Constructor out parameters

Daily Knowledge Drop

Class constructors can contain out or ref parameters, and operate just as they would on a normal method.

Usages

While not a very common feature to make use of in most code, below we'll look at a few use cases when an out parameter on a constructor might be useful. There may definitely be better ways of handling these specific use cases, but using an out parameter is a way to handle them.

Dependent entity

Consider the situation when instantiating a class, another class is created at the same time using the same information.

In the below example, we have a Product class to store product information (including the current price) and a ProductPrice class to store the history of prices for the product.

When creating a Product record, ProductPrice record should be created at the same time:

var product = new Product(Guid.NewGuid(), "Green pants", 99.99m);
var price = new ProductPrice
{
    Id = product1.Id,
    Price = product1.Price
};

This could however also be done using an out parameter. Consider the following constructor:

public Product(Guid id, string name, decimal price, out ProductPrice productPrice)
{
    Id = id;
    Name = name;
    Price = price;

    productPrice = new ProductPrice 
    { 
        Id = id, 
        Price = price 
    };
}

Which can then be called as follows:

var product = new Product(Guid.NewGuid(), "Green pants", 99.99m, out ProductPrice price);

More concise? Yes. A better solution? Debatably.

Generated information

Expanding on the above example, maybe the Id of the Product is generated internally to the class, in the constructor. An out parameter could be used to return the instance Id from the constructor.

Here the Id is generated in the constructor, and assigned to the out parameter as well as the Id property of thd class itself:

public Product(string name, decimal price, out Guid id)
{
    id = Id = Guid.NewGuid();
    Name = name;
    Price = price;
}

The usage of the constructor:

var product = new Product("Green pants", 99.99m, out Guid productId);

Again - more concise than some alternatives? Yes. A better solution? Debatably.

Notes

This is not something I'd ever really considered, or ever required in 20 years of development - however it is an interesting feature. Maybe one day I might have a a practical use to use it in production code.

References

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Daily Drop 147: 26-08-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.