EN CSharp Keyword where generic type constraint

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C#: where — type parameter limit

Handbook C# · Practical Guide

Specifies the type parameter requirements: base class, interface, class/struct, new(), notnull, unmanaged, and other allowed combinations.

Simple example (Example.cs)

Save this example separately as Example.cs. Run it independently of the other examples on this page.

Example.cs:

// PROJECT-EXAMPLE
T Create<T>() where T : new() => new T();
System.Console.WriteLine(Create<Item>().Name);
class Item { public string Name => "Ore"; }

Result:

Ore

Syntax

where T : constraint1, constraint2

Example 1: simple form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Counter counter = Factory.Create<Counter>();
        Console.WriteLine(counter.Value);
    }
}
public class Counter { public int Value => 7; }
public static class Factory { public static T Create<T>() where T : class, new() => new T(); }

Example 2: another practical form

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

public static class Program
{
    public static async Task Main(string[] args)
    {
        Console.WriteLine(Values.Sum<Number>());
    }
}
public interface IValue { int Value { get; } }
public readonly struct Number : IValue { public int Value => 5; }
public static class Values { public static int Sum<T>() where T : struct, IValue { T value = new(); return value.Value; } }

Official source: Microsoft Learn.