C# struct, how to assign a null value?

Question

I have a list:

List<int, SomeStruct>

For some reason, it is not allowing me to assign null values to it. What do I do if I want to have no struct associated?

Korman

Answer 1

Use nullable types:

List<int, SomeStruct?>

You can't assign null to an element of the list because structs are value types, while null means an empty pointer, and so can only be assigned to reference type variables.

Also note that List as you're using it doesn't exist in .NET! Perhaps you want Dictionary?

Thankless

When I do this, any references the the structures member now gives me the error: System.Nullable<> does not contain a definition for 'Member' – Korman

@whydna: Read up on nullable types. Essentially SomeStruct? is syntactic sugar for Nullable<SomeStruct>. You can't access members directly, but that's the price you pay for using null semantics with a value type. – Thankless

Answer 2

In C# a struct is a 'value type', which can't be null.

If you really, really need to be able to null this, then make it into a 'nullable' type by adding a trailing question mark.

You should probably look into the details of this more first, though - it's quite likely you don't really want a 'struct' at all, but would be better with a 'class'.

Whomsoever

Agreed. Problem is I don't own the Struct, I am simply using it :( – Korman

Answer 3

Unless you have defined a custom generic collection, List<T, U> doesn't exist in the System.Collections.Generic namespace. Did you rather mean Dictionary<TKey, TValue>?

You can use Nullable types: Using Nullable Types (C# Programming Guide).

As for any ValueType, you need to specify explicitly that you will allow null values for your type. You can do so by concatenating the '?' character with your type name, or by marking it as Nullable, where T is the name of your structure.

Necrolatry

I couldn't resist to answering when I saw there was going to be three 'Will' answering one question! =P – Necrolatry

Where there are three Wills there are three Ways? – Parkway

Answer 4

All of the existing answers here are correct: in C# struct is a value type and cannot be null.

However some struct implementations (but definitely not all) follow a convention where the default/empty value is stored in a Empty static field. Sometimes there is also an IsEmpty property:

// System.Guid
public struct Guid : IFormattable, IComparable, IComparable<Guid>, IEquatable<Guid>
{
    public static readonly Guid Empty;
    ...
}

// System.Drawing.Point
public struct Point
{
    public static readonly Point Empty;
    public bool IsEmpty { get; }
    ...
}

If the struct has an Empty field or IsEmpty property, then you can use it to test if the struct variable is Empty instead of null. Otherwise use a nullable wrapper as the other answers state.

If you are the programmer of the struct, strongly consider adding an Empty field and IsEmpty property if appropriate.

Tether