where T unmanaged (no references)

Context: The unmanaged constraint (available since C# 7.3) requires the type parameter to be an unmanaged type: a value type that contains no references to managed objects (no reference-type fields). This includes primitive types, enums, and structs composed only of unmanaged fields. This constraint is useful for interop with unmanaged code (P/Invoke, stack allocations, etc.).

Usage Example

using System;
using System.Runtime.InteropServices;

public struct UnmanagedPoint
{
    public int X;
    public int Y;
    // No reference fields
}

public struct PointWithReference
{
    public int X;
    public string Description; // reference -> not allowed
}

public class UnmanagedBuffer<T> where T : unmanaged
{
    public unsafe T* Allocate(int count)
    {
        return (T*)Marshal.AllocHGlobal(count * sizeof(T));
    }
}

class Program
{
    static void Main()
    {
        var buffer = new UnmanagedBuffer<UnmanagedPoint>();
        Console.WriteLine("Unmanaged buffer created (simulated)");

        // This does not compile: PointWithReference contains a reference
        // var errorBuffer = new UnmanagedBuffer<PointWithReference>();
    }
}

Output console

dotnet run
Unmanaged buffer created (simulated)

Important notes

• unmanaged types are a subset of struct.
• Allows using sizeof(T) and stackalloc T[].
• Ideal for high-performance APIs or interop.

Real-world usage example

System.Span<T> – Uses unmanaged constraints internally for some optimizations.
See .NET docs on unmanaged constraint.

Related sections

• 11.4.1 where T : struct
• 11.4.7 Multiple constraints