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NDLRGEN051: Custom constructor guard method is invalid

Cause

No accessible static method matching the required guard-method shape was found on the referenced guard type for the guarded field or property type.

Rule Description

A custom guard method must be an accessible, static, void-returning method with exactly two parameters: a value parameter compatible with the guarded member's type (directly, or via a generic method's type-parameter inference), and a string parameter name as the last parameter. Neither parameter may be passed by ref, out, or in -- the generator emits a direct call and cannot support by-reference parameters.

The generator calls this method directly, so its shape must be fully resolvable at compile time; there is no fallback to reflection for a method that almost matches. The diagnostic message reports the specific reason the best candidate method failed to qualify, for example:

  • No method with that name exists on the guard type.
  • The method is not accessible from the generated constructor.
  • The method is not static.
  • The method does not return void.
  • The method does not have exactly two parameters.
  • The method's second parameter is not a string parameter name.
  • A parameter is passed by ref/out/in.
  • The method's value parameter type is not compatible with the guarded member's type.

How to Fix

Adjust the guard method's signature to match the required shape:

using NexusLabs.Needlr.Generators;

public static class CollectionNotEmptyGuard
{
    // WRONG - NDLRGEN051: not static
    public void Validate(IReadOnlyCollection<string> value, string parameterName) { }
}

[GenerateConstructor]
public partial class OrderService
{
    [ConstructorGuard(typeof(CollectionNotEmptyGuard))]
    private readonly IReadOnlyCollection<string> _orders;
}

using NexusLabs.Needlr.Generators;

// CORRECT - accessible, static, void, (value, string parameterName)
public static class CollectionNotEmptyGuard
{
    public static void Validate<T>(IReadOnlyCollection<T>? value, string parameterName)
    {
        ArgumentNullException.ThrowIfNull(value, parameterName);

        if (value.Count == 0)
        {
            throw new ArgumentException("Collection must not be empty.", parameterName);
        }
    }
}

[GenerateConstructor]
public partial class OrderService
{
    [ConstructorGuard(typeof(CollectionNotEmptyGuard))]
    private readonly IReadOnlyCollection<string> _orders;
}

See Also