where T class (reference type)

Context: The constraint where T : class requires the type parameter to be a reference type (class, interface, delegate, array). It allows null. It is often used for containers that manage shared objects or require references.

Usage Example

using System;
using System.Collections.Generic;

public class Warehouse<T> where T : class
{
    private readonly List<T> _items = new List<T>();

    public void Add(T item)
    {
        if (item == null)
            throw new ArgumentNullException(nameof(item));
        _items.Add(item);
    }

    public T Get(int index) => _items[index];
}

class Program
{
    static void Main()
    {
        var stringWarehouse = new Warehouse<string>();
        stringWarehouse.Add("Hello");
        Console.WriteLine(stringWarehouse.Get(0));

        // This does not compile: int is a struct
        // var intWarehouse = new Warehouse<int>();
    }
}

Output console

dotnet run
Hello

Important notes

class includes interfaces, delegates, and arrays.
Allows checking for null and using the as operator.

Real-world usage example

List<T> – Does not have a class constraint, but many custom collections use it to ensure elements are objects.
See .NET docs on class constraint.

where T class (reference type)

Usage Example

Output console

Important notes

Real-world usage example

Related sections