Trim warnings for MinLengthAttribute

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp
Domain
backend

Research direction

Start by examining System.ComponentModel.DataAnnotations.MinLengthAttribute and the OptionsValidator source-generator path described in the issue. Determine how length validation can avoid the trim warning without requiring an UnconditionalSuppressMessage; done means the validation approach is trim-safe and preserves the intended MinLength behavior.

Written by the indexing model from the issue text.

Description

area-System.ComponentModel.DataAnnotations

The following produces warnings on the MinLength attribute instance:

using Microsoft.Extensions.Options;
using System.ComponentModel.DataAnnotations;

var model = new Model { Name = "abcd" };
var result = model.Validate(null, model);
Console.WriteLine(result.Succeeded);

[OptionsValidator]
partial class Model : IValidateOptions<Model> {
    [MinLength(4)]
    public required string Name { get; set; }
}

Program.cs(10,6): warning IL2026: Using member 'System.ComponentModel.DataAnnotations.MinLengthAttribute.MinLengthAttribute(Int32)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Uses reflection to get the 'Count' property on types that don't implement ICollection. This 'Count' property may be trimmed. Ensure it is preserved

(This is true even when using the options validation source generator - even though the generator generates its own attribute types, the original attribute still exists in user code.)

It seems like the best pattern we can recommend at the moment is to add UnconditionalSuppressMessage to the property, but it seems like there should be a built-in way to do length validation that doesn't require suppressing warnings.

The MinLengthAttribute ctor is ruc because its IsValid implementation reflects over the instance type to get the Count property. Maybe that could be represented using generics, for example:

class MinLengthAttribute<T> : MinLengthAttribute
    where T : ICollection
{
    public MinLengthAttribute(int length) {}
    public override bool IsValid(object? value)
    {
        if (value is ICollection collection)
        {
            return collection.Count >= Length;
        }
        throw ...;
    }
}

Somewhat related to https://github.com/dotnet/runtime/issues/82526.

@eerhardt @tarekgh @eiriktsarpalis

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