : {0}", totalCount);
    }
}

Running GetCount on multiple threads can produce unexpected results because all threads share the same totalCount variable. Static methods are not thread‑safe by default; you must handle synchronization yourself. See K. Scott Allen’s article on Static and Thread Safety for more details.

Downsides of static classes

• Tight coupling – static classes can make your code tightly coupled and harder to test.
• Hidden dependencies – methods may rely on static data set elsewhere, making the order of operations important and potentially causing unexpected results.

public static class TaxStructure
{
    public static int TaxRate { get; set; }
}

public static class CalculateTax
{
    public static void ShowTax()
    {
        Console.WriteLine(TaxStructure.TaxRate);
    }
}

You must set TaxStructure.TaxRate before calling CalculateTax.ShowTax(). If you don’t, or you call them in the wrong order, the output may be incorrect. This hidden dependency can make the code harder to maintain and test.

Further reading

• Static Methods are Death to Testability
• Avoid Utility Classes