Recently, I’ve been working on a couple of C# projects to try and bring myself up to date with .NET Core’s latest development. Disregarding my issues with unit testing, I ran into some old ‘gotchas’ again that have been around for quite some time now, yet they are something I feel like aren’t too well known at large. Here’s my attempt to bring them out.

Primitive types and System.String

It’s common to think of C#’s primitive types as the types that have an alias assigned by default. These contain, for example; int, string, bool and float. However, there is some ambiguity over the definitions, especially for string.


The .NET CLR (Common Language Runtime) defines the following types to be primitive:

  • System.Boolean
  • System.Byte
  • System.SByte
  • System.Int16
  • System.UInt16
  • System.Int32
  • System.UInt32
  • System.Int64
  • System.UInt64
  • System.IntPtr
  • System.UIntPtr
  • System.Char
  • System.Double
  • System.Single

Note that System.String is not listed. I don’t know the exact reasoning for this, but my best guess is that because System.String is a reference type, it doesn’t get to join the list of value types. The Type.IsPrimitive property can be used to check if a given type is a primitive type in the CLR.

C#’s specification

The C# specification (version 4) doesn’t talk about primitive types at all. The specification defines so-called simple types; they are defined as “value types that have a keyword alias in C#”. This immediately excludes System.String - it is a reference type after all.

However, System.String still behaves as if it was one of the simple types:

  • It has a keyword alias (string)
  • It has three (three!) constant literals available - regular, verbatim and interpolated

All in all, you should be fine treating System.String as a ‘primitive’ type in your code, but it’s useful to know what’s actually going on under the hood.

Quick sidenote: the keyword aliases

Using the keyword aliases is as if the compiler automatically prefixes all your classes with

using short = System.Int16;
using int = System.Int32;
using long = System.Int64;
using float = System.Single;
// etc.

However, there is one situation where it matters whether you use the keyword aliases or the types: enum underlying type definition. When defining the underlying type for an enum, you have to use the keyword aliases.

internal enum MyEnum : long // valid
    Value1, ...

internal enum MyEnum : System.Int64 // invalid
    Value1, ...

Value types versus reference types

Most (if not all) C# programmers find themselves pondering over the distinction between these two types of, well, types. I’m going to assume you know the basic differences (how they behave when passed around etc.), but I’ll cover some topics that aren’t immediately obvious.

Where they reside in memory

It’s common to think of value types being in the stack, and reference types being in the heap - always. This is more-or-less true, but not guaranteed. You can easily instantiate a value type and it’ll end up in the stack:

int constructedInt = new System.Int32(5);

You can also box a value type to object: a reference type.

object intObject = 5;

It’s ambiguous where these types exactly reside in memory, but here’s the deal: you don’t have to care. This is C# after all, the compiler and runtime abstract the memory away from you, only throwing OutOfMemoryExceptions when you allocate too much of it.

All value types are immutable by default whereas reference types are mutable

This is false. Well, the ‘by default’ part is. To make a value type immutable, you have to make it such yourself. Most (if not all) value types defined in the CLR or C# specification are immutable. A classic counter-example is the Point-struct in both Windows Forms and Windows Presentation Foundation.

internal struct MutableStruct
    public int Value;
    public MutableStruct(int value)
        Value = value;
var mutable = new MutableStruct(5);
mutable.Value = 10;

This code compiles just fine and behaves as you’d expect it to - it first prints 5, then 10. However, please don’t do this. If you have to modify a struct, provide operators or methods (or both) to derive a completely new instance derived from the original. (Don’t forget to mark your fields as readonly to make them properly immutable.)

Reference types are just as mutable as value types, but they too can be made immutable. The difference is that it’s commonly accepted to have mutable reference types.

Readonly automatic properties

When defining a property that generates its own backing field automatically, you can omit the setter to make the property read-only. Properties are often associated to be a native getter-setter pattern (because that’s what they compile down to), it’s possible to minify the property definition through some syntactic sugar. Rather than having to define the (hopefully) private backing field yourself, you can omit the bodies for the getter and setter to have the compiler generate the backing field for you.

public string MyProperty { get; set; }

As such, it’s possible to omit the setter completely to make the property read-only. However, because of the previously mentioned association with the getter-setter pattern, you’d think the setter is entirely gone and you can’t set its value at all. This is obviously counter-intuitive: you do want to set the value to something. It would make sense to forget the compiler trick - define the backing field and have your getter return that.

private string backingField;

public string MyProperty
        return backingField;

(There are more syntactic sugars available here related to expression bodies, but I’ve decided to omit them to have the example be clear.)

Here, it’s possible to define backingField as readonly and set its value in a constructor, just as you would with any other readonly field.

The thing is; when you omit the setter in an automatic property, you can still set its value in the constructor, just as if it was a readonly field.

public string MyProperty { get; }

public MyClass(string stringValue)
    MyProperty = stringValue;

This is not immediately obvious, because there’s no readonly keyword anywhere indicating the read-only-ness of the member, which is why I wanted to mention it.

Default access modifiers

When omitting the access modifier from a class, struct or a member, the compiler will assign a default one. It’s common to think that members will end up being private, and classes and structs are public. This is not true, however. The compiler will assign the most restrictive modifier possible if the modifier is omitted. Here’s a table of the default access modifiers:

Thing Default access modifier
Top-level class internal
Nested class private
Interface internal
Class members private
Explicitly implemented interface member public
Interface and enum members public
Property getter and setter public
Constructor private
Delegate internal

Important things to note:

  • If you omit a constructor, the compiler will generate a default public constructor with no parameters.
  • It’s not possible to define an access modifier for a namespace. Namespaces are public by default.
  • The getter and setter of a property are the one exception to the rule stated above. It’s possible to define a more restrictive modifier to either of them, such as protected for the getter and private for the setter.


These are by far not all of the ‘gotchas’ you’ll run into, but these are the ones I ran into lately and wanted to bring out.