Asp.Net Core Model Validation Multiple Attributes

Question

I have a Asp.Net Core REST service and I'm using the built in validation. I needed some additional functionality, so I found some examples of validation attributes that I needed, so here is a small part of my model:

    [RequiredIfEmpty("B")]
    [RequiredIfEmpty("C")]
    public string A { get; set; }
    public string B { get; set; }
    public string C { get; set; }

So, pretty obvious what I'm going for. I want to validate that A is specified if B or C is empty.

When I send a JSON request that will fail validation, I only get:

"A is required when B is empty."

I'm expecting to get:

"A is required when B is empty."
"A is required when C is empty."

So, it seems like the validation code does a distinct on the attributes based on type because it ignores the 2nd one. This is further proven if I do:

    [RequiredIfEmpty("B")]
    [RequiredIfEmpty2("C")]
    public string A { get; set; }
    public string B { get; set; }
    public string C { get; set; }

RequiredIfEmpty2 is just derived from RequiredIfEmpty, no additional code. Now I get the expected:

"A is required when B is empty."
"A is required when C is empty."

In this example, I only have 2 dependent properties, so no biggie to create a 2 version, but its very hacky and I don't like it.

I thought about changing the RequiredIfEmpty attribute to take a string[] of properties, but it doesn't appear like the MVC infrastructure would allow multiple error strings returned by a single attribute.

I did report it to Microsoft, but wondering if anybody else can think of a work-around besides having a 2 version?

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

.Net Core + .Net Framework

You can override the Equals and GetHashCode methods in the attribute to create distinctions between AllowMultiple attributes.

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = false)]
public sealed class RequiredIfEmptyAttribute : RequiredAttribute
{
    private object _instance = new object();

    public override bool Equals(object obj) => _instance.Equals(obj);

    public override int GetHashCode() => _instance.GetHashCode();

    // all the rest of the code
}

.Net Framework Only

If you're only targeting the .Net Framework, you can use the TypeId property to be a unique identifier between two attributes of the same type. (MSDN documentation)

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = false)]
public sealed class RequiredIfEmptyAttribute : RequiredAttribute
{
    public override object TypeId { get; } = new object();

    // all the rest of the code
}

I thought about changing the RequiredIfEmpty attribute to take a string[] of properties, but it doesn't appear like the MVC infrastructure would allow multiple error strings returned by a single attribute.

Correct. You cannot return more than one message per attribute. You could implement the IValidatableObject interface to achieve this, however.

edited Jun 20, 2020 at 9:12

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answered Oct 27, 2016 at 0:35

Will Ray

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5 Comments

SledgeHammer Over a year ago

I based my attributes on the MVC Foolproof open source project, and I looked at that code real quick and they were not overriding TypeId, so I guess you found the problem :). Hopefully. I will test tomorrow. Thanks!

Will Ray Over a year ago

@SledgeHammer Glad to help. You might want to double-check on how the TypeId property is set. I'm using a new object, but MSDN uses a GUID. (Not sure if it matters.)

SledgeHammer Over a year ago

Wait... I'm confused. ValidationAttribute doesn't have a TypeId property? That seems like part of .Net 4.x rather then Core?

SledgeHammer Over a year ago

Yup... Core doesn't have that property. I then tried to override GetHashCode() with base.GetHashCode() ^ DependentProperty.GetHashCode(), but that didn't seem to work. But I got it to work by overriding Equals and checking if the DependentProperties are equal.

Will Ray Over a year ago

@SledgeHammer Holy buckets, you're right! I've learned something new today. I went ahead and updated my answer to reflect this.

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