C# Language Overview
With 8-9 years of experience, you likely know most of this intuitively. This section focuses on the conceptual explanations you'll need to articulate clearly in an interview, not just use in code.
Topics in This Section
- Async / Await — The threading model, common mistakes, and best practices
- SOLID Principles — Each principle with a real C# example
- Dependency Injection — DI in .NET Core, lifetimes, and common patterns
- Generics & LINQ — Type safety, deferred execution, and common operators
Key C# Concepts to Know Cold
Value types vs reference types: Value types (int, struct, bool, decimal) live on the stack (or inline in an object). Reference types (class, string, arrays) live on the heap with a pointer on the stack. Important for understanding equality, mutation, and GC behavior.
string vs StringBuilder: string is immutable — every concatenation creates a new string object. Use StringBuilder for building strings in loops.
IEnumerable vs IQueryable: IEnumerable executes in memory (LINQ to Objects). IQueryable translates to SQL (LINQ to SQL, EF Core) — the expression tree is sent to the database.
== vs .Equals(): For value types, both compare by value. For reference types, == compares references by default unless overridden (e.g., string overrides it). .Equals() can be overridden to compare by value.
null vs default: For reference types, default is null. For value types, default is 0/false/etc. In generics, default(T) is the correct way to get the zero value.
C# Version Features Worth Knowing
| Version | Notable Features |
|---|---|
| C# 6 | String interpolation, null-conditional ?., nameof |
| C# 7 | Tuples, pattern matching, out variables |
| C# 8 | Nullable reference types, async streams, switch expressions |
| C# 9 | Records, init-only properties, top-level statements |
| C# 10 | Global usings, file-scoped namespaces, record structs |
| C# 11 | Required members, raw string literals |
| C# 12 | Primary constructors, collection expressions |