EN CSharp Keyword fixed

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C#: fixed

Handbook C# · Practical Guide

For the duration of the block, it keeps the managed data immobile for working through the pointer; fixed buffer — a separate declaration of a structure field.

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
int[] values = [3];
unsafe
{
    fixed (int* pointer = values) *pointer = 7;
}
System.Console.WriteLine(values[0]);

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

Result:

7

Syntax

fixed (Type* pointer = managedValue) { ... }

In .csproj, set true. For managed data, it's usually easier to work through Span without raw pointers.

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)
    {
        string text = "ore";
        unsafe { fixed (char* pointer = text) Console.WriteLine(*pointer); }
    }
}

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)
    {
        int[] values = [2, 4, 6];
        unsafe { fixed (int* pointer = values) { pointer[1] = 9; Console.WriteLine(*(pointer + 1)); Console.WriteLine(sizeof(long)); } }
        Console.WriteLine(values[1]);
    }
}

Example 3

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)
    {
        unsafe { Packet packet = default; packet.Bytes[0] = 7; Console.WriteLine(packet.Bytes[0]); }
    }
}
public unsafe struct Packet { public fixed byte Bytes[4]; }

Official source: Microsoft Learn.