EN CSharp Keyword unmanaged function pointer calling convention

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C#: unmanaged — function pointer call convention

Handbook C# · Practical Guide

unmanaged on the delegate* type specifies an unmanaged call convention. Without a list, the platform agreement is used; Cdecl can be explicitly specified in square brackets. A static C# method with UnmanagedCallersOnly is called via a compatible unmanaged pointer instead of a normal managed call. The project allows unsafe blocks.

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
unsafe
{
    delegate* unmanaged[Cdecl]<int, int> twice = &Tools.Twice;
    System.Console.WriteLine(twice(4));
}
static class Tools { [System.Runtime.InteropServices.UnmanagedCallersOnly(CallConvs = new[] { typeof(System.Runtime.CompilerServices.CallConvCdecl) })] public static int Twice(int value) => value * 2; }

Enable <AllowUnsafeBlocks>true</AllowUnsafeBlocks> in the project. The pointer is valid only within the shown scope.

Result:

8

Syntax

delegate* unmanaged<int, int> operation = &Transform;
delegate* unmanaged[Cdecl]<int, int> callback = &TransformCdecl;

A call with an unmanaged platform agreement

File Example.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14.0</LangVersion>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

</Project>

File Program.cs:

using System;
using System.Runtime.InteropServices;

internal static unsafe class Program
{
    public static void Main()
    {
        delegate* unmanaged<int, int> doubleValue = &DoubleValue;
        Console.WriteLine(doubleValue(12));
    }
    [UnmanagedCallersOnly]
    private static int DoubleValue(int value) => value * 2;
}

Result:

24

A callback with an explicitly specified Cdecl convention

File Example.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <LangVersion>14.0</LangVersion>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

</Project>

File Program.cs:

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

internal static unsafe class Program
{
    public static void Main()
    {
        delegate* unmanaged[Cdecl]<int, int, int> callback = &Difference;
        int result = InvokeCallback(callback, 14, 5);
        Console.WriteLine(result);
    }
    private static int InvokeCallback(delegate* unmanaged[Cdecl]<int, int, int> callback,
        int left, int right) => callback(left, right);
    [UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvCdecl) })]
    private static int Difference(int left, int right) => left - right;
}

Result:

9

Official source: Microsoft Learn.