In this tutorial we will go through of couple different ways of using custom constructor parameters when resolving an instance with Unity:

1. By using the built-in ParameterOverride
2. By creating a custom ResolverOverride.
   

Background

When you’re using a DI-container like Unity, you normally don’t have to worry about how the container resolves the new instance. You have configured the container and the container will act based on your configuration. But there may be cases where you have pass in custom constructor parameters for the resolve operation. Some may argue that this screams of bad architecture but there’s situations like bringing a DI-container to a legacy system which may require these kind of actions.

Resolved class

In this tutorial we are resolving the following test class:

public class MyClass
{
    public string Hello { get; set; }
    public int Number { get; set; }
 
    public MyClass(string hello, int number)
    {
        Hello = hello;
        Number = number;
    }

It is registered to the container using RegisterType-method and without passing in any parameters:

var unity = new UnityContainer();
unity.RegisterType<MyClass>();

So let’s see how we can pass in the “hello” and “number” variables for the MyClass’ constructor when calling Unity’s Resolve.

Unity ResolverOverride

Unity allows us to pass in a “ResolverOverride” when the container’s Resolve-method is called. ResolverOverride is an abstract base class and Unity comes with few of these built-in. One of them is ParameterOverride which “lets you override a named parameter passed to a constructor.” 

So knowing that we need to pass in a string named “hello” and an integer called “number”, we can resolve the instance with the help of ParameterOverride:

[Test]
public void Test()
{
    var unity = new UnityContainer();
    unity.RegisterType<MyClass>();
 
    var myObj = unity.Resolve<MyClass>(new ResolverOverride[]
                                   {
                                       new ParameterOverride("hello", "hi there"), new ParameterOverride("number", 21)
                                   });
 
    Assert.That(myObj.Hello, Is.EqualTo("hi there"));
    Assert.That(myObj.Number, Is.EqualTo(21));
}

We pass in two instances of ParameterOverride. Both of these take in the name and the value of the parameter.

Custom ResolverOverride: OrderedParametersOverride

But what if you don’t like passing in the parameter names and instead you want to pass in just the parameter values, in correct order? In order to achieve this we can create a custom ResolverOverride. Here’s one way to do it:

public class OrderedParametersOverride : ResolverOverride
{
    private readonly Queue<InjectionParameterValue> parameterValues;
 
    public OrderedParametersOverride(IEnumerable<object> parameterValues)
    {
        this.parameterValues = new Queue<InjectionParameterValue>();
        foreach (var parameterValue in parameterValues)
        {
            this.parameterValues.Enqueue(InjectionParameterValue.ToParameter(parameterValue));
        }
    }
 
    public override IDependencyResolverPolicy GetResolver(IBuilderContext context, Type dependencyType)
    {
        if (parameterValues.Count < 1)
            return null;
 
        var value = this.parameterValues.Dequeue();
        return value.GetResolverPolicy(dependencyType);
    }

The parameter values are passed  in through the constructor and put into a queue. When the container is resolving an instance, the parameters are used in the order which they were given to the OrderedParametersOverride.

Here’s a sample usage of the new OrderedParametersOverride:

[Test]
public void TestOrderedParametersOverride()
{
    var unity = new UnityContainer();
    unity.RegisterType<MyClass>();
 
    var myObj = unity.Resolve<MyClass>(new OrderedParametersOverride(new object[] {"greetings", 24 }));
 
    Assert.That(myObj.Hello, Is.EqualTo("greetings"));
    Assert.That(myObj.Number, Is.EqualTo(24));
}

Sample code

The above examples can be found from GitHub.