C# – In C#, what is the difference between public, private, protected, and having no access modifier

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All my college years I have been using public, and would like to know the difference between public, private, and protected?

Also what does static do as opposed to having nothing?

Best Answer

Access modifiers

From docs.microsoft.com:

public

The type or member can be accessed by any other code in the same assembly or another assembly that references it.

private

The type or member can only be accessed by code in the same class or struct.

protected

The type or member can only be accessed by code in the same class or struct, or in a derived class.

private protected (added in C# 7.2)

The type or member can only be accessed by code in the same class or struct, or in a derived class from the same assembly, but not from another assembly.

internal

The type or member can be accessed by any code in the same assembly, but not from another assembly.

protected internal

The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.

When no access modifier is set, a default access modifier is used. So there is always some form of access modifier even if it's not set.

`static` modifier

The static modifier on a class means that the class cannot be instantiated, and that all of its members are static. A static member has one version regardless of how many instances of its enclosing type are created.

A static class is basically the same as a non-static class, but there is one difference: a static class cannot be externally instantiated. In other words, you cannot use the new keyword to create a variable of the class type. Because there is no instance variable, you access the members of a static class by using the class name itself.

However, there is a such thing as a static constructor. Any class can have one of these, including static classes. They cannot be called directly & cannot have parameters (other than any type parameters on the class itself). A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced. Looks like this:

static class Foo()
{
    static Foo()
    {
        Bar = "fubar";
    }
    
    public static string Bar { get; set; }
}

Static classes are often used as services, you can use them like so:

MyStaticClass.ServiceMethod(...);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

Access modifiers

static modifier

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C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

Related Topic

`static` modifier