Using IEnumerable<> as a parameter for a generic method that can get reference and non referen-CodePudding

I'm trying to build a validation attribute that will validate the size of a collection, I want the method to be able to handle reference types like string and also non reference types like int. I tried to use IEnumerable<object> but that stays null in case I pass a IEnumerable<int>.

This is my current code -

[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property)]
public class EnumerableSizeAttribute : ValidationAttribute<IEnumerable<object>>
{
 
    protected override ValidationResult? IsValid(IEnumerable<object>? enumerable, ValidationContext validationContext)
    {
    //check collection size
    }
}

What should I use as parameter to allow me to pass any collection type into the method? Thanks for the help!

CodePudding user response：

Generic variance rules between value and reference types are... complex.

If you need to avoid boxing, then consider making the method itself generic and take IEnumerable<T>, so that the caller specifies it (usually implicitly, so no code change).

If that isn't an option: consider using the non-generic IEnumerable APIs. Or IList or ICollection since you're after the count, which you can then access cheaply via .Count without paying any of the boxing / iteration overheads associated with IEnumerable.

CodePudding user response：

In C# 11/.NET 7 and later you can have generic attributes :

[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property)]
public class EnumerableSizeAttribute<T> : ValidationAttribute<IEnumerable<T>>
{
 
    protected override ValidationResult? IsValid(IEnumerable<T>? enumerable, ValidationContext validationContext)
    {
    //check collection size
    }
}

Inside IsValid you can check whether enumerable is an ICollection<T>, thus using the cheap Count property to check the size :

if(enumerable is ICollection<T> col && col.Count <...)
{
    
}

You may be able to use property pattern matching too, although I'm still not comfortable with the syntax. Something like

if(enumerable is ICollection<T> {Count: < MaxLength} col)
{
    
}