When to use: Tuple vs Class in C# 7.0

As this answer is causing some confusion amongst some folk here, I should clarify that - as per the question - all references to "tuple" here refer to the ValueTuple type and new tuple syntactic sugar features of C# 7 and in no way refer to the old System.Tuple reference types.

So now I am wondering, when Should I use tuples and when Should I create a class in c# 7.0?

Only you can really answer that question as it really depends on your code.

However, there are guidelines and rules you can follow in guiding you in choosing between them:

Tuples are values, so are copied by value, rather than by reference.

Most of the time, this should not be an issue. However, if you are passing around tuples of large structs, this might have an impact on performance. Ref locals/returns can be used to work around these performance issues, though.

Additionally, because they are values, modifying a copy remotely will not change the original copy. This is a good thing, but could catch some folk out.

Tuple element names are not persisted

The names given to elements are used by the compiler and (in most cases) are not available at run-time. This means that reflection cannot be used to discover their names; they cannot be accessed dynamically and they cannot be used in razor views.

Also this is an important consideration with APIs. A tuple returned from a method is the exception to the rule regarding after-compilation name discoverability. The compiler adds attributes to the method that hold information on the tuple names. This means you can safely return a tuple from a public method in one assembly and access its names in another.

Tuples are lightweight

Tuples are much simpler to write than types as they are less verbose and the declaration can be "inlined" (ie declared at the point of use). This works well when declaring a method that returns multiple values, for example.

However, because they are declared at the point of use, if you have MethodA that calls MethodB that calls MethodC and each returns a tuple, you'll need to redefine the tuple at every stage. There isn't (yet) a way of creating an alias of a tuple and re-using it across multiple methods.

Just use common sense

For any situation where you might consider using a tuple: simply ask yourself the question: "will a tuple simplify the code here". If the answer is "yes", then use one. And that ultimately is the primary consideration over whether to use a tuple or a custom class.

Generally speaking, named classes have some significance in the design of your system. They are also more verbose to write. For example, you may have a class called MediaFileOpener. It is important to the design that we know what this class does - we are working with media files!

Anonymous types and tuples are used when there is no design significance and all you want is a lightweight Data Transfer Object (DTO) to move information around.

As rule, if your class requires some documentation to describe what it is for, or if there is behaviour that it provides, use a full class. If all you need is temporary storage or some sort of grouping, use a Tuple. Consider a situation where you want to return more than one value from an async method. Tuple is designed to solve that problem.

Use a Class

If your objects are entities that are widely used throughout your application and are also stored in some kind of persistent storage like a relational database (SQL Server, MySQL, SQLite), a NoSQL Database or Cache (Redis, Azure DocumentDB) or even on simple text files or CSVs.

So yeah, anything persistent should have its own class.

Use a Tuple

If your objects are short lived without a special meaning for your application. For example, if you need to quickly return a pair of coordinates it's better to have something like this:

(double Latitude, double Longitude) getCoordinates()
{
    return (144.93525, -98.356346);
}

than define a separate class

class Coordinates
{
    public double Latitude { get; set; }
    public double Longitude { get; set; }
}

A Tuple will save you time from having to allocate memory on the heap using new for such a simple operation.

Another time when I find tuples useful is when performing multiple mathematical operations on some operands

(double Result1, double Result2, double Result3) performCalculations(int operand1, int operand 2)

It doesn't make sense to define a class in this case. No matter what the calculations are the results do not belong to a class. So the alternative would be to use out variables but I believe tuples are more expressive and result in improved readability.