Extension methods have existed since C# 3, but they only let you add methods. C# 14 introduces extension blocks, which also let you define extension properties and static extension members on an existing type.

The new syntax

Inside a static class you declare an extension block with the "receiver", that is the type being extended. Every member of the block can use that receiver.

public static class StringExtensions
{
    extension(string text)
    {
        // extension property
        public bool IsBlank => string.IsNullOrWhiteSpace(text);

        // extension method, as before but without "this"
        public string Truncate(int max) =>
            text.Length <= max ? text : text[..max] + "...";
    }
}

var title = "Introduction to .NET 10";
Console.WriteLine(title.IsBlank);       // False
Console.WriteLine(title.Truncate(12));  // Introduction...

Static extension members

By omitting the parameter name you extend the type itself, not an instance: you can add "static" methods or properties to a type you do not control.

public static class GuidExtensions
{
    extension(Guid)
    {
        public static Guid FromText(string value) =>
            Guid.TryParse(value, out var g) ? g : Guid.Empty;
    }
}

var id = Guid.FromText("not-valid"); // Guid.Empty

Compatibility

Old extension methods with this keep working and can live alongside the new blocks. There is nothing to rewrite: adopt extension blocks when you need properties or static members, or when you want to group several extensions of the same type.

Tips

• Use extension properties for derived information without side effects.
• Avoid "stuffing" system types with dozens of extensions: prefer dedicated namespaces.
• Remember that an instance member always takes precedence over an extension with the same name.