in C# before the Nullable(?) was invented，Which can be null？which can't be null？

Question

in C# before the Nullable(?) was invented，Which can be null？which can't be null？

and Why are we need to worried about null or not null？

Answer 1

in C# before the Nullable(?) was invented...and Why are we need to worried about null or not null？

The default value of an uninitialized int is zero which is a valid integer value. But what if you need to validate a int that was entered by a user or passed to a service endpoint by code? That's where nullable types are handy. An int? can be null.

The C# reference documentation illustrates the concept with code examples.

Nullable value types (C# reference)

Which can be null？which can't be null？

The default value of a reference type is null while the default value of a value type depends on the value type. An int is zero. A bool is false...

Value types (C# reference)

Reference types (C# reference)

Answer 2

in general null means the value is unknown or undefined.

in C# value types can not be null as value types are stack storage (a value on the stack) which is accessed directly, vs heap storage which is accessed via a pointer.

the C# design of value types did not support a null indicator (which would take extra memory)

although value types can be boxed (stored on the heap) the runtime did not support a null case.

so if you have a bool variable (a value type) it can be true or false, but not null. but what if it was important to know in your code that the value was unknown? this is the use case nullable types where added to support.

Nullable<bool> is an object that support true, false and null. (bool? is just syntax sugar). to use a nullable bool as a bool, it must be cast as a bool:

bool? a = null; 
bool b = (bool) a;   // runtime error because a is null 
bool c = a ?? false; // cast with default value
bool d = a;          // compile error 
bool e = null;       // compile error

Answer 3

Hi 兰树豪,

The replies from Bruce and AgaveJoe are very useful, and I recommend you read them carefully. I made some additions you could also refer to it.

As a value type is implicitly convertible to the corresponding nullable value type, you could assign a value to a variable of a nullable value type as you would do that for its underlying value type.

bool? flag = null;
// An array of a nullable value type:
int?[] arr = new int?[10];

You could use the is operator with a type pattern to both examine an instance of a nullable value type for null and retrieve a value of an underlying type:

int?a = 42;
if (a is null)
{
    Console.WriteLine($" a does not have a value");
}
else
{
    Console.WriteLine($"a is {a.Value}");
}

Regarding the knowledge of null in C#, I highly recommend you read this issue, I believe it will be helpful to you.

Best regards,

Elya

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