Top Mistakes of Beginner C# Developers: Advanced Level

Top Mistakes of Beginner C# Developers: Advanced Level

The transition from writing simple console applications to creating reliable, scalable, and performant solutions in C# is a challenge. Many developers, having mastered the syntax, encounter "pitfalls" that turn code into a source of bugs and performance degradation. In this article, we will analyze 4 key mistakes that beginner C# developers make at an advanced level and show how to avoid them.

1. Incorrect Work with Asynchrony (async/await)

One of the most common problems is misunderstanding the async/await model. A typical mistake is blocking asynchronous code using .Result or .Wait(). This not only kills performance but can also lead to a deadlock.

Example of a mistake:

public class BadAsyncExample{    public string GetData()    {        // Block the thread, waiting for the result of an asynchronous operation        return GetDataAsync().Result; // Deadlock!    }

    private async Task<string> GetDataAsync()    {        await Task.Delay(1000);        return "Data";    }}

Correct approach:

public class GoodAsyncExample{    public async Task<string> GetDataAsync()    {        await Task.Delay(1000);        return "Data";    }

    // The calling code must be asynchronous from start to finish    public async Task ProcessAsync()    {        var data = await GetDataAsync();        Console.WriteLine(data);    }}

Use ConfigureAwait(false) in library code to avoid capturing the synchronization context and reduce the risk of deadlocks.

2. Overusing LINQ and Performance Loss

LINQ is a powerful tool, but its incorrect use can lead to multiple iterations over a collection and performance degradation. The main mistake is misunderstanding deferred execution.

Example of a mistake:

var numbers = Enumerable.Range(1, 1000);

// Filter the collectionvar filtered = numbers.Where(n => n % 2 == 0);

// Multiple iterationsvar count = filtered.Count();   // 1st passvar sum = filtered.Sum();       // 2nd passvar list = filtered.ToList();   // 3rd pass

Each call to Count(), Sum(), or ToList() re-executes the entire LINQ pipeline. This is especially critical when working with databases (Entity Framework), where each pass is a new SQL query.

Correct approach:

var numbers = Enumerable.Range(1, 1000);

// Materialize the result oncevar filtered = numbers.Where(n => n % 2 == 0).ToList();

// Now work with the ready listvar count = filtered.Count;var sum = filtered.Sum();

Always materialize the result of a LINQ query (ToList(), ToArray()) if you plan to use it more than once.

3. Ignoring Null Safety and NullReferenceException

NullReferenceException is an eternal companion of the C# developer. With the advent of nullable reference types in C# 8.0, we have tools to combat this problem, but many beginners ignore them.

Example of a mistake:

public class User{    public string Name { get; set; } // Can be null!}

public void PrintUserName(User user){    Console.WriteLine(user.Name.Length); // NullReferenceException if user == null or Name == null}

Correct approach:

#nullable enablepublic class User{    public string Name { get; set; } = string.Empty; // Do not allow null    public string? MiddleName { get; set; } // Explicitly indicate that it can be null}