AL0001: Prohibit reassignment of primary constructor parameters

Primary constructor parameters should not be reassigned as this can lead to confusion and bugs.

When it triggers

// Error: Primary constructor parameter 'x' should not be reassigned
public class Example(int x)
{
    public void SetX(int value) => x = value;  // AL0001
    public void Increment() => x++;             // AL0001
}

Why this matters

Primary constructor parameters are captured by the compiler and may be used for multiple purposes:

• Field initialization
• Property backing fields
• Constructor logic

Reassigning them creates confusion about which value is actually used and can lead to subtle bugs.

How to fix

Use a separate field if you need mutable state:

public class Example(int x)
{
    private int _x = x;

    public void SetX(int value) => _x = value;
    public void Increment() => _x++;
}

Configuration

[*.cs]
dotnet_diagnostic.AL0001.severity = error