Model validation: RequireIfNull attribute (ASP.NET Core) - asp.net-core

Question

I would need RequiredIfNull attribute for model validation.
How can I add conditional Required attribute. Condition would depend on another property. If that property value is null, then this shouldn't be.
Something like that:
public class MyModel
{
public int? prop1 { get; set; }
[ConditionalRequired(prop1)] //if prop1 == null, then prop2 is required, otherwise MyModel is invalid
public int? prop2 { get; set; }
}

Answer 1

You need a custom validation attribute. For example:
using System.ComponentModel.DataAnnotations;
using System.Reflection;
[AttributeUsage(AttributeTargets.Property)]
public class RequiredIfNullAttribute : ValidationAttribute
{
private const string DefaultErrorMessageFormat = "The {0} field is required.";
public RequiredIfNullAttribute(string otherProperty)
{
if (otherProperty == null)
{
throw new ArgumentNullException(nameof(otherProperty));
}
OtherProperty = otherProperty;
ErrorMessage = DefaultErrorMessageFormat;
}
public string OtherProperty { get; }
protected override ValidationResult IsValid(object value,
ValidationContext validationContext)
{
if (value == null)
{
var otherProperty = validationContext.ObjectInstance.
GetType().GetProperty(OtherProperty);
object otherPropertyValue = otherProperty.GetValue(
validationContext.ObjectInstance, null);
if (otherPropertyValue == null)
{
return new ValidationResult(
string.Format(ErrorMessageString, validationContext.DisplayName));
}
}
return ValidationResult.Success;
}
}
Then in your model:
public class MyModel
{
public int? prop1 { get; set; }
[RequiredIfNull(nameof(prop1))]
public int? prop2 { get; set; }
}
Things will get more complex if you also need to add client side validation.