Need help understanding C# generics

Question

I am self-studying a C# reference and it gives the following information:

1.21.4. Declaring Generic Parameters Generic parameters can be introduced in the declaration of classes, structs, interfaces, delegates (see the upcoming "Delegates" section), and methods. Other constructs, such as properties, cannot introduce a generic parameter, but can use a generic parameter. For example, the property Value uses T:

public struct Nullable<T>
{
  public T Value {get;}
}

First, I get an error trying to compile this saying that it must contain a body because it is neither abstract nor extern or that automatic parameters must have both get and set accessors.

Second, assuming it is wrong and I correct it by adding "set;", I cannot seem to format a call to it successfully.

Answer 1

That is just showing the API of Nullable<T> rather than the implementation. It's not meant to be compiled - System.Nullable<T> is part of the framework, you don't have to implement it yourself.

Answer 2

You appear to be reading "C# 3.0 in a Nutshell". Well, the example is just that - an example. It is only intended to illustrate how the Value property of the Generic Nullable class exposes the generic parameter declared by the containing type.

It isn't meant to be part of a compilable example.

Answer 3

I'm not sure if you just picked a bad example for your struct name (since Nullable is a framework struct), but if not, the error is due to the fact that you have no set accessor in your property. Automatic properties (added in C# 3.0) need both a get and set property. So, if you change the code to:

public struct Nullable<T>
{
  public T Value {get; set; }
}

it should work. In this case, the error had nothing to do with generics. To create an instance, you could use:

Nullable<int> i = new Nullable<int>();

This will make it compile. However, as both Jon and Cerebrus has pointed out, it's probably just an example to show the workings of generics.

Answer 4

For an automatic property you always need both a getter and setter. Without a getter you could set the value, but nothing could ever retrieve it. Without a setter the value would always be the default because nothing could ever set it:

//tradition 
private T _value; 
public T Value 
{ 
    get 
    {
        return _value;
    }
}
//locally, _value can always be set

//Auto-matically implemented property
public T Value { get; }
//there is never a way to set it

//with this it can be privately set, but is get only to everyone else
public T Value{ get; private set; }

Answer 5

Your question here seems to be about auto-properties, rather than generics.

Auto-properties must have a set accessor, although it doesn't have to have the same visibility as the get:

public T Value { get; private set; }

You can call the set in the constructor:

public Nullable ( T value ) {
    Value = value;
}

Although in this case you're dealing with a struct, and they always have a default (parameterless) constructor - you have to be a bit more careful with structs and auto-properties.

In .net there's already a nullable generic:

Nullable<int> i = GetCounterThatMightBeNull();

int j = 0;
if( i.HasValue )
    j = i.Value;
...

This was added in .Net 2 (at the same time as generics), and while early betas looked like the code above they streamlined it the final version:

//int? is shorthand for Nullable<int>
int? i = GetCounterThatMightBeNull(); 

// ?? is shorthand for the null check
int j = i ?? 0;

//or
if( i == null ) {
    //this works even though Nullable<int> is a struct
}