Nicer way of parameter checking?

Question

We use the .NET 2.0 framework with C# 3.0 (I think it's the last version of C# which can run on the 2.0 version of the framework, correct me if I am wrong).

Is there something built into C# which can make this type of parameter validation more convenient?

public ConnectionSettings(string url, string username, string password,
                          bool checkPermissions)
{
    if (username == null) {
        throw new ArgumentNullException("username");
    }

    if (password == null) {
        throw new ArgumentNullException("password");
    }

    if (String.IsNullOrEmpty(url)) {
        throw new ArgumentException("Must not be null or empty, it was " +
            (url == null ? url : "empty"), "url");
    }

    this.url = url;
    this.username = username;
    this.password = password;
    this.checkPermissions = checkPermissions;
}

That sort of parameter validation becomes a common pattern and results in a lot of "almost boilerplate" code to wade through in our public methods.

If there is nothing built in. Are there any great free libraries which we could use?

Answer 1

I normally create static helper methods...

E.g.

public static void CheckNotNull(object value, string parameterName)
{
   if(value == null) { throw new ArgumentNullException(parameterName); }
}

Means you can condense your code down to something similar to below and just makes it a little tidier.

CheckNotNull(username, "username");
CheckNotNull(password, "password"); 

Or you can wrap it up as an extension method:

public static void CheckNotNull<T>(this T value, string parameterName)
{
   if(value == null) { throw new ArgumentNullException(parameterName); }
}

And use like:

username.CheckNotNull("username");
password.CheckNotNull("password");

And if you're feeling really fancy, you could probably interrogate the parameter names by using reflection. Reflection's kinda slow, so you'd only do this if you were going to throw an exception, but it saves you typing the literal parameter name all the time.

Answer 2

You could use an il weaver like Post Sharp, keep in mind that compiler as a service in C# 5 will make this kind of stuff built in.

Personally I would not recommend this approach unless the problem is huge and must be tackled. Usually a few asserts and checking preconditions as you described above is a best practice.

EG:

public ConnectionSettings(
   [NotNullOrEmpty] string url, 
   [NotNull] string username, 
   [NotNull] string password,
   bool checkPermissions)
{
    this.url = url;
    this.username = username;
    this.password = password;
    this.checkPermissions = checkPermissions;
}

You could also integrate this kind of stuff with code contracts which would allow you to perform some rich static analysis.

Answer 3

Here's a nice fluent way of doing this.

What are your favorite extension methods for C#? (codeplex.com/extensionoverflow)

public static class Extensions
{
        public static void ThrowIfArgumentIsNull<T>(this T obj, string parameterName) where T : class
        {
                if (obj == null) throw new ArgumentNullException(parameterName + " not allowed to be null");
        }
}

internal class Test
{
        public Test(string input1)
        {
                input1.ThrowIfArgumentIsNull("input1");
        }
}

Answer 4

You can do it using contracts but it is the same concept.

This should be good practise anyhow, because it clearly shows what are mandatory fields on a public method.

Answer 5

It's not build into the .Net Framework but you can use Fluent Argument Validation

Answer 6

Similar idea to fluent parameter validation as mentioned by Giorgi, but this one avoids redundantly naming the parameter, and the strings that can't be automatically updated by code refactoring tools.

http://charlieflowers.wordpress.com/tag/parameter-validation/

Answer 7

Just to add to Sam's answer above, here's a link which tells you how you can go about implementing such attributes with PostSharp:

http://dpatrickcaldwell.blogspot.com/2009/03/validate-parameters-using-attributes.html

Answer 8

I recently released the first stable version of Guard, a library that helps you write guard clauses like these:

public ConnectionSettings(
    Uri url, string username, string password, bool checkPermissions)
{
    this.username = Guard.Argument(() => username).NotNull();
    this.password = Guard.Argument(() => password).NotNull();
    this.url = Guard.Argument(() => url).NotNull().Https();
    this.checkPermissions = checkPermissions;
}

• It's faster than you may think, this document delves deeper into the performance considerations.
• You can add new validations in the form of extension methods, this document explains it in detail.