Making a Non-nullable value type nullable

Question

I have a simple struct that has limited use. The struct is created in a method that calls the data from the database. If there is no data returned from the database I want to be able to return a null, but Visual Studio complains, Cannot convert null to PackageName.StructName because it is a non-nullable value type.

How can I make it nullable?

Adlay

Answer 1

You want to look into the Nullable<T> value type.

Granth

I used this and it still complains – Furie

Answer 2

public struct Something
{
    //...
}

public static Something GetSomethingSomehow()
{
    Something? data = MaybeGetSomethingFrom(theDatabase);
    bool questionMarkMeansNullable = (data == null);
    return data ?? Something.DefaultValue;
}

Discover

As far as I understand, this code does not return null as Malfist wants to. – Paraguay

IIRC, it was intended as an illustration of the use of nullable types, since the poster didn't seem to understand exactly what they were. – Discover

default(Something) may have been a better default return value in the event that no value exists. – Interpellate

Answer 3

The definition for a Nullable<T> struct is:

struct Nullable<T>
{
    public bool HasValue;
    public T Value;
}

It is created in this manner:

Nullable<PackageName.StructName> nullableStruct = new Nullable<PackageName.StructName>(params);

You can shortcut this mess by simply typing:

PackageName.StructName? nullableStruct  = new PackageName.StructName(params);

See: MSDN

Nidanidaros

Answer 4

Nullable<T> is a wrapper class that creates a nullable version of the type T. You can also use the syntax T? (e.g. int?) to represent the nullable version of type T.

Girlfriend

Calling Nullable<T> a "wrapper class" is incorrect: is a struct, not a class, so it has value-type semantics, not reference-type semantics. It does have some special compiler magic though, in that even though Nullable<T> is a struct, it cannot be used as a generic type argument when the generic type parameter has a T : struct constraint, nor can Nullable<T> be used as T in another Nullable<T>, so Nullable<Nullable<Int32>> won't compile either. – Quern

Answer 5

Use the built-in shortcuts for the Nullable<T> struct, by simply adding ? to the declaration:

int? x = null;

if (x == null) { ... }

Just the same for any other type, struct, etc.

MyStruct? myNullableStruct = new MyStruct(params);

Admiration

Answer 6

You could make something nullable for example like this:

// Create the nullable object.
int? value = new int?();

// Check for if the object is null.
if(value == null)
{
    // Your code goes here.
}

Jacobsohn

Answer 7

You can use default as an alternative

public struct VelocityRange
{
    private double myLowerVelocityLimit;
    private double myUpperVelocityLimit;
}

VelocityRange velocityRange = default(VelocityRange);

Afteryears

The default(T) of a value-type T is its zero-byte representation, which may be significant, especially when dealing with integer value-types, for example default(Int32) == 0 - for that reason I never recommend using default as a stand-in for "null". – Quern