where T BaseClass (base class constraint)

Context: The constraint where T : BaseClass requires the type parameter to inherit from the specified base class (or be that class itself). This allows using members of the base class on instances of T.

Usage Example

using System;

public abstract class Shape
{
    public abstract double Area();
}

public class Circle : Shape
{
    public double Radius { get; set; }
    public override double Area() => Math.PI * Radius * Radius;
}

public class Square : Shape
{
    public double Side { get; set; }
    public override double Area() => Side * Side;
}

public class AreaCalculator<T> where T : Shape
{
    public double CalculateArea(T shape)
    {
        return shape.Area(); // possible because T inherits Shape
    }
}

class Program
{
    static void Main()
    {
        var calc = new AreaCalculator<Circle>();
        double area = calc.CalculateArea(new Circle { Radius = 5 });
        Console.WriteLine($"Circle area: {area:F2}");

        var calcSquare = new AreaCalculator<Square>();
        Console.WriteLine(calcSquare.CalculateArea(new Square { Side = 4 }));
    }
}

Output console

dotnet run
Circle area: 78.54
16

Important notes

• Only one base class is allowed (no multiple inheritance).
• The base class constraint can be an abstract class.

Real-world usage example

Comparer<T> – Requires that T implements IComparable<T> (which is an interface, but the principle is similar).
See .NET docs on base class constraint.

Related sections

• 11.4 Constraints
• 11.4.5 where T : IMyInterface