This article shows how to define a different nullability for the getter and setter of a property in C#.

Summary #

If you don’t want to read the article:

public class MyClass
{
    // getter: not null
    // setter: can be null
    [NotNull]
    public string? SectionName
    {
        get => _sectionName ?? "DefaultSection";
        set => _sectionName = value;
    }

    private string? _sectionName;

    // getter: can be null
    // setter: not null
    [DisallowNull]
    public string? ExplicitName
    {
        get => _explicitName;
        set => _explicitName = value ?? throw new ArgumentNullException();
    }

    private string? _explicitName;
}

Details #

Normally, the getter and the setter of a property have identical nullability (“not null” or “can be null”) - as defined by the property’s type.

But sometimes you may want to have a different nullability for getter or setter:

1. The getter never returns null whereas the setter accepts null as value.
2. The getter can return null but the setter doesn’t accept null.

[NotNull] #

For option 1, add [NotNull] to the property:

// getter: not null
// setter: can be null
[NotNull]
public string? SectionName
{
    get => _sectionName ?? "DefaultSection";
    set => _sectionName = value;
}

private string? _sectionName;

The description of [NotNull] is:

Specifies that an output is not null even if the corresponding type allows it.

[DisallowNull] #

For option 2, add [DisallowNull] to the property:

// getter: can be null
// setter: not null
[DisallowNull]
public string? ExplicitName
{
    get => _explicitName;
    set => _explicitName = value ?? throw new ArgumentNullException();
}

private string? _explicitName;

The description of [DisallowNull] is:

Specifies that null is disallowed as an input even if the corresponding type allows it.

Value Types #

These attributes also work for value types:

public class MyClass
{
    [NotNull]
    public int? BufferSize
    {
        get => _bufferSize ?? 4096;
        set => _bufferSize = value;
    }

    private int? _bufferSize;

    [DisallowNull]
    public int? ExplicitAge
    {
        get => _explicitAge;
        set => _explicitAge = value ?? throw new ArgumentNullException();
    }

    private int? _explicitAge;
}