Question

Normally, ModelBinding Validation of a class member might be done like this example:

public Class someclass
{
    [StringLength(50)]
    public string SomeValue { get; set; }
}

SomeValue is limited to 50 characters at a maximum.

Is it possible to have the constant (50) changed to something else at run-time, say, during the construction of each instance of that class, so that it is possible to have varying instances with different StringLength limitations?

If so, how does one do this?

Answer 1

Yes. But the only way is create your own implementation of the DataAnnotationsModelValidatorProvider and then register it in Global.ascx.cs. You can’t simply remove attributes at runtime BUT interupt the MVC internals that read them:

public class ConventionModelValidatorProvider : DataAnnotationsModelValidatorProvider
{
    protected override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
    {
        List<Attribute> newAttributes = new List<Attribute>(attributes);
        if( mycondition == true )
        {
            //get rid of the existing attribute
            newAttributes.Remove(newAttributes.OfType<StringLengthAttribute>().First());


            //add a new one 
            newAttributes.Add( new StringLengthAttribute(5324));
        }

        return base.GetValidators(metadata, context, newAttributes);
    }
}

Register:

ModelValidatorProviders.Providers.Clear();
ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() );