[Length] Attribute isn't supported

[satma0745]

This issue is a feature request for a support of the [Length] attribute. Judging by the example in the Contribution guide, this issue falls into the Feature Requests category.

The System.ComponentModel.DataAnnotations namespace has 3 (actually even more, but it is not within the scope of this Issue) Attributes responsible for the string length validation:

1. [MinLength(length)] - MS Docs
2. [MaxLength(length)] - MS Docs
3. [Length(minimumLength, maximumLength)] - MS Docs

And while the first two are supported, the latter isn't.
Suppose, we have the following DTO definition:

class Dto
{
    [MinLength(2), MaxLength(10)]
    public string Prop1 { get; set; }

    [Length(2, 10)]
    public string Prop2 { get; set; }
}

Having such model SwaggerGen produces the following OpenApi specification:

"Dto": {
  "type": "object",
  "properties": {
    "prop1": {
      "maxLength": 10,  
      "minLength": 2,
      "type": "string",
      "nullable": true
    },
    "prop2": {
      "type": "string",
      "nullable": true
    }
  },
  "additionalProperties": false
}

Where the prop1 field has both maxLength and minLength values specified, but the prop2 field doesn't have them at all.

[satma0745]

It turns out that the Project has not been updated for a year (at this point in time). This means that the current version for the project is .Net 7.0, while the [Length] attribute was introduced in .Net 8.0. It's impossible to add support for this attribute at the moment.

[satma0745]

If You stumbled upon this Issue and the Project hasn't yet migrated to .Net 8, then you can make use of the following SchemaFilter in Your Solution:

internal sealed class LengthAttributeSupportSchemaFilter : ISchemaFilter
{
    public void Apply(OpenApiSchema schema, SchemaFilterContext context)
    {
        if (context.MemberInfo is null)
        {
            return;
        }

        var lengthAttributes = context.MemberInfo.GetInlineAndMetadataAttributes().OfType<LengthAttribute>();
        foreach (var lengthAttribute in lengthAttributes)
        {
            ApplyLengthAttribute(schema, lengthAttribute);
        }
    }

    private void ApplyLengthAttribute(OpenApiSchema schema, LengthAttribute lengthAttribute)
    {
        if (schema.Type == "array")
        {
            schema.MinItems = schema.MinItems is null 
                ? lengthAttribute.MinimumLength 
                : Math.Max(schema.MinItems.Value, lengthAttribute.MinimumLength);

            schema.MaxItems = schema.MaxItems is null
                ? lengthAttribute.MaximumLength
                : Math.Min(schema.MaxItems.Value, lengthAttribute.MaximumLength);
        }
        else
        {
            schema.MinLength = schema.MinLength is null 
                ? lengthAttribute.MinimumLength 
                : Math.Max(schema.MinLength.Value, lengthAttribute.MinimumLength);

            schema.MaxLength = schema.MaxLength is null
                ? lengthAttribute.MaximumLength
                : Math.Min(schema.MaxLength.Value, lengthAttribute.MaximumLength);
        }
    }
}

Warning

I needed [Length] attribute support for models only, so in this example I'm retrieving the list of attributes from the context.MmemberInfo. If You need support for this attribute in other cases, then You need to change the context.MemberInfo.GetInlineAndMetadataAttributes() call.

[satma0745]

I'm not closing this Issue, since the solution I've provided is temporary. I hope that the Project'll migrate to the .Net 8 eventually and we'll be able to add support for the [Length] attribute on the Project's source-code side.

[soroshsabz]

ITNOA

I need this feature too.

[github-actions]

This issue is stale because it has been open for 60 days with no activity. It will be automatically closed in 14 days if no further updates are made.

[dnperfors]

I wanted to help you out by implementing this, but I need .NET 8 support for this... Guess I will start with reviewing the .NET 8 PR instead :)

[soroshsabz]

@satma0745 @dnperfors As you can see, .NET 8 added in PR #2799 and merge it.

So do you can start implementing this feature?

thanks

[satma0745]

@soroshsabz
Yeah, I'll start working on it

[satma0745]

@soroshsabz Opened a PR: #2882

[martincostello]

Thanks for adding the support for this - the change is available in Swashbuckle.AspNetCore 6.6.2.