Constraints

Context: Constraints restrict the types that can be used as generic type arguments. They allow accessing specific members (constructors, methods, properties) of the type parameter. The where clause is used to declare constraints.

Usage Example

using System;

// Multiple constraints illustrated in a class
public class ConstraintDemo<T> where T : class, new()
{
    public T CreateInstance()
    {
        return new T(); // new() allows using the parameterless constructor
    }
}

class Program
{
    static void Main()
    {
        var demo = new ConstraintDemo<ConstraintDemo<object>>();
        var instance = demo.CreateInstance();
        Console.WriteLine(instance != null);
    }
}

Output console

dotnet run
True

Important notes

Constraints are optional but powerful.
Constraints can be applied to the class or the method.
Multiple constraints are separated by commas.

Real-world usage example

Dictionary<TKey, TValue> – TKey cannot be null (implicit constraint).
See .NET docs on constraints.

Constraints

Usage Example

Output console

Important notes

Real-world usage example

Related sections