Top Mistakes of Beginner C# Developers: From A to Z

Top Mistakes of Beginner C# Developers: From A to Z

Transitioning to C# and the .NET platform is an exciting journey. However, as with any language, beginners often step on the same rake. Some of these mistakes lead to hard-to-find bugs, others to performance degradation. In this article, we will break down the most common mistakes of beginner C# developers, from incorrect use of types to problems with asynchrony. Knowing these pitfalls will help you write cleaner, safer, and more efficient code.

1. Ignoring NullReferenceException and Lack of Checks

This is perhaps the most common mistake every beginner encounters. You call a method or property on an object that is actually null, and the application crashes with NullReferenceException.

Problem: The developer assumes the method always returns an object, or does not check the result of a call.

Example of incorrect code:

public string GetCustomerName(int id)\n{\n    var customer = _repository.FindById(id);\n    // If customer == null, the next line will crash\n    return customer.FullName.ToUpper();\n}

How to fix: Use a conditional operator or the ? operator (null-conditional).

public string GetCustomerName(int id)\n{\n    var customer = _repository.FindById(id);\n    // Safe call: if customer is null, null is returned\n    return customer?.FullName?.ToUpper() ?? "Unknown customer";\n}

Tip: In C# 8.0 and above, use Nullable Reference Types. This will allow the compiler to warn you about potential null values at compile time.

2. Confusion Between Value Types and Reference Types (ref/out)

Many beginners do not understand the difference between passing by value and by reference. This is especially true for the ref and out modifiers.

Problem: You think that passing an object to a method changes the original variable, but this is not always the case. If you assign a new object to the parameter, the original variable will not change (unless you use ref).

Example:

public void ChangeName(string name)\n{\n    name = "New name"; // This will NOT change the original variable!\n}\n\nstring myName = "Old name";\nChangeName(myName);\nConsole.WriteLine(myName); // Output: "Old name"

How to fix: Use ref or out if you really need to change the reference.

public void ChangeName(ref string name)\n{\n    name = "New name";\n}\n\nstring myName = "Old name";\nChangeName(ref myName);\nConsole.WriteLine(myName); // Output: "New name"

Important: out is used when the method must assign a value to the parameter (before exiting the method). ref is used when the variable is already initialized before the call.

3. Incorrect Use of LINQ (Multiple Enumerations)

LINQ is a powerful tool, but it can severely impact performance if you don't understand how deferred execution works.

Problem: Calling .Where() or .Select() does not execute the query immediately. Each time you iterate over the result (e.g., in a foreach loop), the query is executed again.

Example of incorrect code:

var numbers = Enumerable.Range(1, 1000);\nvar filtered = numbers.Where(n => n % 2 == 0);\n\n// First pass — query executes\nint count = filtered.Count();\n\n// Second pass — query executes AGAIN (another time)\nforeach (var num in filtered)\n{\n    Console.WriteLine(num);\n}

How to fix: Materialize