.NET Core 3 Conditional Model Validation Attribute

ConditionalValidationAttribute.cs

using System;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;

namespace Foo {
    // Implementation makes use of the IPropertyValidationFilter interface that allows
    // control over whether the attribute (and its children, if relevant) need to be
    // validated.
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
    public class ConditionalValidationAttribute : Attribute, IPropertyValidationFilter {
        public string OtherProperty { get; set; }
        public object OtherValue { get; set; }

        public ConditionalValidationAttribute(string otherProperty, object otherValue) {
            OtherProperty = otherProperty;
            OtherValue = otherValue;
        }

        public bool ShouldValidateEntry(ValidationEntry entry, ValidationEntry parentEntry) {
            // Default behaviour if no other property is set: continue validation
            if (string.IsNullOrWhiteSpace(OtherProperty)) return true;

            // Get the property specified by the name. Might not properly work with
            // nested properties.
            var prop = parentEntry.Metadata.Properties[OtherProperty]?.PropertyGetter?.Invoke(parentEntry.Model);

            return prop == OtherValue;
        }
    }
}

SampleModel.cs

using System;
using System.ComponentModel.DataAnnotations;

namespace Foo {
    public class Model {
        public bool ShouldCheckOther { get; set; }
        
        [Required]
        [ConditionalValidation(nameof(ShouldCheckOther), true)]
        public string DependentRequired { get; set; }
    }
}