PostSharp Parameter Validation - Optional String with Maximum Length

Question

I am using PostSharp 3.1 for parameter validation on a web service.

I have an optional string parameter which must be less than 50 characters when passed.

I currently have [StringLength(50)] which means that a string must be passed, string.Empty can be passed but null cannot.

null is valid for this parameter.

The way that I need it to work is the same as the [EmailAddress] validation - if null is passed, do not validate, else validate if any string is passed.

Can this be done?

Answer 1

I have written a custom validation attribute as follows:

using System;

using PostSharp.Aspects;
using PostSharp.Patterns.Contracts;
using PostSharp.Reflection;

public class NullOrStringLengthAttribute : LocationContractAttribute, ILocationValidationAspect<string>
{
    private readonly int maximum;

    public NullOrStringLengthAttribute(int maximum)
    {
        this.maximum = maximum;

        this.ErrorMessage = "The parameter '{0}' must be a string with a maximum length of " + maximum + ".";
    }

    public Exception ValidateValue(string value, string locationName, LocationKind locationKind)
    {
        if (value == null)
        {
            return null;
        }

        return value.Length > this.maximum ? this.CreateArgumentException(value, locationName, locationKind) : null;
    }

}

EDIT: Updated to include minimum string length and new PostSharp 4.x error message patterns:

using System;

using PostSharp.Aspects;
using PostSharp.Patterns.Contracts;
using PostSharp.Reflection;

public class NullOrStringLengthAttribute : LocationContractAttribute, ILocationValidationAspect<string>
{
    public NullOrStringLengthAttribute(int maximumLength)
    {
        this.ErrorMessage = string.Format("The {{2}} must be null or a string with a maximum length of {0}.", maximumLength);
        this.MaximumLength = maximumLength;
        this.MinimumLength = 0;
    }

    public NullOrStringLengthAttribute(int minimumLength, int maximumLength)
    {
        if (maximumLength != int.MaxValue)
        {
            this.ErrorMessage = string.Format("The {{2}} must be null or a string with length between {0} and {1}.", minimumLength, maximumLength);
        }
        else
        {
            this.ErrorMessage = string.Format("The {{2}} must be null or a string with a minimum length of {0}.", minimumLength);
        }

        this.MaximumLength = maximumLength;
        this.MinimumLength = minimumLength;
    }

    public int MaximumLength { get; private set; }

    public int MinimumLength { get; private set; }

    public Exception ValidateValue(string value, string locationName, LocationKind locationKind)
    {
        if (value == null)
        {
            return null;
        }

        if (value.Length < this.MinimumLength || value.Length > this.MaximumLength)
        {
            return this.CreateArgumentException(value, locationName, locationKind);
        }

        return null;
    }
}