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How to Resolve Error "CS0227: Unsafe code may only appear if compiling with /unsafe" in C#

The Compiler Error CS0227 is a configuration error. The message reads: "Unsafe code may only appear if compiling with /unsafe".

By default, C# code is "Safe," meaning the runtime manages memory allocation, bounds checking, and garbage collection to prevent common bugs like buffer overflows or access violations. Features that bypass these protections—such as pointers (*, &, ->) and the fixed statement—are considered Unsafe. Even if your code syntax is correct, the C# compiler refuses to build unsafe code unless you explicitly grant permission in the project configuration.

This guide explains how to enable unsafe blocks in Visual Studio, VS Code, and the command line.

Understanding Safe vs. Unsafe Contexts​

The /unsafe compiler flag is a gatekeeper. It ensures that you don't accidentally introduce memory management risks into your project.

  • Safe Code: References objects, uses arrays with bounds checking, automatic garbage collection.
  • Unsafe Code: Uses raw memory addresses (pointers), creates fixed-size buffers, performs pointer arithmetic.

Using the unsafe keyword in your code (e.g., unsafe void Method()) is the code-level opt-in. The /unsafe flag (or AllowUnsafeBlocks) is the project-level opt-in. You need both.

Scenario: Writing Unsafe Code Without Permission​

This error occurs when you write syntactically correct C# code that uses pointers, but the project settings have not been updated to allow it.

Example of code:

public class PointerTest
{
    // The 'unsafe' keyword is present, which is correct syntax.
    public unsafe void UsePointer()
    {
        int value = 10;
        
        // Pointers are valid logic here...
        int* ptr = &value; 
        
        // ...but if the project config is default, this triggers CS0227.
        *ptr = 20;
    }
}

Build Output:

error CS0227: Unsafe code may only appear if compiling with /unsafe

The most robust way to fix this is to edit your project file directly. This ensures the setting works across all environments (Visual Studio, CLI, CI/CD pipelines).

  1. Right-click your project in Solution Explorer and select Edit Project File (or open the .csproj file in your editor).
  2. Locate a <PropertyGroup>.
  3. Add the <AllowUnsafeBlocks> tag and set it to true.
<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    
    <!-- ✅ Correct: This enables the /unsafe compiler switch -->
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

</Project>

Solution 2: Enabling via Visual Studio UI​

If you prefer using the graphical interface in Visual Studio:

  1. Right-click your Project in the Solution Explorer.
  2. Select Properties.
  3. Navigate to the Build tab.
    • In newer VS versions: Go to Build > General.
    • In older VS versions: It may be a checkbox directly on the Build tab.
  4. Find the checkbox labeled Allow unsafe code and check it.
  5. Save the properties (Ctrl+S).
note

This action automatically updates your .csproj file as described in Solution 1.

Solution 3: Command Line Compilation​

If you are compiling single files manually using csc.exe (the C# compiler) instead of dotnet build, you must pass the switch explicitly.

Example of the command:

# ⛔️ Incorrect
csc Program.cs

# ✅ Correct
csc /unsafe Program.cs

Conclusion​

CS0227 is not a code error; it is a permission error.

  1. Check your Code: Ensure you actually need unsafe features (pointers).
  2. Check your Config: Open your .csproj file.
  3. Enable the Flag: Add <AllowUnsafeBlocks>true</AllowUnsafeBlocks>.

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