Partial methods ^{C# 3.0} and partial properties ^{C# 13.0} allow splitting a declaration and its implementation across files. This is especially useful for code generation scenarios where a generator provides the implementation.

C# 14 extends this to constructors and events, completing the set of members that can be partial.

Code

Now

// User-written file
partial class Customer
{
    public partial Customer(string name);
    public partial event EventHandler? NameChanged;
}

// Code-generated file
partial class Customer
{
    private string name;

    public partial Customer(string name)
    {
        this.name = name;
    }

    public partial event EventHandler? NameChanged
    {
        add => nameChangedHandlers += value;
        remove => nameChangedHandlers -= value;
    }
}

Before

// User-written file
partial class Customer
{
    partial void Initialize(string name);

    public Customer(string name)
    {
        Initialize(name);
    }
}

// Code-generated file
partial class Customer
{
    private string name;

    partial void Initialize(string name)
    {
        this.name = name;
    }
}

Notes

• Only the implementing declaration of a partial constructor can include a constructor initializer (: this() or : base())
• Only one partial type declaration can include primary constructor syntax
• The implementing declaration of a partial event must include add and remove accessors

More information

• What's new in C# 14.0
• Partial constructors