AutoFixture是否可以从提供的数据集中生成随机字符串/文本？

Question

It is possible to use AutoFixture to generate random data for a string property .. but it's from a fixed data source?

For example: I have 30 street names hardcoded into a memory collection (array/list/whatever). Then, for my Address instance, the StreetName property isn't just a random string value (which is the default result from AutoFixture) but one of the street names from that hardcoded collection.

My first thought was to use a random number which AutoFixture might be able to create.. and this number is inside the array length/size ... so in effect i'm randomizing an array slot. Then, using this random number, get the value (aka street name) of the collection/array slot (ie. given the indexer, get the value at that index location).

Is this how it should be done?

Answer 1

As with so many other things regarding AutoFixture, things become much easier if you can use more explicit domain modelling . Instead of modelling StreetName as a string , introduce a domain object for it:

public sealed class StreetName
{
    private readonly string value;

    public StreetName(string streetName)
    {
        value = streetName ?? throw new ArgumentNullException(nameof(streetName));
    }

    public override bool Equals(object obj)
    {
        var other = obj as StreetName;
        if (other == null)
            return base.Equals(obj);

        return Equals(value, other.value);
    }

    public override int GetHashCode()
    {
        return value.GetHashCode();
    }

    public override string ToString()
    {
        return value;
    }

    public static implicit operator string(StreetName streetAddress)
    {
        return streetAddress.value;
    }

    public static implicit operator StreetName(string streetAddress)
    {
        return new StreetName(streetAddress);
    }
}

This is one of those modelling steps that are painful in C# and Java, but would be a one-liner in F# or Haskell...

Let's assume, however, that we have a list of predefined street names:

public static class StreetNames
{
    public static IEnumerable<string> Values = new[] {
        "221 B Baker St.",
        "1313 Webfoot Walk",
        "420 Paper St.",
        "42 Wallaby Way"
        /* More addresses go here... */ };
}

You can now trivially tell AutoFixture to pick only from that list, using ElementsBuilder :

var fixture = new Fixture();
fixture.Customizations.Add(
    new ElementsBuilder<StreetName>(StreetNames.Values.Select(s => (StreetName)s)));

At this point, though, it means that when you create StreetName values with AutoFixture, it'll pick from StreetNames.Values , but it still isn't going to do that when you ask it to create Address values. You can address (ha ha) that issue with a little ISpecimenBuilder :

public class StreetNameBuilder : ISpecimenBuilder
{
    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as PropertyInfo;
        if (pi == null || pi.Name != "StreetName" || pi.PropertyType != typeof(string))
            return new NoSpecimen();

        var sn = context.Resolve(typeof(StreetName));
        return (string)(StreetName)sn;
    }
}

Now you can configure your Fixture like this:

var fixture = new Fixture();
fixture.Customizations.Add(
    new ElementsBuilder<StreetName>(StreetNames.Values.Select(s => (StreetName)s)));
fixture.Customizations.Add(new StreetNameBuilder());

It'll now create Address values with StreetName values picked from the predefined list.

If you can't change your domain model, you can still add a class like StreetName . Just add it to your test code base instead of the production code base.