TestDataBuilder.AutoFixture 0.1.0

TestDataBuilder

A small extension that enables AutoFixture to assign values to public readonly properties and fields.

Usage

TestDataBuilder provides an alternative Builder for AutoFixture that can set values for:

• Public properties with public getters and setters
• Public properties with public getters and private setters
• Public properties with only public getters
• Public fields
• Public readonly fields

To create a builder, you can use the following two extension methods for IFixture:

• To create a builder with default customizations:

var builder = fixture.NewTestDataBuilder<T>();

• To create a builder with default customizations:

Fixture fixture = new();
var builder = fixture
    .NewTestDataBuilder<T>(
        x => x.Create(y => y.Property, "defaultValue"),
        x => x.Create(y => y.NextProperty, () => 12 * 60)
    );

Adding default customizations at declaration time can be helpful when you need to use the builder to create multiple objects that share some common characteristics.

When a builder is created, it exposes the following methods:

ITestDataBuilder<T> With<TProperty>(Expression<Func<T, TProperty>> expression, TProperty value);
ITestDataBuilder<T> With<TProperty>(Expression<Func<T, TProperty>> expression, Func<TProperty> factory);
ITestDataBuilder<T> Without<TProperty>(Expression<Func<T, TProperty>> expression);
T Create();

For a smoother transition, the syntax is deliberately similar to that used by AutoFixture.

Limitations

At this time, the library cannot:

• create multiple objects (no equivalent to the CreateMany() method)
• assign values to readonly properties and fields without an explicit setup using the .With() methods
• assign with one declaration a value to a readonly field or property that is a nested object

There are simple strategied to overcome those limitations that will be further detailed in the next section of this guide.

How to crete multiple objects that share some common customizations

The simplest way to create multiple objects that share some traits is to store a customized builder with defaults in a variable and then call the .Create() method as often as required. Each instance thus created will be new.

Fixture fixture = new();
var builder = fixture
    .NewTestDataBuilder<MyClass>(
        x => x.Create(y => y.Property, "defaultValue"),
        x => x.Create(y => y.NextProperty, 12)
    );

var input1 = builder.Create();
var input2 = builder.Create();

Assert.Equal(input1.Property, input2.Property); // true
Assert.Equal(input1.NextProperty, input2.NextProperty); // true
Assert.Equal(input1, input2); //false

How to assign a customized value to a readonly property or field that is a reference type

The only way to assign custom values to a property or field that is a reference type is to create a custom builder for that value and pass an instance to the main builder.

class Root
{
    public NestedItem Item { get; }
}

class NestedItem
{
    public int Value { get; }
}

//...

// test definition
Fixture fixture = new();
var nestedItemBuilder = fixture
    .NewTestDataBuilder<NestedItem>(x => x.Create(y => y.Value, 12));

var rootItem = fixture
    .NewTestDataBuilder<Root>()
    .With(x => x.Item, nestedItemBuilder.Create())
    .Create();