Question

I want to create a custom validation attribute, in which I want to compare the value of my property with another property’s value in my model class.
For example I have in my model class:

...    
public string SourceCity { get; set; }
public string DestinationCity { get; set; }

And I want to create a custom attribute to use it like this:

[Custom("SourceCity", ErrorMessage = "the source and destination should not be equal")]
public string DestinationCity { get; set; }
//this wil lcompare SourceCity with DestinationCity

How can I get there?

Answer 1

Here’s how you could obtain the other property value:

public class CustomAttribute : ValidationAttribute
{
    private readonly string _other;
    public CustomAttribute(string other)
    {
        _other = other;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(_other);
        if (property == null)
        {
            return new ValidationResult(
                string.Format("Unknown property: {0}", _other)
            );
        }
        var otherValue = property.GetValue(validationContext.ObjectInstance, null);

        // at this stage you have "value" and "otherValue" pointing
        // to the value of the property on which this attribute
        // is applied and the value of the other property respectively
        // => you could do some checks
        if (!object.Equals(value, otherValue))
        {
            // here we are verifying whether the 2 values are equal
            // but you could do any custom validation you like
            return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
        }
        return null;
    }
}