Multiple constraints

Context: Multiple constraints can be applied to a single type parameter. The order must be: base class (or class/struct), interfaces, new(). Multiple constraints increase the capabilities of the generic type.

Usage Example

using System;
using System.Collections.Generic;

public interface IComparableWithDisplay<T> : IComparable<T>
{
    void Display();
}

public class Person : IComparableWithDisplay<Person>
{
    public string Name { get; set; }
    public int Age { get; set; }

    public int CompareTo(Person other)
    {
        return Age.CompareTo(other.Age);
    }

    public void Display() => Console.WriteLine($"{Name} ({Age} years)");
}

public class Utility<T> where T : class, IComparableWithDisplay<T>, new()
{
    public T CreateAndDisplay()
    {
        T obj = new T();
        obj.Display();
        return obj;
    }

    public T Max(T a, T b)
    {
        return a.CompareTo(b) > 0 ? a : b;
    }
}

class Program
{
    static void Main()
    {
        var util = new Utility<Person>();
        var p1 = new Person { Name = "Alice", Age = 30 };
        var p2 = new Person { Name = "Bob", Age = 25 };

        Person older = util.Max(p1, p2);
        Console.WriteLine($"Older: {older.Name}");
    }
}

Output console

dotnet run
Older: Alice

Important notes

• Order: base class (or class/struct) first, then interfaces, then new().
• You cannot combine class and struct.
• Constraints increase readability but also rigidity.

Real-world usage example

Dictionary<TKey, TValue> – TKey must implement IEquatable<TKey> or object.Equals (implicit constraint).
See .NET docs on multiple constraints.

Related sections

• 11.4 Constraints
• 11.5 Project: Generic Repository Simulator