C# Programming

107 flashcards by MentisLibrary

C# for working developers: value and reference types, nullable references, classes interfaces and records, generics, the collection types, LINQ and deferred execution, async and await, exceptions, and the .NET runtime and tooling.

What does var mean in C#?

It asks the compiler to infer the variable's type from its initializer. The variable is still statically and permanently typed.

How does string interpolation work?

A string prefixed with $ may embed expressions in braces; the compiler turns them into a formatted string at run time.

const vs readonly

const is a compile-time constant baked into every call site and limited to primitives, strings and null. readonly is set once at run time, in the declaration or a constructor.

What does a switch expression give you over a switch statement?

It produces a VALUE rather than executing statements, requires every arm to be exhaustive or have a discard, and needs no break.

What does the is pattern do?

It tests the run-time type and, in the same expression, declares a typed variable that is in scope afterwards.

What is the difference between ?? and ??=?

?? returns the left operand unless it is null, in which case it returns the right. ??= assigns the right operand only when the left is currently null.

What does ?. do?

It evaluates the member access only when the left side is non-null, otherwise the whole expression is null. The rest of the chain is short-circuited.

What is the difference between out, ref and in parameters?

ref passes an existing variable by reference for reading and writing. out also passes by reference but the callee must assign it. in passes by reference for reading only.

When would you write a local function instead of a lambda?

When the helper is only used inside one method and you want a name, recursion, ref or out parameters, or no delegate allocation.

What does nameof return?

The unqualified name of the symbol as a string, resolved at compile time, so renaming the symbol updates every use.

What does the @ prefix do on a string literal?

It makes the string verbatim: escape sequences are not processed and the literal may span lines, which suits file paths and regular expressions.

How do you iterate a collection and why does foreach work?

foreach calls GetEnumerator and then MoveNext and Current. Any type exposing that shape, usually through IEnumerable<T>, can be iterated.

What is an expression-bodied member?

A concise form for a member whose body is a single expression, written with => instead of braces and a return.

How does an object initializer differ from a constructor call?

It sets accessible properties after the constructor runs, in one expression, without needing a constructor overload for every combination.

What is the core difference between a value type and a reference type?

A value type variable holds the data itself and is copied on assignment. A reference type variable holds a reference, so assignment copies the reference and both names see the same object.

When is a struct the right choice over a class?

When the type is small, logically a single value, immutable, and short lived. Coordinates, money amounts and identifiers fit; anything with identity or inheritance does not.

What is boxing, and why does it cost?

Converting a value type to object or to an interface, which allocates a heap object holding a copy. Unboxing casts it back and copies again.

What is a nullable value type?

T? where T is a value type, which is Nullable<T>: a struct holding a value plus a HasValue flag, so an int can represent absence.

What do nullable reference types actually change?

Nothing at run time. With #nullable enable, the compiler tracks which references you declared as nullable and warns where a possibly-null value is dereferenced.

Why is string immutable, and what follows from that?

Every operation that appears to modify a string returns a new one. That makes strings safe to share across threads and usable as dictionary keys, at the cost of allocations.

When should you reach for StringBuilder?

When the number of concatenations is not known at compile time, typically a loop. It appends into a growable buffer and materializes the string once.

What is an enum backed by, and what does that allow?

An integral type, int by default. Any value of that integral type can be cast into the enum, whether or not it names a member.

What does declaring a type as a record give you?

Value-based equality, a readable ToString, deconstruction, and with expressions for non-destructive copies, all generated by the compiler.

When is a tuple the right return type?

For a private or local multi-value result where naming a type would add noise. Tuple elements can be named, and the result deconstructs at the call site.

Which members does every type inherit from object?

ToString, Equals, GetHashCode and GetType. Overriding Equals without GetHashCode breaks every hash-based collection.

When must you use decimal instead of double?

For money and any base-10 exact arithmetic. decimal is a 128-bit base-10 type, so 0.1 is exact; double is binary floating point and cannot represent it exactly.

Is integer overflow checked in C# by default?

No, arithmetic wraps silently. A checked block or expression makes overflow throw OverflowException instead.

What does dynamic change about a member call?

Type checking is deferred to run time. The compiler emits a call site that resolves the member when it executes, so a typo becomes a RuntimeBinderException.

What is an auto-implemented property?

A property whose backing field the compiler generates. { get; set; } is read and write, { get; } is set only in the constructor, and { get; init; } is set only during initialization.

What can an interface declare, and what can it not hold?

Members a type promises to provide: methods, properties, events and indexers. It holds no instance state, and a class may implement many interfaces.